Sometimes a river will get way out of its banks and stay there for a very long time. Occasionally this kind of flood occurs "off-season"—when the river is not expected to be flooding. When this occurs, scientists refer to it as a great flood. A great flood is a major event for the river-floodplain ecosystem. The water may go well beyond the normal floodplain and uproot or drown trees and other vegetation. The river may gouge a new channel, or deposit huge banks of sand and debris (called natural levees). Fish and waterfowl may have a rich, new habitat for a while. And when the river finally goes back into its banks, it’s a gold rush for the pioneer plants and animals that colonize the muddy land.

Even though a great flood may seem like a major disruption for the river-floodplain ecosystem, it is important to understand that it is a normal event. Totally unpredictable in time and space, but normal. Kind of like adolescence. Every Great River will experience great floods, and their effects may be profound. If seasonal floods renew the fertility of the floodplain, great floods rearrange the entire ecosystem, changing the landscape and allowing the balance among species and their functions to be reset.

Sometimes we hear officials speak of great floods in terms of "100-year" or "500-year" floods. This is very unfortunate terminology because it causes a lot of confusion. Great floods are not on a schedule! The "100-year level" simply means a flood so big that it is only likely to happen once in a hundred years.

Great floods are sometimes predictable weeks to months before they occur. For example, in 1997 the heavy spring flooding along the Red River in Minnesota and North Dakota was predicted two months in advance. Scientists noted the heavy rainfall that the region received in the fall. That water froze and then snow fell on the ice. The spring thaw released not just the winter precipitation, but the fall precipitation into the Red River. There was another great flood in 1993, in the Mississippi basin.

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