Graphic showing concept of subsurface stormflow , or interflow

Interflow, also known as subsurface runoff is relatively rapid flow toward the stream channel that occurs below the surface. It occurs more rapidly than baseflow, but typically more slowly than surface runoff. In some cases interflow may be as important as surface runoff for forecasting rapid rises in the stream channel. In fact, in regions with high infiltration rates and steep terrain, interflow may be the dominant process by which streams react quickly to rainfall or snowmelt.

This process is most likely to occur in humid, deep-soil areas. However, significant interflow contribution may occur in thin-soiled regions when there is an impermeable layer such as bedrock beneath the more permeable surface soil layer.