This chapter covers the characteristic of river basins (also called watersheds) that affect surface runoff and sediment yield. The main watershed characteristics are illustrated in Section 3.1, followed by rainfall precipitation (Section 3.2), interception and infiltration (Section 3.3), excess rainfall (Section 3.4), and surface runoff (Section 3.5). Soil eroded from upland areas is usually the source of most sediments that are transported by rivers to reservoirs and estuaries. Methods are presented to calculate upland erosion (Section 3.6) and to estimate sediment yield from watersheds (Section 3.7).

River-basin characteristics

The hydrologic cycle describes processes that contribute to the source and the yield of water and sediment from upland areas to the fluvial system. Figure 3.1 depicts a portion of a watershed during a precipitation event. Shown in this figure are the processes of condensation, precipitation, interception, evaporation, transpiration, infiltration, subsurface flow, exfiltration, deep percolation, groundwater flow, surface flow, surface-detention storage, channel precipitation, evaporation, and streamflow. All of these processes play a role in hydrology; however, precipitation, infiltration, overland flow, and streamflow are most important in surface runoff, upland erosion, and river mechanics.

River basins or watersheds define areas of the Earth's surface where rainwater drains into a particular stream. The terms basin and catchment are synonymously used in the literature. Watershed characteristics can often be described in geographical terms, including physiography and topography, geology and pedology, and climatology and forestry.