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CHAPTER 6. TECHNOLOGICAL AND MANAGERIAL ASPECTS OF EUTROPHICATION

6.9. Sediment Remediation

6.9.1. Watershed Sediment Control

The prevention of soil erosion in watersheds draining into lakes is an imperative step in the control of non-point (i.e., diffuse) sources of nutrients and agricultural chemicals. Major controls of soil erosion should be for riverbank erosion, animal produced erosion, sheet erosion, and slope erosion.

Accelerated erosion of the banks of rivers, streams, and drainage ditches accounts for large quantities of sediment delivered into rivers and lakes. Man-made changes to river and stream banks, and changes in flow characteristics result in increased erosion of exposed riverbanks. Changes are due to removal of tree cover and removal of secondary and ground cover for agricultural production. Removal of shade increases stream water temperature, which, with a commensurate increase in sediment deposition from erosion, results in the loss of fish habitat and decline in species diversity. Methods for preventing the extreme river erosion include placement of new resistant bank of rocks, gabion, or other appropriate structure. To prevent moderate to low bank erosion, the grading slope of bank and growth of ground cover vegetation, such as natural grasses, is recommended. Re-institution of secondary growth on the bank and bank top also prevents moderate bank erosion.

Cattle, sheep, and other animals with free access to streams and rivers cause bank collapse and damage on a large-scale. To prevent this type of erosion it is recommended to build fences to exclude access of animals to the stream or river. Access for drinking must be provided by construction of an access point. An access point to the stream must be provided by a construction similar to a ford. A graded slope into the river should be built, armoured with large stone, with the armouring to be used on the streambed to prevent animals stirring the riverbed. A fence must be constructed across the river upstream and downstream of the ford to confine the animals to a single segment of the river in which the riverbed has been protected. Alternatively, animals may be excluded from the river completely with water being taken from the stream or other source and transferred to a drinking trough. Intensive animal husbandry in which livestock are confined does not cause excessive problems with erosion. Here the problem is one of disposal of animal wastes and preventing nutrients and bacteria from entering stream courses. This must be accomplished by the construction of septic tanks or similar systems with well-designed and constructed tile drain fields.

Sheet erosion is defined as the run-off of exposed soils from fields with direct loss to streams and rivers. It is less visible than bank erosion or gully erosion of slopes and stream banks. However, it is ubiquitous and results in the continuing loss of valuable soils from agricultural regions. It is normally associated with wet conditions and direct rainfall but wind erosion in arid and semi-arid regions or during periods of extended drought can be extremely spectacular. The solution to sheet erosion is simple and can be stated simply as a strategy for limiting, to the extent possible, the exposure of the soil. Maintenance of vegetative cover must be regarded as the aim of good soil management. Ploughing is essential for many crops but should only be undertaken before seed sowing and never before the onset of extended rainy seasons or cold weather. Ploughing should be along contours and not up and down slopes of even relatively low slope gradients. This will enhance infiltration and slow down soil loss. Once cropped, stubble should be left on the field and ploughed in at a later time, i.e., before sowing the next crop. Crop rotation and fallow periods will enhance the organic content of the soil and improve resistance to erosion. Under special circumstances dilute solutions of polymers may be used as soil stabilizers though these are specialized techniques. Finally, ploughing should not extend to the riverbank, and a minimum of 1 m with an optimum of a 5-m strip should be left at the bank and around each field. This prevents movement of the soil off the field and also aids in helping control stream bank erosion. Where this is practiced, the redevelopment of vegetation and woodland has resulted in the regeneration of a natural ecosystem with a return of hunting opportunities. It has also resulted in improved stream clarity and regeneration of habitat and spawning grounds with an associated increase in fish diversity and quality, and consequently fishing opportunities.

In large-scale watersheds, slope erosion is the major source of sediment delivered to larger river systems. Wholesale deforestation of slope results in massive movement of soils from the slope to the associated stream or river. Large-scale destruction of rivers and associated lakes has occurred from this cause. For example, the loss of salmon in the rivers of Western North America is due to deforestation by clear cutting, resulting in sedimentation and turbidity in the spawning grounds. Also, the increased sediment load, sedimentation and consequent change in the bed form of a river may lead to widespread flooding as observed in the River Ganges. Sediment loss from slope erosion in the Himalayan foothills is taking place due to deforestation resulting from the collection of wood for fires for heating and cooking. The solutions are again simple and relate to the maintenance of vegetative cover for the prevention of erosion. Some methods are as follows. Selective cutting of forest, removing only the mature trees of the species desired. Replanting immediately on removal of trees and the overall application of a sound forest management practice. Where vegetative cover is removed for agriculture the slope must be terraced to stabilize the slope and prevent erosion. Each terrace must have a well-designed drainage system to improve infiltration and prevent the occurrence of total saturation of the soil.

As a general conclusion to the control of soil loss in watersheds a guiding principle should be that land stewardship must conform to the extent possible to the natural system. Where possible, this system should be enhanced to compensate for the inevitable losses that must occur because of man's intervention. It is also clear that planning must be an integral part of landscape management and that such plans can only be implemented with full public participation, education, and collaboration. Measures of success should be instituted which not only determine progress, but also assess the benefits that accrue as a result of sound practice.

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