The river darter may attain a length of two to three inches. Its first dorsal fin has a small, black spot in the front and a large, black spot at the rear. The second dorsal and anal fins have light brown lines. This fish is green on its back and upper sides, and each side has eight to 15 black bars. A small black spot is present at the base of the tail fin. A black, “teardrop” mark can be seen under each eye, and another black mark runs from the front of each eye to the snout. The gill covers are fairly well connected by a membrane across the throat. The lateral line is complete. The anal fin of the male is very large, reaching nearly to the tail fin.
The river darter may be found statewide in Illinois except in the northeastern one-fourth. This fish lives in rivers in areas with a swift current and a bottom of gravel or rock. The river darter stays in water depths over four feet during the day, moving to shallower water at night. It spawns in spring. It eats insect larvae.