LOCATION.-- Lat 42°22'34", long 88°14'11" (NAD of 1983), in SW1/4SW1/4 sec.18, T.45 N., R.9 E., McHenry County, Hydrologic Unit 07120006, on left bank at upstream side of bridge on Chapel Hill Road in Johnsburg, 5.5 mi upstream from Stratton Lock and Dam (previously known as McHenry Dam), and at mile 103.0.

DRAINAGE AREA.-- 1,205 mi².

PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE AND STAGE

DISCHARGE: December 1997 to August 1999.

STAGE: October 1939 to current year.

SURFACE-WATER QUALITY

SEDIMENT: December 1997 to May 1999.

MISCELLANEOUS: Sediment concentration and particle size, water years 1998-99.

REVISED RECORDS.-- WDR IL-75-1: Drainage area.

GAGE.-- Water-stage recorder and phone telemeter. Datum of gage is 733.00 ft above NGVD of 1929 (levels by Illinois Department of Transportation) (732.81 ft NAVD 1988). Oct. 1, 1939 to Dec. 14, 1939, nonrecording gage at same site at datum 2.16 ft higher. Dec. 14, 1939, to Sept. 30, 1971, at datum 2.16 ft higher.

REMARKS.-- Stage regulated by Stratton Lock and Dam, 5.5 mi downstream from station (previously known as McHenry Dam). Suspended-sediment samples were collected twice weekly with more frequent samples collected during high runoff periods.

EXTREMES FOR PERIOD OF RECORD.--

SURFACE-WATER DISCHARGE AND STAGE: Maximum recorded discharge, 4,780 ft³/s, June 17, 1999, gage height, 5.91 ft; maximum gage height, 7.55 ft, present datum, April 6, 1960, discharge unknown; minimum recorded discharge, -1,300 ft³/s, June 25, 1998; minimum gage height, 1.43 ft, present datum, Mar. 9 1967.

SUSPENDED-SEDIMENT CONCENTRATION: Maximum daily, 420 mg/L, May 17, 1999; minimum daily, 2 mg/L, Jan. 20 to Feb. 3, 1998.

SUSPENDED-SEDIMENT LOAD: Maximum daily, 1,680 tons, May 17, 1999; minimum daily, 5.1 tons, Jan. 25 to Jan. 27, 1998.