Coho and other salmon must find water somewhere or they will die. Potentially more dangerous to the health of fish are the long hot dry months of summer. During this season much of the water in the smaller streams and creeks dries up and is not replenished. When there is too little water, the fish often get trapped in very small shallow pools with very little oxygen and temperatures that are too warm to live in. In addition to causing stress in the young fish, low water levels will also make them very visible and give predators such as birds and other land-based animals an easy opportunity for a snack.
Stream gradient is an important factor in the creation of fish habitat.
For example, mountainous areas with long steep streams often consist of rough shallow riffle-type waters, giving salmon fewer opportunities to rest and gather energy in calm pools. The velocity and discharge of water in a stream are also dependent on its gradient. The steeper the terrain, generally the faster water will flow. A high water velocity, combined with a steep stream, may prove to be very challenging or even impassable to a migrating salmon. However, certain species such as cutthroat trout may be living above these steep stream sections, and have no need to swim up or down them.