BRIDGING THE GAP BETWEEN THE PALEO- AND HISTORICAL- FLOOD RECORD OF THE MISSISSIPPI RIVER: ASSESSMENT OF DAMS, LEVEES, AND OTHER SOURCES OF NON-STATIONARITY

Paleo-flood records are commonly used to help inform return-periods for extreme flood events. In order to put these records into context, an understanding of the non-stationarity in the instrumental hydrological record needs to be established. To establish this context for the Mississippi River, we have complied a hydrologic database consisting of seven long-term hydrologic monitoring stations (>130 years; LTHMS) and a geodatabase of dam and levee emplacement along a 2,200 km segment of river between St. Paul, Minnesota and Vicksburg, Mississippi. We use regression and interrupted time series analyses to assess trends in river discharges in order to evaluate the impacts of reservoir expansion and other sources of non-stationarity on river flows. We also developed hydraulic models to quantify the impacts of levees on flood peaks.

Our analyses show substantial changes in river-flow distribution throughout the year and statistically significant increasing trends in river discharges along the majority of the study segment since the mid-20^th century. Changes in flow distribution are attributed to effects of rapid reservoir expansion in the Mississippi River Basin between 1935 and 1980 where maximum reservoir storage was increased by more than tenfold (30 km³ to 345 km³). Since ~1980, statistically significant increasing trends in the mean and minimum discharges were found at nearly all LTHMS (6 out of 7). Above the confluence of the Missouri River, statistically significant increasing trends in maximum annual discharge at the LTHMS were also found. Comparison of pre- and post- rapid reservoir expansion discharges indicate increases of 1% to 38% in the minimum, mean and maximum annual discharges. The increases in discharges are attributed to an increase in precipitation and land use changes in the upper Midwest.

Hydraulic modeling of the impacts of levees on flood flows revealed that levees have likely increased peak flood discharges (<1% up to 10%) and decreased flood-wave travel time from a few hours to a few days. The largest impacts on flood discharges and flood wave travel times occur along the Lower Mississippi River where the floodplain is exceptionally broad. Understanding these trends and changes in the hydrologic record will help create a more coherent long-term flood chronology for the Mississippi.