Why does the Mississippi River flood?

Answer: When reviewing the causes of flooding on the Mississippi River, there are numerous things to consider.  One important fact is to consider where the Mississippi gets its water.  The Mississippi River maintains its flow through water (snow melt and rain in the spring, and rain in the summer) draining into the river from 41 percent of the continental United States.  Thousands of smaller rivers and streams feed into the Mississippi to help maintain its flow and, at times, cause flooding.

The second factor to consider is that locks and dams were not constructed to prevent flooding.  Although they appear to be large flood control structures, they have absolutely no flood control capabilities whatsoever.  Reservoirs, or dams, such as Coralville Reservoir (on the Iowa River), and Saylorville and Coralville Reservoirs (on the Des Moines River) were constructed for flood control and help limit the flow of water going down those tributaries into the Mississippi River.  If the Mississippi River dams could hold back the water, an enormous pool would be created behind the dam and would flood many communities.

The lock and dam system was created to maintain a 9-foot deep channel in the Mississippi River to ensure there will be deep enough water for commercial tows.  The dam's rollers and gates do not reach from the bed (bottom) of the river to above the water line.  They are only placed into a portion of the river to assist in maintaining the 9-foot depth.  The rollers and gates on the dams only restrict water flow; in essence, they put a drag on the flow to keep the pool behind the dam high enough to maintain the 9-foot deep navigation channel.  Once the 9-foot channel is obtained, all the excess water flow passes through the dam under the gates or rollers.  In high water situations, such as the floods of 1993, 1997 and 2001, the dam gates and rollers are taken completely out of the water and all the flow passes through.

Many people believe that the locks and dams create flooding problems, but that is not true.   As mentioned above, the locks and dams on the Upper Mississippi River were built to provide 9-foot depth for commercial navigation.  These dams are designed to maintain fixed pool levels by passing all the inflows entering the system and are fully open during high flows.  There is insufficient storage at these dams to cause any change in flood levels.  For example, if all the pools behind the dams were fully drained, it would take only a few hours to fill the vacated space during a major flood.

Contrary to popular belief, flooding is an act of nature.  Several factors such as excessive snow melt, rainfall into the river and its tributaries, runoff, a loss of wetlands, and urban construction all contribute to increased flooding on river systems.  Preventing flood-related damages can be accomplished by building structures, such as reservoirs, levees, channels, and floodwalls that change the characteristics of floods.  In addition, non-structural measures, such as flood plain evacuation and flood proofing help reduce the flood risk to humans.

The U.S. Army Corps of Engineers fights the nation's flood problems not only by constructing and maintaining structures, but also by providing detailed technical information on flood hazards.  Under the Flood Plain Management Services Program, the Corps provides, on request, flood hazard information, technical assistance, and planning guidance to other federal agencies, states, local governments, and private citizens.