ROCK ISLAND, Ill. – The results of a U.S. Army Corps of Engineers’ nine-month study to enhance understanding of flooding on the Des Moines River at and below the Corps’ Saylorville Lake and Lake Red Rock reservoirs show that flood flow frequencies on the Des Moines River have increased.
Corps officials met yesterday with representatives from the City of Des Moines and Ottumwa; Polk and Wapello counties; the Iowa Departments of Natural Resources and Transportation; the Federal Emergency Management Agency; and staff members from Senator Tom Harkin, Congressmen Leonard Boswell and Dave Loebsack, and Governor Terry Branstad; to discuss the results of the Des Moines River Regulated Flow Frequency Study and begin assessing possible impacts to existing Corps’ and local flood-risk-management projects and Corps reservoir operations.
In recent decades, the Des Moines River has risen higher and more often prompting the Corps to re-evaluate flow frequency estimates. This study included a scientific assessment to estimate the frequency and magnitude of future reservoir outflows and downstream river flows by updating and analyzing the period of record to account for an additional 14 years of data (1995-2008), including the Midwest Flood of 2008.
Consistent with the large flood events the basin has experienced over the past several decades, the study results show that flood flow frequencies have increased over previous Corps estimates and that floods like the 1993 and 2008 events are more likely than previously estimated. Thus, floodplains adjacent to the Des Moines River and some areas once thought to be outside of the floodplain or that are protected by a flood-risk-management project are at a greater risk of flooding than previously estimated.
Study Summary
The study involved an extensive review of historic stream gage records, reservoir inflows and outflows, and river flow data for a 92-year period of record from 1917-2008. Data from the 2010 flood event was not included as the study was well underway prior to the event.
The study results help to better identify the likelihood of flooding, allowing residents and property owners to better assess their individual flood risks. Further, the results assist the Corps, other agencies, and local governments in communicating those flood risks to individual property owners and communities; as well as provide design information for new projects or modification of existing projects.
While there may be many underlying reasons why river flows and flooding have increased, the study was not designed or conducted to define the cause(s). The scope of the study was to examine river and reservoir data and project future flood probabilities.
The following tables summarize new and previous flow frequencies at Saylorville Lake, Lake Red Rock and SE 6th Street in Des Moines, Iowa.
Flood flow frequencies increase; 1993 and 2008 flood events more likely
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Saylorville Lake Release1
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Lake Red Rock Release2
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Flood Event Probability
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Previous Value
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New Value
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Previous Value
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New Value
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0.50 (2-year)
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13,000 cfs
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12,300 cfs
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27,500 cfs
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25,700 cfs
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0.10 (10-year)
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17,000 cfs
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17,300 cfs
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31,500 cfs
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31,000 cfs
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0.02 (50-year)
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27,000 cfs
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45,200 cfs
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50,800 cfs
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68,700 cfs
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0.01 (100-year)
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34,000 cfs
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53,400 cfs
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70,300 cfs
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93,600 cfs
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0.005 (200-year)
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39,000 cfs
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61,900 cfs
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94,800 cfs
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137,100 cfs
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0.002 (500-year)
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50,000 cfs
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73,800 cfs
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Not Estimated
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137,100 cfs
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Table 1 - New and previous Saylorville Lake and Lake Red Rock release estimates as determined by the study.
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Saylorville Lake Release1
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Lake Red Rock Release2
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Flood Event Probability
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With Reservoir
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Without Reservoir3
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With Reservoir
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Without Reservoir3
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0.50 (2-year)
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12,300 cfs
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16,000 cfs
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25,700 cfs
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39,700 cfs
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0.10 (10-year)
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17,300 cfs
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34,600 cfs
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31,000 cfs
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81,200 cfs
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0.02 (50-year)
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45,200 cfs
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53,200 cfs
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68,700 cfs
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121,000 cfs
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0.01 (100-year)
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53,400 cfs
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61,700 cfs
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93,600 cfs
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138,000 cfs
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0.005 (200-year)
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61,900 cfs
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70,300 cfs
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137,100 cfs
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156,000 cfs
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0.002 (500-year)
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73,800 cfs
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82,300 cfs
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137,100 cfs
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180,000 cfs
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Table 2 - The reduction in Des Moines River flows attributed to the operation of Saylorville Lake and Lake Red Rock projects as determined by the study.
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SE 6th Street, Des Moines 4
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SE 6th Street, Des Moines 4
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Flood Event Probability
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Previous Value
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New Value
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0.50 (2-year)
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25,400 cfs
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26,300 cfs
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0.10 (10-year)
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40,400 cfs
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44,100 cfs
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0.02 (50-year)
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72,000 cfs
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83,300 cfs
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0.01 (100-year)
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87,000 cfs
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107,500 cfs
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0.005 (200-year)
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106,000 cfs
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122,100 cfs
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0.002 (500-year)
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Not Estimated
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142,000 cfs
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Table 3 - New and previous flood flow frequency estimates on the Des Moines River at SE 6th Street in downtown Des Moines, as determined by the study.
1 Saylorville Lake’s maximum flood pool is 890 feet National Geodetic Vertical Datum (NGVD29). When water levels reach elevation 884 feet NGVD29, water flows over the spillway. The maximum release through the Lake’s conduit is approximately 21,000 cfs. Values above 21,000 cfs include flows over the spillway. The highest recorded outflow was 47,000 cfs on June 12, 2008.
2 Lake Red Rock’s maximum flood pool is 780 feet NGVD29. When water levels reach the maximum flood pool water flows through the controlled spillway comprised of Tainter gates. The maximum release through the Lake’s sluice gates is approximately 37,700 cfs. Flows above 37,700 cfs are directed through the Tainter gates. The highest recorded outflow was 104,500 cfs on July 13, 1993.
3 The values under “Without Reservoir” represent the predicted flows on the Des Moines River if Saylorville Lake and Lake Red Rock projects did not exist. As of 2009, these projects have prevented more than $730 million in flood-related damages along the Des Moines River. Note that these figures have not been indexed for 2011 price levels.
4 The highest recorded flow at SE 6th Street, Des Moines, Iowa, was 116,000 cfs on July 11, 1993.
NOTE: cfs = The rate of flow past a given point, measured in cubic feet per second. One cubic foot = 7.48 gallons.