The temperatures a fish can withstand also depend on its maturity. Generally, eggs or immature fish are less tolerant of temperature extremes and fluctuations than adults. It is best if there are trees that overhang the stream to provide shade. Shade keeps the water cooler. Therefore, salmon, trout, smelt, and their relatives are generally confined to the cold water areas of the world.
Water enables fish to move and migrate.
A change in water flow can signal the salmon and trout to migrate. These fish have adapted to respond to these changes which stimulate them to migrate. Through the trial and error of many generations, these fish know the time is right and there will be enough water for them to make the journey. Those that don't migrate at the right time won't survive as well and may not spawn, removing their genes from the population. These flow changes tell the young when its time to go to sea and the older fish when they can swim to their home streams to spawn.