Animals living in a young river are sensitive to water temperature and its quality. The Japanese fresh water crab is an index species which indicates high quality of water. Different from other fresh water crabs the Japanese fresh water crab does not go to sea and stay in fresh water throughout of its life. Seema is the fish belongs to salmon family living in Japan. Since seema can live only in cold water, they are found in a young river. A young river is usually covered with brunches of trees which keep water cool by preventing that sun lights warm up the surface of water. Fish and other animals depend on leaves and insects fallen from those brunches. The larvae of the caddisflies are aquatic insects living in a young river. In Nagano prefecture, Japan those larvae are local delicacy.

The Japanese fluvial sculpin has frog like face. Some of them spend all their life in a river while others live a part of their life in sea. They feed on water bugs such as the larvae of caddisflies. After female attached eggs on rocks male protect eggs until babies come out of eggs. The Kajika frog also lives in a young river. Freshwater prawns are usually found in a mature and an old river except hira freshwater prawns who climb up the rivers against water current and live in a young river.