Heavy Rainfall Increases Coralville Lake Outflow

Heavy Rainfall Increases Coralville Lake Outflow

Heavy rainfall over portions of north central Iowa on Monday has increased runoff into Coralville Reservoir from its 3,084 square-mile watershed.

To manage pool levels and maintain flood storage capacity, the U.S. Army Corps of Engineers will gradually increase Coralville Lake outflows on the Iowa River from a current rate of 10,000 cubic feet per second (cfs) to 15,000 cfs, and then increase the outflow to 18,000 cfs on July 3. This outflow will remain in effect until further notice.

Increased inflow into the Coralville Reservoir on the Iowa River is causing the pool level to rise significantly. Today, the Coralville Lake level is at 702.17 feet and will continue to rise over the next several weeks. Inflow into the reservoir is expected to raise the pool level to approximately 711.5 feet on or around July 9. This level represents use of 97 percent of flood storage capacity with a pool level .5 feet below the emergency spillway at 712 feet.  The record high stage at Coralville Reservoir was 717.02 feet on June 15, 2008.

If water overtops the spillway, increased flooding can be expected in the Iowa City area. To reduce the risk of this occurring, the Corps increased the release rate from the Coralville Reservoir on Tuesday, July 1, to 8,000 cfs and will continue to increase the release rate to 18,000 cfs. Additional rainfall and revised National Weather Service forecasts may require increasing outflows beyond 18,000 cfs in an effort to keep the pool level below the spillway.

Downstream residents should be alert to rising water levels as the reservoir outflow is increased.

USACE, July 3, 2014