Chicago, river, formed in Chicago by the junction of its North Branch (24 mi/39 km long) and South Branch (10 mi/16 km long), and flowing southeast via a canal into the Des Plaines River at Lockport, Ill. The river formerly flowed east, then northeast via a channel, into Lake Michigan. Its course was reversed by the Chicago Sanitary and Ship Canal, 30 mi (48 km) long, 22 ft (6.7 m) deep, and from 162 to 290 ft (49–88 m) wide, built (1892–1900) on the South Branch to prevent the pollution of Lake Michigan by Chicago's sewage; locks prevent the river from entering the lake. The use of Lake Michigan's water to flush the canal was a heated political issue finally settled in 1930 when the U.S. Supreme Court ordered a reduction in the amount of water being diverted from the lake. This decision forced Chicago to build sewage treatment plants. The channels of the Chicago River and the North Branch have been improved to aid deep-draft vessels and barges.

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