The Green River is an 89-mile-long (143 km) tributary of the Rock River in northwestern Illinois in the United States. Via the Rock, it is part of the Mississippi River watershed. Much of the Green's course has been straightened and channelized. The Green River was created to drain water from former swamps to create better farmland. The Green River is home to many fish species including largemouth bass, smallmouth bass, hybrid bass, common carp, northern pike, and creek chub.