LOCATION.-- Lat 41°53'18", long 87°37'16" (NAD of 1927), in NE1/4SW1/4 sec.10, T.39 N., R.14 E., Cook County, Hydrologic Unit 07120003, on left downstream side of bridge within the Columbus Drive bridge structure in Chicago, 0.5 mi downstream from Chicago River Lock and Dam.

DRAINAGE AREA.-- Indeterminate.

PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE AND STAGE

DISCHARGE: October 1996 to current year.

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 relations between this site and flow data from the Metropolitan Water Reclamation District of Greater Chicago-Chicago River Controlling Works.

EXTREMES FOR PERIOD OF RECORD.-- Maximum daily discharge, 1,370 ft³/s, Aug. 6, 1997, minimum daily, -2,450 ft³/s, Feb. 21, 1997, due to regulation to prevent flooding.