"Big Muddy" is misbehaving again. Flood waters have been raising havoc in the upper reaches of the Mississippi, and the flood crest (highest water) is inexorably moving down river at about 40 miles a day. In some places, the river is cresting higher than in the flood of 1993, which was one of the worst in the past 100 years. There's so much water in the tributaries of the Mississippi, and the main stem itself, that water levels will remain exceptionally high for weeks after the crest passes. If rainfall is above normal in the days to come, then the flooding could be of historic proportions. Human chains have been piling up tens of thousands of sand bags along river and stream banks in an attempt to keep the river from invading their community. Flooding has also been unsafe for river barge traffic - more than 400 miles of the river has been closed to shipping. Thus far, Minnesota, Wisconsin, Illinois, and Iowa have all declared states of emergency in the counties bordering the Mississippi.

Cities and towns that raised money to build flood walls and levees are breathing a little easier than cities like Davenport, Iowa where no such protection exists. Davenport is one the cities that make up the Quad Cities, and it's being hit hard by the flooding. A portion of the city near LeClaire Park and Main Street is underwater and the Minor League baseball park in Davenport where the Quad Cities River Bandits plays has been ruined. The River Bandits have been flooded out for the rest of the season - they'll play their home games in a different park 25 miles from the Mississippi. Yesterday, the river was very close to reaching its projected 22.5 foot crest, which is just shy of the record 22.63 feet crest set in July 1993 and more than 7 feet above flood level. The crest could last up to 36 hours before receding. As was the case 8 years ago, the question as to whether or not a flood wall should be built in Davenport will likely be the subject of spirited debate by its citizens after the flood waters retreat.

For cities like Prairie du Chien, Wisconsin, for instance, this has been the worst floods since the severe flooding of 1965. Earlier in the week, strong winds, gusting to 50 miles per hour, compounded the high water by pushing 3-4 foot waves against the manmade sandbag dikes. In places, the dikes were overwhelmed by the waves, and even when the bags were piled high enough, the tremendous pressure exerted by the high volume of water squeezed through the sandbags, compromising the integrity of the dikes. Once the river finds a weak spot in the dikes, theres not much anyone can do to stop the Mississippi from spreading out. All along the upper Mississippi, beleaguered flood fighters have been working long hours using more sand bags to plug leaks in dikes and worn out levees in an all-out effort to contain the bloated Mississippi River.

A number of the same problems and concerns that are occurring now were issues during the 1993 flooding of the Mississippi. In 1993, 10 states in the upper Mid West and central plains had to deal with flooding. There were several principal factors that led to the 1993 flooding. For several consecutive months in early 1993, above normal precipitation in the upper Mid West swelled the Mississippi and Missouri Rivers and their tributaries. Then during the spring and summer, when anomalously heavy rains fell, there was no place for the excess water to go. Much of the Mid West received over 150% of normal rainfall, and parts of North Dakota, Kansas, and Iowa received more than double their typical rainfall. Individual storms frequently dumped large volumes of precipitation that local streams simply couldn't handle. One other factor leading to flooding in 1993 was cooler than normal temperatures acted to retard evaporation rates. Thus, there was less evaporation of rain water and the rainfall could not be readily absorbed by the saturated ground. So, more ran-off into streams.

In contrast to 1993, this year's flooding has a snow component that was missing from the floods of 8-years ago. Heavy winter snowfall in the headwaters area of the Mississippi produced a healthy snowpack, and even though no record snowfalls occurred, and the snowpacks didn't build to record depths, there was plenty of snow to cause problems if it melted quickly. If 12 inches of snow covers an area of say about 200 miles by 200 miles in the upper Mid West, and if an equivalent of about 4 cubic inches of water is stored in each 1" X 1" X 12" slice of snow, then this could result in as much as 2 1/2 cubic miles of water when the snow melted. Of course, sublimation and evaporation would remove some of the snow and some of the meltwater during the ablation process, and some of the water would be absorbed by the ground. Nevertheless, there's still a tremendous amount of water now available to runoff toward streams that wasn't there before the snow began to melt.

In general, the snowpack can be thought of as a reservoir that effectively stores water. If the snow melts gradually, water is slowly released. This year, a late March cold spell kept the pack in a frozen state, and when unseasonably warm temperatures arrived, much of the snow melted simultaneously. Because the ground was still cold and hard, most of the melting snow didn't penetrate into the soil but instead ran-off directly into streams. On top of this, heavy rains in early April added still more water to swollen streams and rivers. By the way, rain falling on snow doesn't accelerate snowmelt by very much. Normally, the majority of the energy that's available to melt snow comes from short-wave solar radiation (from 0.2 to about 2.2 microns), which is absorbed by the snowpack. Convective and latent energy exchanges between the snowpack and the air above are of secondary importance.

The Mississippi River drainage basin has been altered over the previous century by draining wetlands, construction of housing developments, allowing building in the floodplains, and by attempting to confine the flow of the Mississippi and its tributaries. Increasing the amount of impervious area and decreasing the amount of land in marsh and woodlands will only increase the potential for flooding of the "Big Muddy" in the future.

For more about this, see this site about the 1993 floods http://lists.uakron.edu/geology/natscigeo/Lectures/streams/Miss_flood.htm#causes 

and see the following site for this year's flooding

http://epod.usra.edu/  (for April 25 and 26).