Different species of salmon will have specific preferences to the size of their stream. For example, coho will spend much of their time in small streams and even smaller tributaries, whereas chinook are much less picky about the size of their spawning stream. As young fry, salmon spend much of their time in quiet deep pools in various types of streams. Here they can feed on the many insects and larvae that surround them. Slow moving pools also allow salmon to seek shelter from predators.
Fast water provides streams with valuable oxygen when it gets mixed with the air.
As the water flows between and over rocks air gets mixed into the water, raising the amount of oxygen usable to fish. Fish of all ages generally don't have any problem swimming up fast moving water, unless it is too fast, such as a hydro-dam or waterfalls. Larger salmon that have returned to their river to spawn and lay eggs are very determined and may be able to jump and swim up waterfalls 3 or 4 meters high. When streams flood, salmon often seek shelter away from the fast waters. During much of the winter, heavy rains in British Columbia cause frequent flooding in many streams and rivers.