Hydrologists measure discharge in units of volume per time, usually in cubic feet per second. The amount of baseflow to a river is relatively constant over time, but is dependent on the regional water-table elevation, which can vary by several feet over one year. However, the discharge of a river varies over time, as the amount of overland flow changes. For example, after a rainstorm or during snowmelt, the amount of water running over the land to the river increases, and discharge of the river then increases. The discharge of a river depends on characteristics of the watershed, such as the number of acres, the amount of precipitation received, vegetation, soils, and slope.
Rivers increase in discharge and size with increasing distance downstream.
The amount of overland flow reaching a river not only depends on the watershed area and amount of precipitation received, but also on the amount of water that soaks into the soil rather than running over the land surface. The water that soaks into the soil is known as infiltration. Infiltration is an important part of the hydrologic cycle because it recharges aquifers, the underground rocks and sediment that contain groundwater. Factors that affect infiltration in a watershed include vegetation and soil types. Hydrologic restoration When repairing a dam, the water level in the impoundment must be temporarily lowered to allow for construction work.