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The study area includes river gages on the Des Moines River below Saylorville Reservoir.  Analysis of the river system was performed to obtain more precise, reliable river flow data.
 

In 1958, Congress authorized construction of Saylorville Lake on the Des Moines River about 11 miles upstream from the City of Des Moines. The drainage area above the dam is 5,823 square miles.
 

The principal purpose of the Saylorville Project is to furnish needed additional storage to supplement the flood control capacity of the downstream Red Rock Dam and to provide flood protection to the City of Des Moines. The permanent conservation pool (elevation 836 feet National Geodetic Vertical Datum, NGVD) forms a lake with storage of about 73,600 acre feet1 and extends some 24 miles upstream from the dam.  The flood control pool stores 567,000 acre feet1 and extends some 54 miles upstream from the dam.

The reservoir was constructed in 1977 to have a total capacity of 676,000 acre-feet at full flood control pool elevation 890 feet and covers about 16,700 acres. The conservation pool was raised from 833 to 836 feet in 1983 to provide a water supply for the City of Des Moines and the Iowa Southern Utilities near Ottumwa Iowa. Pneumatic gates were installed on top of Saylorville spillway in 1994. The regulation plan for Saylorville did not change with the installation of the pneumatic gates. The introduction of pneumatic gate operation does not influence the downstream regulated flow frequency.

When forecasts indicate the pool is rising above 884 feet the pneumatic gates are raised. This allows the pool to rise to 890 feet without water flowing over the spillway. If the forecast is for the pool to rise above 890 feet then the pneumatic gates are lowered gradually starting when the pool reaches 889 feet and are fully lowered when the pool reaches 890 feet. The outflow remains at 21,000 cubic feet per second (cfs) as the pool rises from 884 feet to 889 feet and from 21,000 cfs to 42,000 cfs as the pool rises from 889 feet to 890 feet. Since the installation of the pneumatic gates all of the 21,000 cfs flow from 884 feet to 889 feet is through the conduit instead of a gradual closing the conduit as the flow increases over the spillway to maintain a constant 21,000 cfs.

The highest recorded inflow was 60,600 cfs on June 10, 2008.  The highest recorded outflow was 47,000 cfs on June 12, 2008.  The record high pool elevation at Saylorville Lake was 892.03’ NGVD29 on July 11, 1993.  Since the dam was completed, the pool has reached the spillway six times in 1984, 1991, in April and July 1993, June 2008, and in July 2010.

The Red Rock Dam and the Lake Red Rock Project on the Des Moines River are located chiefly in Marion County, but extend into Jasper, Warren and Polk Counties. The dam is approximately 60 miles downstream from the City of Des Moines.

The drainage area above the dam site is 12,323 square miles. A permanent lake of 265,500 acre feet storage is formed behind the dam. With the flood control pool full (elevation 780.0 feet) the reservoir storage is 1,484,900 acre feet above the conservation pool of 742 feet National Geodetic Vertical Datum
(NGVD). Flood protection is provided to 36,000 acres of agricultural lands in the Des Moines River basin and to the Cities and Towns of Ottumwa, Eldon, Eddyville, Keosauqua and Farmington.

The highest recorded inflow was 134,900 cfs on July 11, 1993.  The highest recorded outflow was 104,500 cfs on July 13, 1993.

As of 2009, these projects have prevented more than $730 million in flood-related damages along the Des Moines River (these figures have not been indexed for 2011 price levels).

1 NOTE:  An acre-foot is one acre of water one foot deep.  One acre foot is equivalent to 325,851.4 U.S. gallons.