A flow of water is generally considered a flood when it covers land that is normally not covered by water. This assumes someone has determined what is normal.
The Minnesota River is a slow-moving, serpentine stream set into the bottom of River Warren's deepest cut. While the normal elevation of the river is maybe 10 feet below the surrounding flood plain, the flood plain is some 200 feet below the table land just a couple miles away. The river drains 16,770 square miles.
Floods occur when a watershed suddenly has to drain more water than the normal channels can carry. In the Minnesota River basin, this water comes from snowmelt and rain storms. When any subwatershed experiences high water, it heads quickly for River Warren. Deep ravines, created by 9,000 years of erosion, funnel massive flows, often causing flash floods along the way. The water then rushes into the Minnesota.
To a certain extent, the river can handle these individual events and suffer only minor, localized high water. Weather is fickle, however, and sometimes, as happened this year, entire regions get inundated. When large chunks of the watershed get hit at the same time, so does the Minnesota River. This is when we get a grim, though relatively unimpressive, reminder of what River Warren once was.
A flood on the Minnesota River affects the Mississippi as well. This year's months long devastation along the entire Upper Mississippi is largely the result of heavy rains in the Minnesota River basin and adjacent watersheds that flow through Iowa.
This brings us back to what is normal. Northern Iowa was, not too long ago, mostly marshland. For a while, it was again. Where's the flood? If the marshlands had been left undrained, then we'd have had a flood. Look at River Warren. Stand on the bluff, anywhere. Where's the flood? It could rain for 40 days and 40 nights and not come close to filling River Warren. On the other hand, the Minnesota and Mississippi Rivers would be totally rebuilt in the process.
If we compare it to your typical storm sewer, River Warren is a main trunk pipeline designed to adequately handle any foreseeable load. The Minnesota and Mississippi Rivers depict the daily dribble found in any good trunk line. The trickle collects silt and debris from the incoming tributary lines until a good storm hits. With the sudden increase in flow, the lines fill up and the debris is flushed away by the torrent. After the rain stops, flow levels drop back down to a trickle, and the cycle begins anew.