When a watershed becomes covered by about ten percent impervious cover, its streams will degrade. As the percentage of impervious cover increases, degradation tends to increase accordingly. This means that streams will begin to experience declines even at rural residential densities. Once an area is covered by suburban-style development, its streams will no longer support healthy fish populations. The two primary fish habitat are pools and riffles.
Riffles are shallow, gravelly, fast-water areas that are the main food production areas of streams.
Pools, which form in deeper, slower flowing areas, are the main fish rearing and resting areas for most salmonids. Scouring stormwater flows destroy pools and riffles. Eroded sand and silt blankets over critical spawning gravels. Frequent and prolonged high flows cause spawning gravels to be replaced by cobble too large to be used by fish for spawning. In extreme cases, all gravels may be scoured down to bare glacial till or bedrock.