"First flush" is a term used to describe the flushing action storm water has on accumulated pollutants from roads, parking lots, driveways, lawns etc. – the first time it rains after a dry spell. This effect is even stronger the first time it rains after an extended dry period. That's because the initial stormwater runoff dislodges and removes most of the accumulated pollutants from these surfaces and carries them into lakes, ponds or canals. Once the first flush of polluted storm water is washed away, the water quality of storm water improves.