In addition to the land animals, such as raccoons that visit for food and water, rivers and streams support many species of aquatic animals. Some swim freely in the water, while others live along the muddy bottom. Some prefer life in midstream or on rocks beneath a fast-moving flow; others seek the shallows or quiet pools. Shallows are often warm and exposed to sunlight, but animals that prefer cool shady spots can find them along the banks beneath overhanging trees. Many river and stream animals are adapted to life under high-speed conditions. Salmon and trout, for example, are torpedoshaped and can swim against the current more easily than some other fish. Some, like torrent beetles and stonefly larvae, have low-slung or flattened bodies that enable them to cling to rocks without being washed away.

Water quality often determines which species will be supported in a particular...