Factors Affecting Runoff And Sediment Yield

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The amount of surface-water runoff and sediment carried by the runoff varies significantly among drainage basins and rivers. The variation results from geologic, topographic, climatic, vegetation, and land-use characteristics of a particular drainage basin and changes in these factors over time. Even the most casual observer can see the difference in the amount of sediment carried by the same river in flood state and at low flow, since floodwaters are usually muddier.

Geologic Factors. The major geologic factors affecting surface-water runoff and sedimentation include rock and soil type, mineralogy, degree of weathering, and structural characteristics of the soil and rock. Fine-grained, dense clay soils on shale and exposed rock types with few fractures generally allow little water to move downward and become part of the subsurface flow. The runoff from precipitation falling on such materials is comparatively rapid, and there are usually many streams. Conversely, sandy soils on sandstone and well-fractured rocks absorb a larger amount of precipitation and have less surface runoff and fewer streams. The upper parts of basins A and B are underlain by shale, and the lower parts are underlain by sandstone. Because the shale has a greater potential to produce runoff than the more porous sandstone, the drainage density that is, the length of stream channel per unit area is much greater in the shale areas than in the sandstone areas. The drainage density for any area of land is determined by measuring the total length of stream channel from all streams in the area and dividing this length by the area.

Topographic Factors. Relief refers to the difference in elevation between the highest and lowest points of any landform of interest. The greater the relief of a drainage basin, the more likely the streams in the basin are to have a steep gradient and a high percentage of steep, sloping land adjacent to the channel. Relief and slope are important because they affect the velocity of water in a stream, the rate at which water infiltrates the soil or rock, and the rate of overland flow. These characteristics then affect the rate at which surface and subsurface runoff enters a stream.

Climatic Factors. Climatic factors affecting runoff and sediment transport include the type of precipitation that occurs, the intensity of the precipitation, the duration of precipitation with respect to the total annual climatic variation, and the types of storms. In general, production of large volumes of water and sediment is associated with infrequent high-magnitude storms that occur on steep, unstable topography underlain by soil and rocks with a high erosion potential.

Vegetation Factors. Vegetation can influence runoff and sediment yield. Vegetation is capable of affecting stream flow in several ways:

Vegetation may decrease runoff by increasing the amount of rainfall intercepted and removed by evaporation.

• Decrease or loss of vegetation due to climatic change, wildfire, or land use, such as grazing by sheep, will increase runoff or production of sediment or both.

• Streamside vegetation reduces stream-bank erosion because its roots bind and hold soil particles in place.

• In forested watersheds, large woody debris, such as stems and pieces of wood, may profoundly affect the stream-channel form and processes, In steep mountain watersheds, many of the pool environments that are-important for fish habitats may be produced by large woody debris Land-Use Factors. Agriculture and urban development are land uses that have an effect on runoff and sediment yield. Agriculture generally increases runoff and sediment yield as land is plowed for crops. Urbanization, with construction of streets, parking lots, and buildings for homes and industry may also greatly increase runoff because of the large amount of impervious pavement covering the land.

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