First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.
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. Source: Wikipedia.
Scientists in the Puget Sound found that stormwater runoff is deadly to salmon. Read the news story here.
First flush monitoring of the Russian River and some of its tributaries (including Colgan Creek) was done in 2007 and 2008. Read the results in these two reports.
2007 First Flush for Russian River and tributaries.
2008 First Flush for Russian River and tributaries