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.