The Iowa River is one of several southeastern flowing rivers that drain the eastern half of Iowa to join the Mississippi River along the state’s eastern boundary. Record floods that swept the nation in the early 1930’s prompted Congress to establish the Flood Control Act of 1938. In an attempt to reduce flooding on the Mississippi River, the U.S. Army Corps of Engineers was authorized to construct several dams on tributary rivers, including the Iowa River. In addition to moderating flows on the Iowa River, Coralville Lake’s less evident, but equally important, role is part of the comprehensive flood control system for the Mississippi River. Construction of the dam started in 1949, but was delayed by the Korean Conflict.
The focus of this story is on the effects of two flooding episodes in 1993 and again in 2008. Both times water breeched the concrete spillway and revealed a layer of bedrock from the ancient Devonian seafloor which existed in this region 375 million years ago.
The area in the lower right corner of the previous image is labeled with the dam and spillway for the lake. Zooming into the Google Earth map to the dam gives the image below. Some key points are labeled A thru F where I have photographs or video for illustration. At point A, there is a tower to control the release gates to regulate the flow out of the lake. Three gates can be opened independently.
During drought, the minimum flow rate of 150 cubic feet per second provides a sufficient flow to meet downstream domestic and industrial water needs. During time of flood, the outflow can be increased to about 20,000 cfs via a 23 foot diameter flow pipe through the dam. Inflow to the lake can exceed 40,000 cfs during rainy flood episodes. The lake level rises and potentially goes over the spillway to the right of point B.
How I See It 