Illinois River 20 miles (32 km) of its course, the Illinois is separated from the Mississippi River by only about five miles, by a peninsula of land that makes up Calhoun County. The Illinois joins the Mississippi near Grafton, approximately 25 miles (40 km) northwest of downtown St. Louis and about 20 miles upstream from the confluence of the Missouri River and the Mississippi. South of Hennepin, the Illinois River is following the ancient channel of the Mississippi River. The Illinoian Stage, about 300,000 to 132,000 years ago, blocked the Mississippi near Rock Island, diverting it into its present channel. After the
Muddy River (Washington) rapidly, passing under Forest Road 25 and then receiving Clear Creek from the left about from the mouth. At the road crossing, Muddy Creek Picnic Site is on the left. Below the confluence with Clear Creek, the river meanders through Cedar Flats, leaves the national forest slightly more than a mile from the mouth, and passes a United States Geological Survey (USGS) stream gauge, on the left from the mouth. Muddy River enters the Lewis River about from the larger stream's mouth on the Columbia River. The USGS monitors the flow of Muddy River at a station upstream of the