Trends in flood stages: Contrasting results from the Mississippi and Rhine River systems

An obstacle to testing human impacts on flooding is that anthropogenic and natural hydrologic changes occur simultaneously and are often indistinguishable. We attempted to isolate the impacts of overlapping mechanisms on flooding by analyzing 73-188 years of stage measurements from the Mississippi River system and the German Rhine. Although typically under-utilized in hydrologic analysis, stage data document the cumulative impacts of both upstream changes such and climate and land use as well as modifications of the river channel and floodplain. Analysis of trends in peak stages and flood frequencies shows contrasting results on the Mississippi and Rhine: increasing trends at most Mississippi stations, but no significant trends at most Rhine stations. On the Mississippi and its tributaries, statistically significant changes in peak stages and peak-stage frequencies were identified at 10 of 14 stations, with all. but one exhibiting increased flooding. On the Rhine River, systematic change occurred at only,1 of 8 stations analyzed. On both river systems, channel, and floodplain modifications dominated net hydrologic response, overwhelming the effects of climate change, land-use shifts, and dam construction.. Specific-gage analysis, which isolates the impacts of instream river modifications, documented declining flow conveyance at all stations where flood levels and frequencies increased. Increased flooding at the Mississippi River sites appears to be driven by the history of aggressive channel engineering, in particular channel constriction to increase navigation depths. In contrast, navigation infrastructure on the Rhine has not substantially degraded the river's capacity to efficiently convey flood flows. (c) 2006 Elsevier B.V. All rights reserved.