A river is largely divided into three parts, a young river, a mature river and an old river. Each part of rivers has its own environment of which animals are also unique. Although ospreys are usually found in coast, occasionally they also appear in the middle part of a river or a mature river, because they are fish hunters. Some snakes such as Japanese rat snakes are also fond of staying in near water.

They eat small animals living in a riparian forest which is formed along the mature river by flood and sand sedimentation. Riparian forests protect land against flood and control mounts of sands supplying from the river. Eurasian harvest mice are known as the mice build their nests with true grasses in a riparian forest and feed on insects and seeds. Raccoon dogs appear in a riparian forest as well to forage water bugs and fish. Foxes are frequent visitors too. Although Japanese mitten crabs eat earthworms and small fish from time to time their main diets are fallen leaves and twigs. Young ayu fish grow in a mature river in which algae ayu feeding on develop on the surface of rocks.