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COURSE: MWM 705 - WATERSHED DEGRADATION AND REHABILITATION
BY: MR.DR. KENNEDY OBIERO (PHD STUDENT, DEPARTMENT OF GEOGRAPHY, KENYATTA UNIVERSITY)
CLICK ON TOPIC 5 or Watershed Sedimentation on the left handside on this page to see details of the respective watershed degradation and sedimentation

COURSE: MWM 705 - WATERSHED DEGRADATION AND REHABILITATION
BY: MR. KENNEDY OBIERO (PHD STUDENT, DEPARTMENT OF GEOGRAPHY, KENYATTA UNIVERSITY)
TOPIC 5 or Watershed Sedimentation on the this page - To read on Effectsto see details of Firethe respective watershed degradation and Grazing on Watershed Degradationsedimentation

KENYATTA UNIVERSITY
Department of Geography
Msc. Integrated Watershed Management
MWM 705: Watershed Degradation and Rehabilitation
Assignment Title
Topic 2: Sedimentation in Watersheds
Student Name: Clement Mromba
Reg No: I56EA/12155/2009
Course Instructor: Mr. Kennedy Obiero
Date of submission: 1st March 2010
Table of Contents
1.0. Introduction. 2
1.1. Classification of watersheds. 3
2.0. Sedimentation. 4
2.1. Direct causes of sedimentation in watersheds. 5
2.2. Indirect causes of watershed sedimentation. 6
3.0. Processes of sedimentation in watersheds. 7
4.0. Estimation of sedimentation in watersheds. 8
5.0. Effects of sedimentation in watersheds. 8
6.0. Controlling sedimentation in watersheds. 9
7.0. Conclusion. 12
8.0. References. 12
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The term watershed is used (especially in North America and Europe) to indicate an area of land from which all water falling as rain or snow would flow toward a single point (Figure 1) (Musy, 2001).This includes both surface water flow, such as rivers and stream and the underground movement of water. Watershed, drainage basin and catchment are used synonymously and all of them refer to the area of land drained by a river system (Langbein, 1995).
Figure 1: The Watershed, hydrologic cycle and its components. Source: (Musy, 2001)
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Watersheds are classified basing on hydrological pattern and size. Basing on hydrological pattern, there are three types of watersheds:
i. Exorheic/Open watershed, which empty to the sea and represent the major part of the drainage of all of the continents except Australia.
ii. Endorheic/Closed watershed, which discharge/empty into an inland, into closed lake basins, and are mainly (but not exclusively) restricted to the arid and semi-arid regions.
iii. Arheic regions, which is the region within which no rivers arise (the lower part of the Nile, Oranje and Niger, all in Africa, are a good examples of this category of basin).
iv. Multiple open watersheds empty into the ocean form more than one source. Within watershed areas you will find other wetland areas like ponds, swamps and marshes.
Basing on size there are four types of watersheds; Sub-watershed (100-500 sq.km), Milli-Watershed (10-100 sq.km), Micro-Watershed (1-10 sq.km) and Mini-Watershed (Less than 1 sq.km) (Figure 2) (Langbein, 1995).
Figure 2: Classification of watershed according to their sizes (Langbein, 1995)
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Sedimentation is the process which involves detachment of soil materials, transport of soil materials, and deposition of eroded materials. These materials are called sediments; sediments involve particles of organic (living) and inorganic (nonliving) origin that accumulate in loose form on the surface, riverbed and seabed. Sedimentation in the watershed comes from four different sources, such as rock (weathering), organic material (biological), dissolved compounds in water and outer space. There are different types of sediments (Table 1) and their corresponding symbols such as; S=sand, M=mud, CL = clay, Si=silt, St=stones, G=gravel, P=pebbles, Rk=rock, Co=coral and Wd=weed (Wetzel, 2001).
Sediments
Terrigenous
Biogenous
Hydrogenous
Origin
Material eroded from the continents
Skeletal structures of organisms
Formed from dissolved salts
Mode of transport
Rivers, wind, glaciers
Settles from top of water column
Not transported, precipitates in place
General occurrence
Along continental margins; finest size reaches deep ocean basin
Intermediate depths at all latitudes where lithogenous sediments are not important (e.g., flanks of oceanic ridges); upwelling regions
In deep ocean basins where input of other sediments is minimal
Usual constituents
Silicate minerals (e.g. quartz, feldspar, clay minerals)
Calcium carbonate (coccoliths and foraminifera) and silica (diatoms and radiolaria)
Highly variable; major examples are manganese nodules, phosphorite nodules, and evaporites
Relative abundance
~50 %
~50%
< 1%
Table 1: Different types of sediment with their sources (Wetzel, 2001)
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Watershed degradation and ultimate sedimentation are closely related, it is the result of the interaction of many environmental, economic, social, cultural and political conditions in any given region (Freedman, 1995). There are two main causes of watershed sedimentation. The primary and most common reasons for sedimentation are known as the direct causes. Logging, overpopulation, urbanization, dam construction etc are under direct causes. The other main cause of sedimentation is known as natural/Mother Nature causes (Freedman, 1995).
Poor farming practices: Farming is still predominantly practiced especially with traditional methods which are causing watershed sedimentation. Peasants tend to cope with fluctuating climatic conditions by conquering new low-fertility lands through slash-and-burn (Brenda, 2002). Through this process, peasants end by burning the limited soil nutrients, thus lowering the potential primary biological productivity level. Old agricultural lands receive very little rehabilitation care through the use of chemical fertilizers, which are generally too expensive for peasants. Organic fertilization with harvested straws and animal dung is limited. Straw is consumed by domestic animals soon after it is harvested on farms and they then leave much dispersed dung. As a result, old farmlands tend to lose most of their fertility. Some are abandoned as they yield far below expectation. Because of these weak land management practices, farmlands are exposed to water erosion during the wet season and to wind erosion after harvest. Both of these erosion processes are known to be strong causes of nutrient transportation leaving degraded soils behind and sediment loads in watershed (Thangam, 1979).
Rapid population growth/overpopulation has resulted to the conversion of forest areas to non-forest lands for settlement and farming. Together with this is urbanization and residential area expansion. This takes a significant loss of forest lands both for harvesting forest products as more people need more timber to build their houses and for developing the greater area their houses, malls, business centers will be built (Thiam, 2002).
Overgrazing is another disastrous activity in a watershed. Thus, watershed forests are destroyed and converted to cattle pasture to supply the burgeoning demand for meat. In Central America, almost half of the rainforests have been slashed and burned for cattle farming in order to comply with foreign demands. Twenty-five per cent of the Amazon's forests have also been destroyed for cattle ranches causing sedimentation in Amazon basin (Thiam, 2002).
While most causes of sedimentation occur due to human activities, there are uncontrolled causes such as mass wasting, volcanic eruption, and earthquakes. Forest fires are started by lightning, and strong winds help to spread the flames. Drought in the forest has increased the amount of flammable bush and debris on the forest floor. Forest fires destroy immeasurable amount of valuable timber. They kill not only trees but also other living things. Meanwhile, volcanic eruption is one of the several natural forces capable of causing damage to forests. The ashes emitted during the eruption coat tree leaves, which then interfere with photosynthesis. Animal population is also devastated. The organisms that survive have to cope with the changed habitat and reduced food supplies.
_Toc254553903
Lack of government legislation for land reforms has also cleared the forest especially in developing countries like of the South East Asian nations. People in that region are among the poorest in the world and are desperate for a piece of land. Unequal distribution of resources has led these people to find their way to exploit the forests (Myers, 1991).
Another reason that denudes the forest is exploitative economic development schemes and the powerlessness of government to safeguard its resources. Poor countries in their attempt to increase their revenues are in a way exploiting their resources like the forests. Timber is exported to reduce the national debt. Countries rich in mineral resources open their doors to multinational mining corporations that clear the forests as they go with their operations. The government especially those belonging in the Third World cannot curb commercial logging and implement a total log ban in exchange to higher foreign exchange rates. Development projects like dams, roads, and airports contracted by the government also cause deforestation which finally results into watershed sedimentation (Myers, 1991).
Factors which determines the speed and rate of sedimentation in watersheds
To start with topography or terrain of the watershed's land. If the area is steep, the water there is likely to flow quickly and cause flooding and erosion, whereas flat watersheds have often have slower flowing rivers. Another feature of a watershed's physical landscape is its geology or soil type. Sandy soils for example absorb water quickly and has low surface run-off, while hard, clay soils are less permeable. Both of these have implications for runoff, erosion and ground water. In a watershed where rock layer is impermeable it will have more surface run-off which will trigger more erosion. Vegetation covering watersheds reduces erosive agent speed and erosion, conserve soil moisture and protect river bank. In Vegetated watershed tree or glasses act as barriers of strong wind and running water. Plant leaves intercept strong rain drops which always detaches soil in bare land. Nature of materials in terms of Size, shape and density plays significant role in watershed sedimentation. Larger materials such as big stones and boulders are heavy, so it is difficult to be washed away unless there is more force used by erosive agents. But when these heavy materials are able to be transported they are easily deposited or sunk as compared to right materials like wood and leaves (Tangtham, 1993).
_Toc254553904
In many watersheds, sedimentation is pronounced during rainfall events. This lead to increased runoff on more impermeable surfaces and large amounts of sediment get washed from the higher areas of the watershed directly to the bottom of the watershed (Mobile Press Register, 1998).
Also, smaller river channels are responsible for delivering important sediment, water, and nutrients downstream to larger channels within the stream’s network.
By removing the vegetation, the most important factor in stream bank and land or surface stability, it exposes riverbank or land surface to different erosive agents such as wind, moving water and ice, leaves land surface bare, disintegrate rock into small fragments, soil lack support from vegetation, and finally, soil become weak and easy to be washed away (Moore, 2003).
Thus, increases the amount of erosion from the sides of the stream into the bottom of it. Once we get a significant rain, the sediment from the bottom gets transported downstream, until the velocity of the water weakens or the weight of the sediment becomes too heavy to carry. For sedimentation to occur it passes the following process; Erosion: Washing away of materials from one point to another (Highland-low-lowland). Transport: Carrying materials by floatation, solution, suspension, bouncing and dragging from one point to another and finally deposition: Settling of particles in a river or low land (Sehgal and Abrol, 1994).
_Toc254553905
Wilcock (1997) used transport observations from four streams and one flume study to demonstrate that grouped sand and gravel transport rates collapse about two common relationships (one for sand, one for gravel) when the bed shear stress is scaled by an empirically defined reference shear stress for the sand and gravel. He came up with this equation;
W∗i=
The sediment transport rate is measured by;
Where, W*i is sediment transport rate, s is is sediment density, is fluid density, g is acceleration of gravity, fi is the proportion of fraction i in the bed, qbi is transport rate in units of mass per unit width and time, and the subscript i represents either the sand or the gravel fraction, = 0.04 which is a value of clean sand.
OR
Estimate of sedimentation and volume of sediments in watershed is measured by;
_Toc254553906
Sediment deposition in a watershed alter the watershed environment significantly, resulting in multifarious physical, biological, safety and economic impacts on the rivers, the dam structure, biodiversity, and the environment in general (Power, 1988). The physical impacts include the following; modification of regime of the river discharge, transformation of river channel morphology in the zone downstream of the river and for some distance upstream. The biological impacts include the following; effect on aquatic migration (upstream-downstream), effect on water quality and quantity, effect on aquatic life (plants and animals), effect on soil quality to support agricultural activities (Sehgal and Abrol, 1994).
Economic and social impacts include the following; reduction of storage capacity which then affects the ability of the river to meet the needs for which it is expected which include also (1) energy production, (2) agriculture and industries, (3) discharge regulations, (4) recreation, (5) fire fighting and (6) public health. Severe flooding increases property damage and loss of human life and millions of dollars are spent each year to remove sediments from storm drain systems, reservoirs, water treatment plants and to repair flood damage (Sehgal and Abrol, 1994).
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The proper measures of controlling sedimentation are through its causes and sources. Series of physical, socio-economic and mechanical measures can be taken. Katyal, et.al. (1994) suggested the following sediment control measures:
Plan the development to fit the site characteristics: Site characteristics such as topography, soils, drainage patterns, and covers should be considered when developing a site. Areas which are prone to erosion should be left undisturbed and undeveloped if possible. Entrance and exits points for runoff should be protected from erosion and equipped with sediment control devices.
Minimize the extent of the disturbed area and the duration of exposure and stabilize disturbed areas as soon as possible; by staging construction and preserving existing vegetation, erosion can be reduced significantly. Once a land surface is disturbed, we should minimize the duration of exposure by protecting it from erosion if possible. Typically, if an area is not going being worked on in more than 45 days, it should be protected by erosion control mats. The State of Maryland has demonstrated the effectiveness of stage construction by enacting strict regulation on grading practices.
Direct runoff away from problems areas: Concentrated flows if possible should be diverted away from problems areas as discussed in the last section.
Keep runoff velocity low: Runoff velocity should be kept as low as possible. For drainage ways such as ditches, high velocity can be reduced by a series of rock check dams which break the flow velocity. Overland flow velocity can be reduced by minimizing slope length and steepness.
Maintaining forests or vegetative cover: including crop residue, is particularly important in reducing wind erosion. It anchors the soil, increases surface roughness, reduces wind speed, conserves soil moisture and adds organic matter which helps bind the soil particles into aggregates. Clearing and cultivating land removes the vegetative cover for part or all of the year. Large, open fields are especially erosion prone because long, unobstructed distances allow the wind's velocity to increase. Tree roots help stabilize stream banks, and tree shade helps reduce algae growth in streams in some cases. Streamside vegetation also traps sediments before they reach the stream and absorbs nitrates from groundwater. Clearing trees removes these benefits.
Maintained forest covers in southern slopes of Mt. Kilimanjaro in Tanzania (Author, 2010)
Contour bunds: The practice of tillage along the contour (or across the slope) and of flat seeding followed by ridging after crop establishment, appears to improve the effectiveness of tillage for rainwater conservation. Ridging and furrowing create a mosaic of mini-catchments to trap rainwater and control soil erosion. Typically, between 30% and 40% of the total precipitation is converted into runoff (Huda et al., 1988). Bunds of different kinds (contour or graded) have been important means, not for in-situ rainwater conservation, but also for runoff harvesting and erosion control. Strengthening the property bunds to as a means of rainwater harvesting may offer a viable solution (Kerr and Sanghi, 1996). In a large catchment, runoff is directed to concentrate at the drainage point of a watershed by the construction of terraces.
Agro forestry: Agroforestry is a system of land use with simultaneous cultivation of trees or bushes and of arable crops or pasture plants. The system fulfils many of the economic, social and environmental requisites (Katyal et al., 1994). Integration of arable crops with trees provides an opportunity to harness the potential of the crop when the trees are young and do not yet yield an economic benefit. Even under severe circumstance, a tree often survives and provides fodder and fuel (Katyal et al., 1994). It makes the environment more hospitable. Additionally, trees make use of off-season rains, recycle nutrients from deeper soil layers, and suppress weeds because of their large canopy. Agroforestry may gain greater acceptance with economically attractive, multipurpose tree species, provide farmers have access to markets for these products. Trees yielding fruits, oil, pesticides, and pharmaceuticals, flavoring agents, timber, fuel and fodder should be considered (Brenda, 2002). Other measures of controlling sedimentation in watersheds may include; environmental education, law enforcement, construction of river bank, construction of water ways, diversification of economy.
Role of Agroforestry in controlling sediments in watersheds. Source: Author (2009)
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By studying the key watershed features in addition to activities along waterways scientists, other researchers and city governments can work to keep them healthy because a small change in one portion of a watershed can drastically affect other parts. The immediate effects of sedimentation may not yet be felt, but if this generation doesn’t feel it the next generation and their children will be the ones to suffer. The only way to ensure that we will not encounter any of the consequences of sedimentation is to stop destroying the biosphere or watershed resources all together.
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Brenda McLellan (2002) Agriculture and Rural Development: Impact of Forest Harvesting.
Alberta. ca Printers, USA. (Retrived on September 14, 2009).
FAO (2005). Global Forest Resources Assessment Report: Impact of Deforestation in Tropical
Forest sub-regions. Amsterdam.
Freedman, B. (1995). Environmental Ecology. 2nd ed. San Diego: Academic Press.
Huda, A.K.S Pathak, P., Rego, T.J and Virmani, S.M. (1988). Agroclimatic considerations for
improved soil and water management and efficient fertilizer use in semiarid India.
Fertilizer News 33: 51-57.
Katyal, J.C., Das, S.K., Korwar, G.R and Osman, M. (1994). Technology for mitigating stresses:
Alternative land uses. In Stressed Ecosystems and Sustainable Agriculture (Virmani
S.M., Katyal, J.C., Eswaran, H and Abrol, I.P eds.). Oxford and IBH Publishing Co. Pvt.
Ltd.New Delhi, India. pp 291-306.
Kerr, J.M., Sanghi, N.K and Sriramappa, G. (1996). Subsidies in watershed development
programs in India: Distortions and opportunities. International Institute for Environment
and Development Gatekeeper Series # 61. London, UK.
Langbein, W. B. and Iseri, K.T. (1995). Manual of Hydrology: Part 1. General Surface-Water
Techniques. Geological survey water-supply paper 1541-Methods and practices of the
Geological Survey. HTML version http://water.usgs.gov/wsc.
Mobile Register (1998). West Mobile’s muddy waters. Section A, Page 1. Retrieved
February 18, 2008 from University Library Archives.
Moore, R. D and Richardson, J. S. (2003). Progress Towards Understanding the Structure,
Function, and Ecological Significance of Small stream Channels and their Riparian
Zones. Canadian Journal of Forest Research 33(8): 1349-1359. Retrieved February 18,
2008 from EBSCO.
Musy, A. (2000) e-drologie. Ecole Polytechnique Fédérale, Lausanne, Suisse.
Myers, N. (1991). "Trees by the Billions". International Wildlife, 112(7): 12-15. Bonn Press,
German.
O'Sullivan, P. E. & C. S. Reynolds, 2004. The Lakes Handbook. Limnology and limnetic
ecology. Blackwell Publishing, Malden, MA, USA.
Power, G. (1988). “Siltation is a threat to Whole World’s Storage dams”. World Water, June
1988, Thomas Telford, London, pp17-19.
Sehgal, J.L and Abrol, I.P. (1994). Soil degradation in India: Status and impact. Oxford and IBH
Publishing Co. Pvt. Ltd., New Delhi, India.
Thangam, E.S. (1979). "Shifting Cultivation in Arunachal Pradesh" Proceedings of Agro-forestry
seminar at Imphal, India.
Thiam, A.K. (2002). The causes and spatial pattern of land degradation risk in southern
Mauritania using multitemporal avhrr-ndvi imagery and field data. Wiley
InterScienceUniversity of Central Arkansas, Conway, USA.
Wetzel, R. G., 2001. Limnology: Lake and river ecosystems. Academic Press, San Diego.
Wilcock PR. 1997. A Method for Predicting Sediment Transport in Gravel-bed Rivers. Report to
the United States Forest Service, Rocky Mountain Forest and Range Experiment
Station: FortCollins, CO.

THIS MATERIAL HAS BEEN PREPARED AND PRESENTED BY CLEMENT
WATERSHED SEDIMENTATION
KENYATTA UNIVERSITY
Department of Geography
Msc. Integrated Watershed Management
MWM 705: Watershed Degradation and Rehabilitation
Assignment Title
Topic 2: Sedimentation in Watersheds
Student Name: Clement Mromba
Reg No: I56EA/12155/2009
Course Instructor: Mr. Kennedy Obiero
Date of submission: 1st March 2010
Table of Contents
1.0. Introduction. 2
1.1. Classification of watersheds. 3
2.0. Sedimentation. 4
2.1. Direct causes of sedimentation in watersheds. 5
2.2. Indirect causes of watershed sedimentation. 6
3.0. Processes of sedimentation in watersheds. 7
4.0. Estimation of sedimentation in watersheds. 8
5.0. Effects of sedimentation in watersheds. 8
6.0. Controlling sedimentation in watersheds. 9
7.0. Conclusion. 12
8.0. References. 12
_Toc254553899
The term watershed is used (especially in North America and Europe) to indicate an area of land from which all water falling as rain or snow would flow toward a single point (Figure 1) (Musy, 2001).This includes both surface water flow, such as rivers and stream and the underground movement of water. Watershed, drainage basin and catchment are used synonymously and all of them refer to the area of land drained by a river system (Langbein, 1995).
Figure 1: The Watershed, hydrologic cycle and its components. Source: (Musy, 2001)
_Toc254553900
Watersheds are classified basing on hydrological pattern and size. Basing on hydrological pattern, there are three types of watersheds:
i. Exorheic/Open watershed, which empty to the sea and represent the major part of the drainage of all of the continents except Australia.
ii. Endorheic/Closed watershed, which discharge/empty into an inland, into closed lake basins, and are mainly (but not exclusively) restricted to the arid and semi-arid regions.
iii. Arheic regions, which is the region within which no rivers arise (the lower part of the Nile, Oranje and Niger, all in Africa, are a good examples of this category of basin).
iv. Multiple open watersheds empty into the ocean form more than one source. Within watershed areas you will find other wetland areas like ponds, swamps and marshes.
Basing on size there are four types of watersheds; Sub-watershed (100-500 sq.km), Milli-Watershed (10-100 sq.km), Micro-Watershed (1-10 sq.km) and Mini-Watershed (Less than 1 sq.km) (Figure 2) (Langbein, 1995).
Figure 2: Classification of watershed according to their sizes (Langbein, 1995)
_Toc254553901
Sedimentation is the process which involves detachment of soil materials, transport of soil materials, and deposition of eroded materials. These materials are called sediments; sediments involve particles of organic (living) and inorganic (nonliving) origin that accumulate in loose form on the surface, riverbed and seabed. Sedimentation in the watershed comes from four different sources, such as rock (weathering), organic material (biological), dissolved compounds in water and outer space. There are different types of sediments (Table 1) and their corresponding symbols such as; S=sand, M=mud, CL = clay, Si=silt, St=stones, G=gravel, P=pebbles, Rk=rock, Co=coral and Wd=weed (Wetzel, 2001).
Sediments
Terrigenous
Biogenous
Hydrogenous
Origin
Material eroded from the continents
Skeletal structures of organisms
Formed from dissolved salts
Mode of transport
Rivers, wind, glaciers
Settles from top of water column
Not transported, precipitates in place
General occurrence
Along continental margins; finest size reaches deep ocean basin
Intermediate depths at all latitudes where lithogenous sediments are not important (e.g., flanks of oceanic ridges); upwelling regions
In deep ocean basins where input of other sediments is minimal
Usual constituents
Silicate minerals (e.g. quartz, feldspar, clay minerals)
Calcium carbonate (coccoliths and foraminifera) and silica (diatoms and radiolaria)
Highly variable; major examples are manganese nodules, phosphorite nodules, and evaporites
Relative abundance
~50 %
~50%
< 1%
Table 1: Different types of sediment with their sources (Wetzel, 2001)
_Toc254553902
Watershed degradation and ultimate sedimentation are closely related, it is the result of the interaction of many environmental, economic, social, cultural and political conditions in any given region (Freedman, 1995). There are two main causes of watershed sedimentation. The primary and most common reasons for sedimentation are known as the direct causes. Logging, overpopulation, urbanization, dam construction etc are under direct causes. The other main cause of sedimentation is known as natural/Mother Nature causes (Freedman, 1995).
Poor farming practices: Farming is still predominantly practiced especially with traditional methods which are causing watershed sedimentation. Peasants tend to cope with fluctuating climatic conditions by conquering new low-fertility lands through slash-and-burn (Brenda, 2002). Through this process, peasants end by burning the limited soil nutrients, thus lowering the potential primary biological productivity level. Old agricultural lands receive very little rehabilitation care through the use of chemical fertilizers, which are generally too expensive for peasants. Organic fertilization with harvested straws and animal dung is limited. Straw is consumed by domestic animals soon after it is harvested on farms and they then leave much dispersed dung. As a result, old farmlands tend to lose most of their fertility. Some are abandoned as they yield far below expectation. Because of these weak land management practices, farmlands are exposed to water erosion during the wet season and to wind erosion after harvest. Both of these erosion processes are known to be strong causes of nutrient transportation leaving degraded soils behind and sediment loads in watershed (Thangam, 1979).
Rapid population growth/overpopulation has resulted to the conversion of forest areas to non-forest lands for settlement and farming. Together with this is urbanization and residential area expansion. This takes a significant loss of forest lands both for harvesting forest products as more people need more timber to build their houses and for developing the greater area their houses, malls, business centers will be built (Thiam, 2002).
Overgrazing is another disastrous activity in a watershed. Thus, watershed forests are destroyed and converted to cattle pasture to supply the burgeoning demand for meat. In Central America, almost half of the rainforests have been slashed and burned for cattle farming in order to comply with foreign demands. Twenty-five per cent of the Amazon's forests have also been destroyed for cattle ranches causing sedimentation in Amazon basin (Thiam, 2002).
While most causes of sedimentation occur due to human activities, there are uncontrolled causes such as mass wasting, volcanic eruption, and earthquakes. Forest fires are started by lightning, and strong winds help to spread the flames. Drought in the forest has increased the amount of flammable bush and debris on the forest floor. Forest fires destroy immeasurable amount of valuable timber. They kill not only trees but also other living things. Meanwhile, volcanic eruption is one of the several natural forces capable of causing damage to forests. The ashes emitted during the eruption coat tree leaves, which then interfere with photosynthesis. Animal population is also devastated. The organisms that survive have to cope with the changed habitat and reduced food supplies.
_Toc254553903
Lack of government legislation for land reforms has also cleared the forest especially in developing countries like of the South East Asian nations. People in that region are among the poorest in the world and are desperate for a piece of land. Unequal distribution of resources has led these people to find their way to exploit the forests (Myers, 1991).
Another reason that denudes the forest is exploitative economic development schemes and the powerlessness of government to safeguard its resources. Poor countries in their attempt to increase their revenues are in a way exploiting their resources like the forests. Timber is exported to reduce the national debt. Countries rich in mineral resources open their doors to multinational mining corporations that clear the forests as they go with their operations. The government especially those belonging in the Third World cannot curb commercial logging and implement a total log ban in exchange to higher foreign exchange rates. Development projects like dams, roads, and airports contracted by the government also cause deforestation which finally results into watershed sedimentation (Myers, 1991).
Factors which determines the speed and rate of sedimentation in watersheds
To start with topography or terrain of the watershed's land. If the area is steep, the water there is likely to flow quickly and cause flooding and erosion, whereas flat watersheds have often have slower flowing rivers. Another feature of a watershed's physical landscape is its geology or soil type. Sandy soils for example absorb water quickly and has low surface run-off, while hard, clay soils are less permeable. Both of these have implications for runoff, erosion and ground water. In a watershed where rock layer is impermeable it will have more surface run-off which will trigger more erosion. Vegetation covering watersheds reduces erosive agent speed and erosion, conserve soil moisture and protect river bank. In Vegetated watershed tree or glasses act as barriers of strong wind and running water. Plant leaves intercept strong rain drops which always detaches soil in bare land. Nature of materials in terms of Size, shape and density plays significant role in watershed sedimentation. Larger materials such as big stones and boulders are heavy, so it is difficult to be washed away unless there is more force used by erosive agents. But when these heavy materials are able to be transported they are easily deposited or sunk as compared to right materials like wood and leaves (Tangtham, 1993).
_Toc254553904
In many watersheds, sedimentation is pronounced during rainfall events. This lead to increased runoff on more impermeable surfaces and large amounts of sediment get washed from the higher areas of the watershed directly to the bottom of the watershed (Mobile Press Register, 1998).
Also, smaller river channels are responsible for delivering important sediment, water, and nutrients downstream to larger channels within the stream’s network.
By removing the vegetation, the most important factor in stream bank and land or surface stability, it exposes riverbank or land surface to different erosive agents such as wind, moving water and ice, leaves land surface bare, disintegrate rock into small fragments, soil lack support from vegetation, and finally, soil become weak and easy to be washed away (Moore, 2003).
Thus, increases the amount of erosion from the sides of the stream into the bottom of it. Once we get a significant rain, the sediment from the bottom gets transported downstream, until the velocity of the water weakens or the weight of the sediment becomes too heavy to carry. For sedimentation to occur it passes the following process; Erosion: Washing away of materials from one point to another (Highland-low-lowland). Transport: Carrying materials by floatation, solution, suspension, bouncing and dragging from one point to another and finally deposition: Settling of particles in a river or low land (Sehgal and Abrol, 1994).
_Toc254553905
Wilcock (1997) used transport observations from four streams and one flume study to demonstrate that grouped sand and gravel transport rates collapse about two common relationships (one for sand, one for gravel) when the bed shear stress is scaled by an empirically defined reference shear stress for the sand and gravel. He came up with this equation;
W∗i=
The sediment transport rate is measured by;
Where, W*i is sediment transport rate, s is is sediment density, is fluid density, g is acceleration of gravity, fi is the proportion of fraction i in the bed, qbi is transport rate in units of mass per unit width and time, and the subscript i represents either the sand or the gravel fraction, = 0.04 which is a value of clean sand.
OR
Estimate of sedimentation and volume of sediments in watershed is measured by;
_Toc254553906
Sediment deposition in a watershed alter the watershed environment significantly, resulting in multifarious physical, biological, safety and economic impacts on the rivers, the dam structure, biodiversity, and the environment in general (Power, 1988). The physical impacts include the following; modification of regime of the river discharge, transformation of river channel morphology in the zone downstream of the river and for some distance upstream. The biological impacts include the following; effect on aquatic migration (upstream-downstream), effect on water quality and quantity, effect on aquatic life (plants and animals), effect on soil quality to support agricultural activities (Sehgal and Abrol, 1994).
Economic and social impacts include the following; reduction of storage capacity which then affects the ability of the river to meet the needs for which it is expected which include also (1) energy production, (2) agriculture and industries, (3) discharge regulations, (4) recreation, (5) fire fighting and (6) public health. Severe flooding increases property damage and loss of human life and millions of dollars are spent each year to remove sediments from storm drain systems, reservoirs, water treatment plants and to repair flood damage (Sehgal and Abrol, 1994).
_Toc254553907
The proper measures of controlling sedimentation are through its causes and sources. Series of physical, socio-economic and mechanical measures can be taken. Katyal, et.al. (1994) suggested the following sediment control measures:
Plan the development to fit the site characteristics: Site characteristics such as topography, soils, drainage patterns, and covers should be considered when developing a site. Areas which are prone to erosion should be left undisturbed and undeveloped if possible. Entrance and exits points for runoff should be protected from erosion and equipped with sediment control devices.
Minimize the extent of the disturbed area and the duration of exposure and stabilize disturbed areas as soon as possible; by staging construction and preserving existing vegetation, erosion can be reduced significantly. Once a land surface is disturbed, we should minimize the duration of exposure by protecting it from erosion if possible. Typically, if an area is not going being worked on in more than 45 days, it should be protected by erosion control mats. The State of Maryland has demonstrated the effectiveness of stage construction by enacting strict regulation on grading practices.
Direct runoff away from problems areas: Concentrated flows if possible should be diverted away from problems areas as discussed in the last section.
Keep runoff velocity low: Runoff velocity should be kept as low as possible. For drainage ways such as ditches, high velocity can be reduced by a series of rock check dams which break the flow velocity. Overland flow velocity can be reduced by minimizing slope length and steepness.
Maintaining forests or vegetative cover: including crop residue, is particularly important in reducing wind erosion. It anchors the soil, increases surface roughness, reduces wind speed, conserves soil moisture and adds organic matter which helps bind the soil particles into aggregates. Clearing and cultivating land removes the vegetative cover for part or all of the year. Large, open fields are especially erosion prone because long, unobstructed distances allow the wind's velocity to increase. Tree roots help stabilize stream banks, and tree shade helps reduce algae growth in streams in some cases. Streamside vegetation also traps sediments before they reach the stream and absorbs nitrates from groundwater. Clearing trees removes these benefits.
Maintained forest covers in southern slopes of Mt. Kilimanjaro in Tanzania (Author, 2010)
Contour bunds: The practice of tillage along the contour (or across the slope) and of flat seeding followed by ridging after crop establishment, appears to improve the effectiveness of tillage for rainwater conservation. Ridging and furrowing create a mosaic of mini-catchments to trap rainwater and control soil erosion. Typically, between 30% and 40% of the total precipitation is converted into runoff (Huda et al., 1988). Bunds of different kinds (contour or graded) have been important means, not for in-situ rainwater conservation, but also for runoff harvesting and erosion control. Strengthening the property bunds to as a means of rainwater harvesting may offer a viable solution (Kerr and Sanghi, 1996). In a large catchment, runoff is directed to concentrate at the drainage point of a watershed by the construction of terraces.
Agro forestry: Agroforestry is a system of land use with simultaneous cultivation of trees or bushes and of arable crops or pasture plants. The system fulfils many of the economic, social and environmental requisites (Katyal et al., 1994). Integration of arable crops with trees provides an opportunity to harness the potential of the crop when the trees are young and do not yet yield an economic benefit. Even under severe circumstance, a tree often survives and provides fodder and fuel (Katyal et al., 1994). It makes the environment more hospitable. Additionally, trees make use of off-season rains, recycle nutrients from deeper soil layers, and suppress weeds because of their large canopy. Agroforestry may gain greater acceptance with economically attractive, multipurpose tree species, provide farmers have access to markets for these products. Trees yielding fruits, oil, pesticides, and pharmaceuticals, flavoring agents, timber, fuel and fodder should be considered (Brenda, 2002). Other measures of controlling sedimentation in watersheds may include; environmental education, law enforcement, construction of river bank, construction of water ways, diversification of economy.
Role of Agroforestry in controlling sediments in watersheds. Source: Author (2009)
_Toc254553908
By studying the key watershed features in addition to activities along waterways scientists, other researchers and city governments can work to keep them healthy because a small change in one portion of a watershed can drastically affect other parts. The immediate effects of sedimentation may not yet be felt, but if this generation doesn’t feel it the next generation and their children will be the ones to suffer. The only way to ensure that we will not encounter any of the consequences of sedimentation is to stop destroying the biosphere or watershed resources all together.
_Toc254553909
Brenda McLellan (2002) Agriculture and Rural Development: Impact of Forest Harvesting.
Alberta. ca Printers, USA. (Retrived on September 14, 2009).
FAO (2005). Global Forest Resources Assessment Report: Impact of Deforestation in Tropical
Forest sub-regions. Amsterdam.
Freedman, B. (1995). Environmental Ecology. 2nd ed. San Diego: Academic Press.
Huda, A.K.S Pathak, P., Rego, T.J and Virmani, S.M. (1988). Agroclimatic considerations for
improved soil and water management and efficient fertilizer use in semiarid India.
Fertilizer News 33: 51-57.
Katyal, J.C., Das, S.K., Korwar, G.R and Osman, M. (1994). Technology for mitigating stresses:
Alternative land uses. In Stressed Ecosystems and Sustainable Agriculture (Virmani
S.M., Katyal, J.C., Eswaran, H and Abrol, I.P eds.). Oxford and IBH Publishing Co. Pvt.
Ltd.New Delhi, India. pp 291-306.
Kerr, J.M., Sanghi, N.K and Sriramappa, G. (1996). Subsidies in watershed development
programs in India: Distortions and opportunities. International Institute for Environment
and Development Gatekeeper Series # 61. London, UK.
Langbein, W. B. and Iseri, K.T. (1995). Manual of Hydrology: Part 1. General Surface-Water
Techniques. Geological survey water-supply paper 1541-Methods and practices of the
Geological Survey. HTML version http://water.usgs.gov/wsc.
Mobile Register (1998). West Mobile’s muddy waters. Section A, Page 1. Retrieved
February 18, 2008 from University Library Archives.
Moore, R. D and Richardson, J. S. (2003). Progress Towards Understanding the Structure,
Function, and Ecological Significance of Small stream Channels and their Riparian
Zones. Canadian Journal of Forest Research 33(8): 1349-1359. Retrieved February 18,
2008 from EBSCO.
Musy, A. (2000) e-drologie. Ecole Polytechnique Fédérale, Lausanne, Suisse.
Myers, N. (1991). "Trees by the Billions". International Wildlife, 112(7): 12-15. Bonn Press,
German.
O'Sullivan, P. E. & C. S. Reynolds, 2004. The Lakes Handbook. Limnology and limnetic
ecology. Blackwell Publishing, Malden, MA, USA.
Power, G. (1988). “Siltation is a threat to Whole World’s Storage dams”. World Water, June
1988, Thomas Telford, London, pp17-19.
Sehgal, J.L and Abrol, I.P. (1994). Soil degradation in India: Status and impact. Oxford and IBH
Publishing Co. Pvt. Ltd., New Delhi, India.
Thangam, E.S. (1979). "Shifting Cultivation in Arunachal Pradesh" Proceedings of Agro-forestry
seminar at Imphal, India.
Thiam, A.K. (2002). The causes and spatial pattern of land degradation risk in southern
Mauritania using multitemporal avhrr-ndvi imagery and field data. Wiley
InterScienceUniversity of Central Arkansas, Conway, USA.
Wetzel, R. G., 2001. Limnology: Lake and river ecosystems. Academic Press, San Diego.
Wilcock PR. 1997. A Method for Predicting Sediment Transport in Gravel-bed Rivers. Report to
the United States Forest Service, Rocky Mountain Forest and Range Experiment
Station: FortCollins, CO.

THIS MATERIAL HAS BEEN PREPARED AND PRESENTED BY CLEMENT
WATERSHED SEDIMENTATION
KENYATTA UNIVERSITY
Department of Geography
Msc. Integrated Watershed Management
MWM 705: Watershed Degradation and Rehabilitation
Assignment Title
Topic 2: Sedimentation in Watersheds
Student Name: Clement Mromba
Reg No: I56EA/12155/2009
Course Instructor: Mr. Kennedy Obiero
Date of submission: 1st March 2010
Table of Contents
1.0. Introduction. 2
1.1. Classification of watersheds. 3
2.0. Sedimentation. 4
2.1. Direct causes of sedimentation in watersheds. 5
2.2. Indirect causes of watershed sedimentation. 6
3.0. Processes of sedimentation in watersheds. 7
4.0. Estimation of sedimentation in watersheds. 8
5.0. Effects of sedimentation in watersheds. 8
6.0. Controlling sedimentation in watersheds. 9
7.0. Conclusion. 12
8.0. References. 12
_Toc254553899
The term watershed is used (especially in North America and Europe) to indicate an area of land from which all water falling as rain or snow would flow toward a single point (Figure 1) (Musy, 2001).This includes both surface water flow, such as rivers and stream and the underground movement of water. Watershed, drainage basin and catchment are used synonymously and all of them refer to the area of land drained by a river system (Langbein, 1995).
Figure 1: The Watershed, hydrologic cycle and its components. Source: (Musy, 2001)
_Toc254553900
Watersheds are classified basing on hydrological pattern and size. Basing on hydrological pattern, there are three types of watersheds:
i. Exorheic/Open watershed, which empty to the sea and represent the major part of the drainage of all of the continents except Australia.
ii. Endorheic/Closed watershed, which discharge/empty into an inland, into closed lake basins, and are mainly (but not exclusively) restricted to the arid and semi-arid regions.
iii. Arheic regions, which is the region within which no rivers arise (the lower part of the Nile, Oranje and Niger, all in Africa, are a good examples of this category of basin).
iv. Multiple open watersheds empty into the ocean form more than one source. Within watershed areas you will find other wetland areas like ponds, swamps and marshes.
Basing on size there are four types of watersheds; Sub-watershed (100-500 sq.km), Milli-Watershed (10-100 sq.km), Micro-Watershed (1-10 sq.km) and Mini-Watershed (Less than 1 sq.km) (Figure 2) (Langbein, 1995).
Figure 2: Classification of watershed according to their sizes (Langbein, 1995)
_Toc254553901
Sedimentation is the process which involves detachment of soil materials, transport of soil materials, and deposition of eroded materials. These materials are called sediments; sediments involve particles of organic (living) and inorganic (nonliving) origin that accumulate in loose form on the surface, riverbed and seabed. Sedimentation in the watershed comes from four different sources, such as rock (weathering), organic material (biological), dissolved compounds in water and outer space. There are different types of sediments (Table 1) and their corresponding symbols such as; S=sand, M=mud, CL = clay, Si=silt, St=stones, G=gravel, P=pebbles, Rk=rock, Co=coral and Wd=weed (Wetzel, 2001).
Sediments
Terrigenous
Biogenous
Hydrogenous
Origin
Material eroded from the continents
Skeletal structures of organisms
Formed from dissolved salts
Mode of transport
Rivers, wind, glaciers
Settles from top of water column
Not transported, precipitates in place
General occurrence
Along continental margins; finest size reaches deep ocean basin
Intermediate depths at all latitudes where lithogenous sediments are not important (e.g., flanks of oceanic ridges); upwelling regions
In deep ocean basins where input of other sediments is minimal
Usual constituents
Silicate minerals (e.g. quartz, feldspar, clay minerals)
Calcium carbonate (coccoliths and foraminifera) and silica (diatoms and radiolaria)
Highly variable; major examples are manganese nodules, phosphorite nodules, and evaporites
Relative abundance
~50 %
~50%
< 1%
Table 1: Different types of sediment with their sources (Wetzel, 2001)
_Toc254553902
Watershed degradation and ultimate sedimentation are closely related, it is the result of the interaction of many environmental, economic, social, cultural and political conditions in any given region (Freedman, 1995). There are two main causes of watershed sedimentation. The primary and most common reasons for sedimentation are known as the direct causes. Logging, overpopulation, urbanization, dam construction etc are under direct causes. The other main cause of sedimentation is known as natural/Mother Nature causes (Freedman, 1995).
Poor farming practices: Farming is still predominantly practiced especially with traditional methods which are causing watershed sedimentation. Peasants tend to cope with fluctuating climatic conditions by conquering new low-fertility lands through slash-and-burn (Brenda, 2002). Through this process, peasants end by burning the limited soil nutrients, thus lowering the potential primary biological productivity level. Old agricultural lands receive very little rehabilitation care through the use of chemical fertilizers, which are generally too expensive for peasants. Organic fertilization with harvested straws and animal dung is limited. Straw is consumed by domestic animals soon after it is harvested on farms and they then leave much dispersed dung. As a result, old farmlands tend to lose most of their fertility. Some are abandoned as they yield far below expectation. Because of these weak land management practices, farmlands are exposed to water erosion during the wet season and to wind erosion after harvest. Both of these erosion processes are known to be strong causes of nutrient transportation leaving degraded soils behind and sediment loads in watershed (Thangam, 1979).
Rapid population growth/overpopulation has resulted to the conversion of forest areas to non-forest lands for settlement and farming. Together with this is urbanization and residential area expansion. This takes a significant loss of forest lands both for harvesting forest products as more people need more timber to build their houses and for developing the greater area their houses, malls, business centers will be built (Thiam, 2002).
Overgrazing is another disastrous activity in a watershed. Thus, watershed forests are destroyed and converted to cattle pasture to supply the burgeoning demand for meat. In Central America, almost half of the rainforests have been slashed and burned for cattle farming in order to comply with foreign demands. Twenty-five per cent of the Amazon's forests have also been destroyed for cattle ranches causing sedimentation in Amazon basin (Thiam, 2002).
While most causes of sedimentation occur due to human activities, there are uncontrolled causes such as mass wasting, volcanic eruption, and earthquakes. Forest fires are started by lightning, and strong winds help to spread the flames. Drought in the forest has increased the amount of flammable bush and debris on the forest floor. Forest fires destroy immeasurable amount of valuable timber. They kill not only trees but also other living things. Meanwhile, volcanic eruption is one of the several natural forces capable of causing damage to forests. The ashes emitted during the eruption coat tree leaves, which then interfere with photosynthesis. Animal population is also devastated. The organisms that survive have to cope with the changed habitat and reduced food supplies.
_Toc254553903
Lack of government legislation for land reforms has also cleared the forest especially in developing countries like of the South East Asian nations. People in that region are among the poorest in the world and are desperate for a piece of land. Unequal distribution of resources has led these people to find their way to exploit the forests (Myers, 1991).
Another reason that denudes the forest is exploitative economic development schemes and the powerlessness of government to safeguard its resources. Poor countries in their attempt to increase their revenues are in a way exploiting their resources like the forests. Timber is exported to reduce the national debt. Countries rich in mineral resources open their doors to multinational mining corporations that clear the forests as they go with their operations. The government especially those belonging in the Third World cannot curb commercial logging and implement a total log ban in exchange to higher foreign exchange rates. Development projects like dams, roads, and airports contracted by the government also cause deforestation which finally results into watershed sedimentation (Myers, 1991).
Factors which determines the speed and rate of sedimentation in watersheds
To start with topography or terrain of the watershed's land. If the area is steep, the water there is likely to flow quickly and cause flooding and erosion, whereas flat watersheds have often have slower flowing rivers. Another feature of a watershed's physical landscape is its geology or soil type. Sandy soils for example absorb water quickly and has low surface run-off, while hard, clay soils are less permeable. Both of these have implications for runoff, erosion and ground water. In a watershed where rock layer is impermeable it will have more surface run-off which will trigger more erosion. Vegetation covering watersheds reduces erosive agent speed and erosion, conserve soil moisture and protect river bank. In Vegetated watershed tree or glasses act as barriers of strong wind and running water. Plant leaves intercept strong rain drops which always detaches soil in bare land. Nature of materials in terms of Size, shape and density plays significant role in watershed sedimentation. Larger materials such as big stones and boulders are heavy, so it is difficult to be washed away unless there is more force used by erosive agents. But when these heavy materials are able to be transported they are easily deposited or sunk as compared to right materials like wood and leaves (Tangtham, 1993).
_Toc254553904
In many watersheds, sedimentation is pronounced during rainfall events. This lead to increased runoff on more impermeable surfaces and large amounts of sediment get washed from the higher areas of the watershed directly to the bottom of the watershed (Mobile Press Register, 1998).
Also, smaller river channels are responsible for delivering important sediment, water, and nutrients downstream to larger channels within the stream’s network.
By removing the vegetation, the most important factor in stream bank and land or surface stability, it exposes riverbank or land surface to different erosive agents such as wind, moving water and ice, leaves land surface bare, disintegrate rock into small fragments, soil lack support from vegetation, and finally, soil become weak and easy to be washed away (Moore, 2003).
Thus, increases the amount of erosion from the sides of the stream into the bottom of it. Once we get a significant rain, the sediment from the bottom gets transported downstream, until the velocity of the water weakens or the weight of the sediment becomes too heavy to carry. For sedimentation to occur it passes the following process; Erosion: Washing away of materials from one point to another (Highland-low-lowland). Transport: Carrying materials by floatation, solution, suspension, bouncing and dragging from one point to another and finally deposition: Settling of particles in a river or low land (Sehgal and Abrol, 1994).
_Toc254553905
Wilcock (1997) used transport observations from four streams and one flume study to demonstrate that grouped sand and gravel transport rates collapse about two common relationships (one for sand, one for gravel) when the bed shear stress is scaled by an empirically defined reference shear stress for the sand and gravel. He came up with this equation;
W∗i=
The sediment transport rate is measured by;
Where, W*i is sediment transport rate, s is is sediment density, is fluid density, g is acceleration of gravity, fi is the proportion of fraction i in the bed, qbi is transport rate in units of mass per unit width and time, and the subscript i represents either the sand or the gravel fraction, = 0.04 which is a value of clean sand.
OR
Estimate of sedimentation and volume of sediments in watershed is measured by;
_Toc254553906
Sediment deposition in a watershed alter the watershed environment significantly, resulting in multifarious physical, biological, safety and economic impacts on the rivers, the dam structure, biodiversity, and the environment in general (Power, 1988). The physical impacts include the following; modification of regime of the river discharge, transformation of river channel morphology in the zone downstream of the river and for some distance upstream. The biological impacts include the following; effect on aquatic migration (upstream-downstream), effect on water quality and quantity, effect on aquatic life (plants and animals), effect on soil quality to support agricultural activities (Sehgal and Abrol, 1994).
Economic and social impacts include the following; reduction of storage capacity which then affects the ability of the river to meet the needs for which it is expected which include also (1) energy production, (2) agriculture and industries, (3) discharge regulations, (4) recreation, (5) fire fighting and (6) public health. Severe flooding increases property damage and loss of human life and millions of dollars are spent each year to remove sediments from storm drain systems, reservoirs, water treatment plants and to repair flood damage (Sehgal and Abrol, 1994).
_Toc254553907
The proper measures of controlling sedimentation are through its causes and sources. Series of physical, socio-economic and mechanical measures can be taken. Katyal, et.al. (1994) suggested the following sediment control measures:
Plan the development to fit the site characteristics: Site characteristics such as topography, soils, drainage patterns, and covers should be considered when developing a site. Areas which are prone to erosion should be left undisturbed and undeveloped if possible. Entrance and exits points for runoff should be protected from erosion and equipped with sediment control devices.
Minimize the extent of the disturbed area and the duration of exposure and stabilize disturbed areas as soon as possible; by staging construction and preserving existing vegetation, erosion can be reduced significantly. Once a land surface is disturbed, we should minimize the duration of exposure by protecting it from erosion if possible. Typically, if an area is not going being worked on in more than 45 days, it should be protected by erosion control mats. The State of Maryland has demonstrated the effectiveness of stage construction by enacting strict regulation on grading practices.
Direct runoff away from problems areas: Concentrated flows if possible should be diverted away from problems areas as discussed in the last section.
Keep runoff velocity low: Runoff velocity should be kept as low as possible. For drainage ways such as ditches, high velocity can be reduced by a series of rock check dams which break the flow velocity. Overland flow velocity can be reduced by minimizing slope length and steepness.
Maintaining forests or vegetative cover: including crop residue, is particularly important in reducing wind erosion. It anchors the soil, increases surface roughness, reduces wind speed, conserves soil moisture and adds organic matter which helps bind the soil particles into aggregates. Clearing and cultivating land removes the vegetative cover for part or all of the year. Large, open fields are especially erosion prone because long, unobstructed distances allow the wind's velocity to increase. Tree roots help stabilize stream banks, and tree shade helps reduce algae growth in streams in some cases. Streamside vegetation also traps sediments before they reach the stream and absorbs nitrates from groundwater. Clearing trees removes these benefits.
Maintained forest covers in southern slopes of Mt. Kilimanjaro in Tanzania (Author, 2010)
Contour bunds: The practice of tillage along the contour (or across the slope) and of flat seeding followed by ridging after crop establishment, appears to improve the effectiveness of tillage for rainwater conservation. Ridging and furrowing create a mosaic of mini-catchments to trap rainwater and control soil erosion. Typically, between 30% and 40% of the total precipitation is converted into runoff (Huda et al., 1988). Bunds of different kinds (contour or graded) have been important means, not for in-situ rainwater conservation, but also for runoff harvesting and erosion control. Strengthening the property bunds to as a means of rainwater harvesting may offer a viable solution (Kerr and Sanghi, 1996). In a large catchment, runoff is directed to concentrate at the drainage point of a watershed by the construction of terraces.
Agro forestry: Agroforestry is a system of land use with simultaneous cultivation of trees or bushes and of arable crops or pasture plants. The system fulfils many of the economic, social and environmental requisites (Katyal et al., 1994). Integration of arable crops with trees provides an opportunity to harness the potential of the crop when the trees are young and do not yet yield an economic benefit. Even under severe circumstance, a tree often survives and provides fodder and fuel (Katyal et al., 1994). It makes the environment more hospitable. Additionally, trees make use of off-season rains, recycle nutrients from deeper soil layers, and suppress weeds because of their large canopy. Agroforestry may gain greater acceptance with economically attractive, multipurpose tree species, provide farmers have access to markets for these products. Trees yielding fruits, oil, pesticides, and pharmaceuticals, flavoring agents, timber, fuel and fodder should be considered (Brenda, 2002). Other measures of controlling sedimentation in watersheds may include; environmental education, law enforcement, construction of river bank, construction of water ways, diversification of economy.
Role of Agroforestry in controlling sediments in watersheds. Source: Author (2009)
_Toc254553908
By studying the key watershed features in addition to activities along waterways scientists, other researchers and city governments can work to keep them healthy because a small change in one portion of a watershed can drastically affect other parts. The immediate effects of sedimentation may not yet be felt, but if this generation doesn’t feel it the next generation and their children will be the ones to suffer. The only way to ensure that we will not encounter any of the consequences of sedimentation is to stop destroying the biosphere or watershed resources all together.
_Toc254553909
Brenda McLellan (2002) Agriculture and Rural Development: Impact of Forest Harvesting.
Alberta. ca Printers, USA. (Retrived on September 14, 2009).
FAO (2005). Global Forest Resources Assessment Report: Impact of Deforestation in Tropical
Forest sub-regions. Amsterdam.
Freedman, B. (1995). Environmental Ecology. 2nd ed. San Diego: Academic Press.
Huda, A.K.S Pathak, P., Rego, T.J and Virmani, S.M. (1988). Agroclimatic considerations for
improved soil and water management and efficient fertilizer use in semiarid India.
Fertilizer News 33: 51-57.
Katyal, J.C., Das, S.K., Korwar, G.R and Osman, M. (1994). Technology for mitigating stresses:
Alternative land uses. In Stressed Ecosystems and Sustainable Agriculture (Virmani
S.M., Katyal, J.C., Eswaran, H and Abrol, I.P eds.). Oxford and IBH Publishing Co. Pvt.
Ltd.New Delhi, India. pp 291-306.
Kerr, J.M., Sanghi, N.K and Sriramappa, G. (1996). Subsidies in watershed development
programs in India: Distortions and opportunities. International Institute for Environment
and Development Gatekeeper Series # 61. London, UK.
Langbein, W. B. and Iseri, K.T. (1995). Manual of Hydrology: Part 1. General Surface-Water
Techniques. Geological survey water-supply paper 1541-Methods and practices of the
Geological Survey. HTML version http://water.usgs.gov/wsc.
Mobile Register (1998). West Mobile’s muddy waters. Section A, Page 1. Retrieved
February 18, 2008 from University Library Archives.
Moore, R. D and Richardson, J. S. (2003). Progress Towards Understanding the Structure,
Function, and Ecological Significance of Small stream Channels and their Riparian
Zones. Canadian Journal of Forest Research 33(8): 1349-1359. Retrieved February 18,
2008 from EBSCO.
Musy, A. (2000) e-drologie. Ecole Polytechnique Fédérale, Lausanne, Suisse.
Myers, N. (1991). "Trees by the Billions". International Wildlife, 112(7): 12-15. Bonn Press,
German.
O'Sullivan, P. E. & C. S. Reynolds, 2004. The Lakes Handbook. Limnology and limnetic
ecology. Blackwell Publishing, Malden, MA, USA.
Power, G. (1988). “Siltation is a threat to Whole World’s Storage dams”. World Water, June
1988, Thomas Telford, London, pp17-19.
Sehgal, J.L and Abrol, I.P. (1994). Soil degradation in India: Status and impact. Oxford and IBH
Publishing Co. Pvt. Ltd., New Delhi, India.
Thangam, E.S. (1979). "Shifting Cultivation in Arunachal Pradesh" Proceedings of Agro-forestry
seminar at Imphal, India.
Thiam, A.K. (2002). The causes and spatial pattern of land degradation risk in southern
Mauritania using multitemporal avhrr-ndvi imagery and field data. Wiley
InterScienceUniversity of Central Arkansas, Conway, USA.
Wetzel, R. G., 2001. Limnology: Lake and river ecosystems. Academic Press, San Diego.
Wilcock PR. 1997. A Method for Predicting Sediment Transport in Gravel-bed Rivers. Report to
the United States Forest Service, Rocky Mountain Forest and Range Experiment
Station: FortCollins, CO.

COURSE: MWM 705 - WATERSHED DEGRADATION AND REHABILITATION
BY: MR. KENNEDY OBIERO (PHD STUDENT, DEPARTMENT OF GEOGRAPHY, KENYATTA UNIVERSITY)
CLICK ON TOPIC 5 on the left handside on this page - To read on Effects of Fire and Grazing on Watershed Degradation

THIS IS SOME CONTENT ON THE TOPIC ON FIRE AND WATERSHED DEGRADATION. YOU CAN READ MORE ON THE SAME
By Obiero Kennedy (Unit Lecturer)
TOPIC 5 - EFFECTS OF FIRE AND GRAZING ON WATERSHED DEGRADATION
TOPIC 5 – EFFECTS OF FIRE AND GRAZING ON WATERSHED DEGRADATION
GRAZING AND WATERSHED DEFORESTATION
- In watershed are graziers, popularly known as OLE_LINK2OLE_LINK1are part of ecosystems. They depend on grazing for their body organic and inorganic requirements.
- Their nutrients, proteins and energy requirements are obtained from grass and related plants on which they feed.
- In natural conditions, the PRIMARY CONSUMERS for a whole host of wild species that graze and browse on vegetation.
- Each of them is a selective feeder and consequently maintains an ecological balance. NO OVERGRAZING with watershed vegetation. .
Human Influence on Watershed Grazing and Degradation
- When natural species are replaced by domesticated cattle and sheep, the watershed ecological balance is upset.
- There arises much greater grazing pressure on certain species which are actively selected by domesticated animals.
- When grazing is light, intermittent and carefully controlled, considering carrying capacities, pressure caused by the domesticated animals is kept at the minimum.
- HOWEVER, due to heavier grazing with either more stock or longer grazing periods, there is a decrease in the favoured species and deterioration in the value of grazing land.
- In dense cattle areas, there has been OVERGRAZING resulting in a very great decrease in palatable grass species.
- In some areas this trend leads to reduction in vegetation cover that lead to SOIL EROSION, INCREASED SUSPENDED SEDIMENTS IN WATER and SEDIMENTATION.
- During the wet season, there is sufficient grazing in most watersheds to support wildlife, cattle, sheep and goats populations.
- HOWEVER, during the dry season, the reverse is true.
Consequences of Cattle, Sheep and Goat Grazing in Watersheds
- The effects especially by goats are more devastating and dramatic. All plants within the goats’ reach may be eaten resulting to loss of the herb stratum and prevention of tree regeneration.
- The grazing animals lead to the elimination of the most palatable herbs and grasses. They tend to select species they prefer and leave the tougher less tasty plants.
- With the removal of native plant species, weedy species invade the watersheds which are less nutritional.
- As overgrazing progresses, hungry animals strip the ground bare and their hooves HASTEN SOIL EROSION further degrading the watershed.
Obiero, 2008
- About 1/3 of the world’s rangelands are degraded by overgrazing making overgrazing the largest cause of soil erosion.
- The process of overgrazing especially in rangelands initiates a desert-producing cycles that feeds on itself known as desertification.
- With nothing to hold back surface runoff, rain drains off quickly before it can infiltrate. There is reduced infiltration to nourish plants.
- Groundwater recharge/replenishment is also reduced.
- Spring and wells are also likely to dry up
- Will create a MICRO-CLIMATE not suitable for seed germination. The dry barren ground/surface reflects more of the sun’s heat, changing wind patterns, driving away moisture-laden clouds, leading to further desiccation (Saigo, 2001).
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Watershed Degradation due to Over-grazing per Continent
Source: Saigo, 2001
Discussion
- African watersheds are at a greater threat to degradation through overgrazing than in other continents.
- Countries with most damage include: Sudan, Zambia, Somalia, Kenya among others.
- In Asia, Iraq is the most susceptible.
PROBE CLASS FOR REASONS.
FIRE
- Fire is also one aspect that contributes to watershed degradation.
- Some fires are caused by natural causes such as lightning.
- However, the largest percentage of fire is caused by human beings.
- Annual burning in some regions such as the savanna has been going on for many centuries.
- Severity of burning depends on season (e.g., on set of dry season), time of the day e.g., morning or mid-day or evening.
- Moisture content of grass/vegetation will influence the severity of fire.
- The presence of wind will fan the fire and increase its severity although it may also decrease its duration.
Why Burning
- Burning clears the land for cultivation especially in areas occupied by cultivators or farmers.
- It drives game from cover thus facilitating their capture.
- It is said to decrease ticks and other parasitic populations.
- It has been used in warfare.
- Man appears to enjoy the sight of a good blaze especially at night.
Effects of Fire
- Fire stimulates the renewed growth of some plant species in watersheds.
- It severely damages the woody plant species in watersheds. Some species are affected more than others.
- The damage resulting from fire depends on the intensity of the burning both in terms of frequency and severity.
- With severe annual fires, vegetation degenerates trees reduce leading to shrub or grass.
- Micro-organisms are destroyed particularly earthworms and microbial population.
- The organic matter and nitrogen content of the soil decreases.
- Light burning to some extent increase soil fertility.