LOCATION.-- Lat 41°38'53", long 87°33'50" (NAD of 1927), in SE1/4SE1/4 sec.36, T.37 N., R.14 E., Cook County, Hydrologic Unit 07120003, on right bank, 0.7 mi downstream of 130^th Street in Chicago, and 1100 ft below the downstream lock gates at the Thomas J. O'Brien Lock and Dam, and at mile 6.9.

DRAINAGE AREA.-- Indeterminate.

PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE AND STAGE

DISCHARGE: October 1996 to September 2003.

GAGE.-- Water stage recorder, acoustical flowmeter, and phone telemeter. Datum of gage is 579.48 ft above NGVD of 1929 (Chicago City datum).

REMARKS.-- This gaging station provides flow data to determine Illinois' diversion of Lake Michigan water. A U.S. Supreme Court Decree limits Illinois' diversion to an average of 3,200 ft³/s. Illinois' diversion includes water diverted from the lake for domestic water supply, for navigation, and water-quality improvement in the Chicago Sanitary and Ship Canal system, and stormwater runoff from a 673 mi² diverted watershed area. Estimated daily discharges determined from regression equations between USGS mean daily discharges and mean daily discharges for the Obrien Lock and Dam as reported by the Metropolitan Water Reclamation District.

EXTREMES FOR PERIOD OF RECORD.-- --Maximum daily discharge, 1069 ft³/s (estimated), June 21,1997; mimimum daily, -769 ft³/s, Feb. 21, 1997, due to regulation to prevent flooding.