Erosion also occurs on a larger scale episodically due to channel bank and hill slope failures, landsliding, forest fires, and debris flows. Land use practices such as logging and clearing, grazing, road construction, agriculture, and urbanization activities also affect sediment production and delivery from a watershed. Sediment production may vary significantly with long-term cycles in drainage system development and rejuvenation, and zones of sediment production and/or deposition may shift in location with time (e.g., headward movement of nick points and/or channel migration and avulsions).
On very small scales of time and space, erosion rate is roughly equivalent to sediment yield. However, for larger basins, as the area and time scales to be considered increase, transport and deposition processes influence both the sediment yield and the timing of sediment discharge from a basin. Deposition occurs at the base of steep slopes; in lakes, reservoirs, and wetlands; in river flood plains; and on point bars in stream channels (lateral accretion and bed aggradation).
Episodic and spatially varying processes dominate sediment and water flow, but theories and quantification of these processes are not well developed. Biota play essential roles in the production, transport and storage of sediment and water, but knowledge of biological functions is poorly integrated into quantitative procedures for estimating sediment and water budgets.
Sediment Yield and Transport
Spatial and temporal variations in physical and biological features of the watershed make estimation of sediment yield an extremely difficult and imprecise task. Important variables include soils and geology, relief, climate, vegetation, soil moisture, precipitation, drainage density channel morphology, and human influences. Dominant processes within a watershed may be entirely different between physiographic or ecological provinces, and may change with time. The problem becomes even more complex when grain size distributions and sediment yield for particular events must be estimated for input to sedimentation transport simulation models. There is no widely accepted procedure for computing basin sediment yield and grain size distribution directly from watershed characteristics without measured information.
Sediment transport is influenced primarily by the action of wind and water, and deposition occurs in a number of locations where energy for transport becomes insufficient to carry eroded sediments. Colluvial deposits, floodplain, and valley deposits, channel aggradation, lateral channel accretion, and lake and reservoir deposits are examples of typical geomorphic deposition processes. The stability and longevity of sediment deposits vary. Lake and reservoir deposits tend to be long-term, whereas some channel and floodplain deposits may be remobilized by the next large scale flood event, only to be deposited downstream. The spatial and temporal variability of sediment production, transport and deposition greatly complicates the task of estimating sediment yield from a watershed.
For more detailed discussion of each geomorphic process important to the determination of erosion, transport, and deposition of sediment see the following references:
Application of Methods and Models for Prediction of Land Surface Erosion
and Yield, TD 36, March 1995, Hydrologic Engineering Center