ROCK ISLAND, Ill. – The U.S. Army Corps of Engineers, Rock Island District, initiated a temporary modification to outflows today at Saylorville and Red Rock reservoirs to preserve flood storage capacity. This also allows the Corps to identify any unknown impacts as it evaluates the modification as a permanent change to the reservoirs’ water control plans.
The temporary modification will be in effect through December 15.
Under the current water control plan during the growing season, normal flood operations release between 12,000 cubic feet per second (cfs) and a maximum of 16,000 cfs at Saylorville (depending upon Red Rock’s pool level) and a maximum of 18,000 cfs at Red Rock.
Following coordination with, and concurrence of, county and city officials from the cities of Des Moines, Johnston and Polk City; Mahaska and Marion counties, it was determined that increased release rates will have no flood-related impacts to communities and areas along the Des Moines River below the reservoirs.
The purpose of the temporary modification is to reduce the possibility of higher release rates due to excessive rainfall. When reservoir levels or inflows permit, outflows will be a maximum of 16,000 cfs at Saylorville (regardless of Red Rock’s pool level) and a maximum of 22,000 cfs at Red Rock. If rainfall increases reservoir elevations to require release rates above these thresholds, releases will be made in accordance with the existing water control plans.
At Saylorville Reservoir, the current pool level is 840.82 feet mean sea level (msl) with a target conservation pool level of 836 feet msl.
At Red Rock Reservoir, the current pool level is 747.47 feet msl with a target conservation pool level of 742 feet msl. Increasing the current outflow of 18,000 cfs to 22,000 cfs would increase downstream water levels approximately 1.2 feet.
A maximum draw down rate of approximately 1 foot per day will be maintained at both reservoirs to prevent erosion and bank sloughing within the reservoir. Increased outflows will continue until target pool levels are met. When met, reservoir outflows will fluctuate to maintain pool levels.
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Red Rock’s forecast is available at: http://rivergages.mvr.usace.army.mil/WaterControl/Districts/MVR/Forecast/pelforecast.html.
Saylorville’s forecast is available at: http://rivergages.mvr.usace.army.mil/WaterControl/Districts/MVR/Forecast/sayforecast.html.
Cubic Feet per Second (cfs): The rate of flow past a given point, measured in cubic feet per second. One cubic foot of water equals about 7 1/2 gallons and weighs 62 pounds.
Mean Sea Level (msl): A local datum based on repeated measurements of sea level throughout all of its normal cycles such as tides and seasonal change. The basis for elevations on a map.