Missouri River Dams Key to 2011 Flood

A series of flood control reservoirs backed up by massive dams is a key factor driving the high water currently swelling the Missouri River.

The abnormally high flow on the upper Missouri River and its tributaries has come on the heels of high winter and spring precipitation in the river's catch basin.

Image credit: US Army Corps of Engineers.

The US Army Corps of Engineers operate six dams, located in eastern Montana and the Dakotas, on the upper main stem of the Missouri, which are designed to control river flow during times of low and high runoff.

When flows into the reservoirs are high, as they have been during this spring, operators allow inflow to exceed outflow, thereby storing the excess in the lakes behind the dams.

Yet each reservoir has a built-in maximum capacity to store water. When this is reached, operators of the dam's flow structure must allow outflow to equal inflow, or the dam would be threatened with over topping--and potentially catastrophic results.

Dams are built with emergency spillways to prevent catastrophic failure once their ability to effect control on flow is exhausted.

Following many weeks of high rainfall and runoff, now further magnified by runoff from Rocky Mountain snow melt, the six flood-control reservoirs have effectively reached their storage capacity. The Army Corps have had to boost outflows to exceptionally high levels to prevent even higher "spikes" in flow below the dams.

Five of the six dams have been scheduled to release water at a rate of about 150,000 cubic feet per second (cusecs) from mid June until at least mid July. At this rate of flow, the river levels below the respective dams are above flood stage, sometimes high enough to threaten property.

Lowest of the dams, known as Gavin Point, is key to water levels on the Missouri River downstream. High flow released at Gavins Point effects river banks in South Dakota, Nebraska, Iowa, Missouri and Kansas.

Unprotected flood plain and some roads have already gone under water, with even higher water levels still forecast to come..

High flows out of Gavins Point was a contributor to the flooding that breached two levees near the three-way meeting point of Iowa, Missouri and Nebraska on Monday. One was a reported 300-foot "full breach". These failures allowed the river to flood crops, and threatened to send floodwaters into the nearby town of Hamburg, Iowa.

But dam releases are not the only factor down stream, as there are many other tributaries, great and small, that flow into the Missouri and thus add to its flow. Recent weeks have seen high flows on the James, Big Sioux and other tributaries compounding high flows down stream of Gavins Point.

Going forward, the outlook for the Missouri River is not good. Forecasters at AccuWeather.com foreseen rainfall above to much above normal in much of the river's watershed through late next week.

Any heavy rainfall within watersheds of these tributaries can have a significant bearing upon flood levels on the Missouri itself.

The Army Corps have forecast flood crests above flood stage along the full reach of the Missouri from Gavins Point to its mouth at the Mississippi River above St. Louis. In some reaches of the Missouri, the flood crest could rival highest historical crests.

Summer rainfall across the Missouri's vast catch basin will determine the ultimate seriousness of the Flood of 2011.

Related to the Story: Severe Weather Center Global Warming Center Extreme Weather with Henry Margusity The WeatherMatrix Blog With Jesse Ferrell