The most frequent peak flow events are generally produced within this elevation band. At elevations above 800 m asl, the snowpack tends to accumulate throughout the winter. Rain in the middle elevation band usually blends to snow at upper elevations, and when rainfall does occur, the snowpack is deep and cold enough to resist melt. Water is delivered to stream channels by a combination of overland flow and subsurface flow. Subsurface flow can occur near the surface just below the forest floor, through macropores or pipes, or as deep groundwater flow through the pore spaces in soil and bedrock.
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.