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ILLINOIS, a river of the United States, and the largest in the state to which it gives its name. It is formed in Grundy co., in the N. E. part of the state, about 45 m. S. W. of Lake Michigan, by the union of Kankakee and Des Plaines rivers, the former of which rises in the N. part of Indiana and the latter in the S. E. part of Wisconsin. The Kankakee receives the Iroquois, and from that point to its junction with the Des Plaines is sometimes known as the Iroquois. The Illinois flows nearly W. to Hennepin, in Putnam co., and thence S. W. and finally S. until it unites with the Mississippi between Calhoun and Jersey counties, 20 m. above the mouth of the Missouri. It is about 500 m. long, and is navigable at high water for 245 m. It is deep and broad, in several places expanding into basins which might almost be called lakes. Peoria, the most important city on its banks, is built on the shore of one of these basins. Its principal affluents are the Fox, Spoon, Crooked creek, the Mackinaw, Sangamon, and Vermilion. Above the mouth of the Vermilion, in La Salle co., it is obstructed by rapids, and a canal has been built from this point to Chicago, a distance of 96 m. Uninterrupted water communication is thus secured between the lakes and the Mississippi. The Illinois was ascended by Marquette in 1673, and explored in 1679-'80 by La Salle and Hennepin, who entered it by the Kankakee, which they reached from Lake Michigan by means of the St. Joseph river and a short portage, and sailed in canoes, La Salle as far as the present site of Peoria, and Hennepin to the Mississippi. In 1682 La Salle navigated the whole course of the river.