River fish eat crustaceans such as crabs and mollusks. Large river fish such as gar eat other fish. Trout eat smaller fish but also eat insects such as dragonflies, mayflies and caddisflies. They also sometimes feed on zooplankton, which are tiny animals that are swept along by the river's current.

Arctic grayling, which is found in rivers in the mountains of the northwestern United States, Canada and Russia and the Missouri river basin, also eat zooplankton as well as fish eggs and insect larvae. Large members of the species might even take animals as large as lemmings.

Most members of the sucker family have down-turned mouths that identify them as bottom feeders. They eat the animals that live on the river bottom, such as worms and crustaceans, and also come to the surface to prey on insects and smaller fish.

Minnows can be carnivorous or can include aquatic plants as part of their diet. Some specialize in snails, algae or plants. Minnows that eat plant food have digestive systems that are adapted to digesting plants. Since these minnows also tend to be omnivores, they have molar-like teeth that are good for grinding their food. The carp, which is one of the largest minnows, is a bottom feeder.