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.
Water is very important for spawning.
The velocity, or speed, of the water in the stream is generally more important than the depth of the water at a good spawning site. The velocity of the water determines the amount of water which will pass over the incubating eggs. The speed of the water can't be so high that it will wash away the gravel but must be high enough to provide enough oxygen to the eggs. The speed of the water also helps maintain the right mixture of gravel sizes for spawning. The oxygen requirements of eggs and alevins increase with increasing temperature.