LOCATION.-- Lat 41°34'11", long 88°04'42", in SW1/4 sec.27, T.36 N., R.10 E., Will County, Hydrologic Unit 07120004, on upper end of 9-foot navigation channel at Lockport Lock and Power Plant on Chicago Sanitary and Ship Canal, 1.1 mi upstream from Des Plaines River, and 1.7 mi southwest of Lockport.

DRAINAGE AREA.-- 740 mi².

PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE

DISCHARGE: February 1900 to 1982. Prior to October 1936 monthly discharge only, published in WSP 1308.

REVISED RECORDS.-- WSP 955: 1940-41(m). WSP 1005: 1943. WSP 1175: 1945. WSP 1308: 1940-50(m). WSP 1728: 1951-53(m).

GAGE.-- Discharge computed from flow through turbines, sluice gates, locks, leakage, and over dam

REMARKS.-- Daily total discharge includes domestic and industrial pumpage, inflow from Des Plaines River basin, and diversion from Lake Michigan. This total is obtained by adding the daily discharges, based on half-hourly discharge readings at dams, sluice gates, lockages, exciter turbines in operation and main turbines in operation as corrected for variable efficiencies, and the leakage of lock gates, dams, sluice gates, and turbines not in operation. Domestic pumpage is the amount of pumpage from Lake Michigan and any such ground sources supplied by infiltration from Lake Michigan by the State of Illinois and its municipalities, political subdivisions, agencies, and instrumentalities, the sewage and sewage effluent derived from which enters the Sanitary and Ship Canal above Lockport. Total diversion is total amount of water diverted from Lake Michigan by the State of Illinois and its municipalities, political subdivisions, agencies, and instrumentalities. The Supreme Court of the United States, on December 1, 1980, amended its Decree of June 12, 1967, allowing the State of Illinois to use a 40-year period to maintain its annual average diversion rate from Lake Michigan of 3,200 ft³/s. This diversion rate includes water diverted from the Lake for domestic consumption, for navigation and waste dilution purposes, plus that which is diverted as surface runoff and ground-water base flow. The amended Decree became effective December 15, 1980, superseding the method of computing the annual average diversion rate of the Decree of June 12, 1967 (which became effective on march 1, 1970), the Decree of April 21, 1930, and the subsequent Federal permit of June 26, 1930, issued by the Secretary of War.

EXTREMES FOR PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE: Maximum daily discharge, 34,948 ft³/s, May 16, 1974; minimum daily, 48 ft³/s, July 28, 1982.