​ILLINOIS RIVER, the largest river in Illinois; formed in Grundy county by the union of the Des Plaines and Kankakee rivers, and flowing a general W. and S. W. course, enters the Mississippi river at Grafton, between Jersey and Calhoun counties. During its course it receives several rivers of considerable size, and affords communication between important towns and villages, being navigable for steamboats for about 286 miles above its mouth.