Hydrological Cycle of a Watershed
Channel flow or discharge consists of channel intercept, overland flow, subsurface storm flow, and base flow. Precipitation is not uniform throughout the year creating variations in discharge. When rainfall is not present, the primary source of river and stream discharges is base flow from stored ground water. Furthermore spring and summer snowmelt adds to the overall discharge. During fall and early winter rain-on-snow events create flooding in the lower floodplains of the watershed.
Typically, precipitation is limited during late summer months. This creates low flows for fish and dry conditions for upland forest vegetation. Annual water budgets for a typical Pacific Northwest watershed (View Seattle Watershed Budget) has excess precipitation from September/October to May. In contrast, water is limiting vegetation during summer.