CONSEQUENCES OF HUMAN-ALTERED FLOODS: LEVEES, FLOODS, AND FLOODPLAIN FORESTS ALONG THE WISCONSIN RIVER

Abstract

Flood-control levees are generally thought to increase flood height and velocity for a given discharge. While extensive areas of floodplain in the United States are leveed, the ecological impacts of levees have largely been ignored relative to other anthropogenic impacts to large river floodplains. We examined a century of flood control along the Wisconsin River by comparing simulated flood regimes under “levee” and “levee-removal” scenarios. We also used field sampling to determine if levees had altered the distribution of dominant floodplain forest trees. Increases in flood stage (height) due to levees were minor, only a few centimeters. This was primarily due to the location of the levees, set back hundreds of meters into the floodplain in some areas. Increases in overbank flood velocities due to levees were minimal compared to increases caused by channel constriction and by increased flood magnitude. Generally, levees had a greater impact on stage and overbank flood velocities of larger magnitude events. The mean number of floods and number of days flooded were lower in areas outside (on upland sides) of levees, and stream power was zero in these areas due to a lack of any inundation. These areas also had lower importance values (IV) for several flood-tolerant tree species (Acer saccharinum and Fraxinus pennsylvanica) and higher IVs for some flood-intolerant species (Quercus velutina and Q. ellipsoidalis). Furthermore, areas inside levees (between the levee and the channel) were no different from completely unleveed areas in the number of floods, number of days flooded and in IVs of several dominant tree species. The levee location (set back into the floodplain) resulted in a similar historic flood regime, and thus, similar abundances of floodplain tree species in areas inside levees as compared to completely unleveed areas. Setback levees can provide an important compromise by maintaining the relative abundance of tree species normally found in unleveed areas, while also allowing some flood control. Floodplain restoration involving levee removal should generally target the removal of mainline levees (those adjacent to the channel) rather than removal of setback levees.