CONTRASTING FLOOD RESPONSES TO RIVER ENGINEERING: MISSISSIPPI AND RHINE RIVER SYSTEMS

Analyses of long-term gaging records from the Mississippi and Missouri rivers and from the Rhine and other European rivers document contrasting responses to river and floodplain modification. On the Mississippi and Missouri rivers, flood conveyance capacity has systematically degraded over time, with increases in flood heights of up to 3 m for equal quantities of water. Preliminary results from a new GIS database of river construction history document that this degradation corresponds to progressive navigation and flood-control engineering. Flood increases have been incorporated into calculations of flood frequencies and suggest, for example, a 100-year flood level at the city of St. Louis of 15.4 ± 0.9 m, sharply higher than results from a current government study of Mississippi system flood probabilities which excludes several mechanisms of flood magnification. In contrast, analysis of nine long-term stations on the River Rhine documents little or no magnification of flooding through at least the 20th century. With the exception of Maxau, Germany, all stations analyzed show flood stage trends that have been flat or have fallen over time, documenting that 20th century regulation of the German Rhine has caused little or no flood magnification. The Mississippi and the Rhine have similar histories of river regulation and floodplain utilization, but the Rhine has been engineered for smaller vessels, with depths and an engineered channel geometry that better match the natural limitations of the system. Furthermore, whereas the Mississippi system has been characterized by continued encroachment on the floodway up to and including the present, the Rhine states have since the 1980s enacted a series of accords to increase the floodplain storage capacity of the Rhine system. Although analyses of precipitation data and the basin-scale rainfall-runoff relationship document modest increases in flooding on the Rhine, these increases appear to be due to climate change and shifts in land use, mainly agricultural, over the course of 20th century.