The term "first flush effect" refers to rapid changes in water quality (pollutant concentration or load) that occur after early season rains. Soil and vegetation particles wash into streams; sediments and other accumulated organic particles on the river bed are re-suspended, and dissolved substances from soil and shallow groundwater can be flushed into streams. Recent research has shown that this effect has not been observed in relatively pervious areas.
The term is often also used to address the first flood after a dry period, which is supposed to contain higher concentrations than a subsequent one. This is referred to as "first flush flood." There are various definitions of the first flush phenomenon.