Overland flow occurs over saturated soil, usually adjacent to stream channels. The rate at which runoff is converted to stream flow is governed in part by stream drainage density. A watershed with a high drainage density will produce higher peak flows than one with a low drainage density, all other factors being equal. Also, the greater the rainfall and the rate of snowmelt, the higher the resulting peak flow. Low flows are sustained by deep groundwater.
Trees play a critical role in governing runoff rates.
Trees also control snowmelt by casting shade on the snowpack and reducing wind speeds at the snow surface. Trees withdraw water from the soil through transpiration. Forestry activities and stream flows The stream flow section of the CWAP focuses on two issues surrounding peak stream flows. Clearcuts and roads tend to increase peak flows, thereby increasing rates of channel change and sediment movement within the channel. The presence of hillside roads increases peak flows.