Illinois River is a tributary of the Michigan River, approximately 71 miles (114 km) long,[2] in Jackson County in north central Colorado.

It drains part of the North Park basin south of Walden.

It descends northward through a winding gorge, emerging into North Park at approximately 8,000 feet (2,400 m) above sea level.

It flows northward through the valley as a winding stream, past Rand.

A portion of the valley of the river south of Walden is located within the Arapaho National Wildlife Refuge.