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.
Impervious surfaces can also cause stream water temperatures to rise, damaging fish habitat.
Increased streambank erosion leads to a loss of important vegetation. Less shade from streamside trees and shrubs causes water temperatures to rise. In addition, local air temperatures have a strong influence on the water temperature in headwater streams. Just as paved areas warm the air temperature, they affect water temperature as well. Often, as urbanization increases, streams are diverted, channelized, dammed, and piped.