Corps of Engineers outflow trigger: When the Burlington gage is projected to exceed 18 ft, COE written policy is to reduce the outflow of the Coralville reservoir to 1000 cubic feet per second (cfs) for up to 7 days to mitigate the impact of high water.
Actual flood mitigation: Since the flow in the Iowa River and the capacity of the Coralville reservoir are small compared to the flow of the Mississippi at 18 ft (248,000 cfs), the mitigation effected by this policy is small. There are two common scenarios. In the first, although the Mississippi is flooding the Iowa River basin is relatively dry, so an outflow reduction to 1000 cfs represents little change. A recent example is the crest of the Burlington gage at 18.94 ft on 21 April 1997, which reflects a reduction of 0.10 feet compared with natural flows. Even this reduction used 5.8% of the capacity of the reservoir. In the second, the Mississippi and the Iowa Rivers are both flooding, so that the Coralville Reservoir is quickly filled beyond the emergency level, in which case the regulation does not call for any attempt at flood mitigation at Burlington. A recent example is the the crest in of the Burlington gage at 23.80 ft on 14 July 1993, which occurred at a time when no mitigation was attempted.