LOCATION.--Lat 41°22'11", long 92°58'48" referenced to North American Datum of 1927, in NE 1/4 NW 1/4 sec.19, T.76 N., R.18 W., Marion County, IA, Hydrologic Unit 07100008, at outlet works near right end of Red Rock Dam on the Des Moines River, 3.0 mi upstream from Lake Creek, 4.5 mi southwest of Pella, and 139.2 mi upstream from mouth. (map)

DRAINAGE AREA.--12,323 mi².

SURFACE-WATER RECORDS

PERIOD OF RECORD.--Reservoir storage (acre-ft) records from March 1969 to September 2000; elevation records from October 2000 to current year.

GAGE.--Water-stage recorder. Elevation records referenced to National Geodetic Vertical Datum of 1929 (levels by U.S. Army Corps of Engineers).

COOPERATION.--Records provided by U.S. Army Corps of Engineers.

REMARKS.--Reservoir is formed by earthfill dam completed in 1969. Storage began in March 1969. Releases controlled through 14 concrete conduits extending through the concrete ogee spillway section into the stilling basin. Inlet invert elevation at 690 ft. Maximum design discharge through the conduits is 37,500 ft³/s, but normal flood control operation limits maximum outflow to 30,000 ft³/s. Spillway section consists of 5 tainter gates, 41 ft wide and 45 ft high, on concrete ogee crest at elevation 736 ft. The storage capacity of the reservoir at full flood-control pool level, elevation 780 ft, is 1,489,900 acre-ft, surface area 65,440 acres. Conservation pool level, elevation 742 ft, is 265,500 acre-ft, surface area 19,100 acres. Reservoir is used for flood control, low-flow augmentation, conservation and recreation. Normal operation will maintain an elevation of 742 ft, with minimum release of 300 ft³/s and maximum release of 30,000 ft³/s during the non-growing season, providing discharges at Ottumwa and Keosauqua do not exceed 30,000 ft³/s and 35,000 ft³/s, respectively. Storage tables for water years 1985-1986 published as day second-ft instead of acre-ft storage.

U.S. Army Corps of Engineers - Rock Island District