.[1] Along >4000 km of the Mississippi River system, we document that climate, land-use change, and river engineering have contributed to statistically significant increases in flooding over the past 100-150 years. Trends were tested using a database of >8 million hydrological measurements. A geospatial database of historical engineering construction was used to quantify the response of flood levels to each unit of engineering infrastructure. Significant climate-and/or land use-driven increases in flow were detected, but the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction. In the area of the 2008 Upper Mississippi flood, for example, about 2 m of the flood crest is linked to navigational and flood-control engineering. Systemwide, large increases in flood levels were documented at locations and at times of wing-dike and levee construction.

The goal of this study was to construct a large, data-rich model to test hydrological responses to engineering modifications on over 3200 km of the Mississippi and Lower Missouri Rivers. We compiled model explanatory variables from a geospatial database quantifying construction of all bridges, wing dikes, bendway weirs, levees, artificial meander cutoffs, channel constriction and navigational dams over the past 100-150 years. Response variables were derived from 68 rated and un-rated hydrologic stations in the study area, with responses analysed across a range of discharges from within-channel flows up to moderate floods. Correlation analysis, multiple linear regression and stepwise regression analyses document strong and consistent responses to construction history, both in individual reach-scale models and systemwide. Meander cutoffs are associated with degradation and acceleration of flow that has reduced stages across the full discharge range. Navigational dams on the Upper Mississippi River increased low-flow stages and flood levels to a lesser extent, with little or no post-dam change. One of the strongest signals was the hydrologic response to wing-dike construction, which resulted in large back-water increases in stage upstream of wing dikes and mixed effects downstream, including the overlapping effects of incision and velocity losses. Levees were associated with local flow concentration, overbank storage loss and floodplain conveyance loss depending on reachscale conditions. The results presented here (1) quantify incremental and cumulative hydrologic responses to a range of engineering activities and (2) provide an empirical tool for verifying and assessing hydraulic and other models of river-system change.

Abstract:A refined specific-gauge approach was developed to quantify changes over time in hydrological response on 3260 km of the Mississippi River system using long-term data observed at 67 hydrologic measurement stations. Of these stations, 49 were unrated (stage-only) stations, for which over 2 000 000 'synthetic discharges' were generated based on measured discharge values at nearby rated stations. The addition of these synthetic discharges nearly tripled the number of stations in the study area for which specific-gauge analysis could be performed. In order to maintain spatial homogeneity across such a broad study area, discharges were normalized to multiples of mean daily flow (MDF). Specific-gauge analysis calculates stage changes over time for invariant discharge conditions. Two discharges were analysed: low-flow and flood conditions at each station. In order to avoid the large errors associated with extrapolation of annual rating curves, a new 'enhanced interpolation' technique was developed that calculates continuous specific-stage time series, even for rare discharges. Thus enhanced, specific-gauge analysis is a useful reconnaissance tool for detecting geomorphic and hydrologic trends over time. Results show that on the Middle Mississippi River and Lower Missouri River, flood stages increased at all stations in spite of widespread incision of the river bed. On the Lower Mississippi River, both low-flow and flood stages decreased, mainly the result of artificial meander cutoffs in the late 1920s and 1930s, except downstream of Natchez, MS, where net aggradation was observed. On the Upper Mississippi River, the specific-gauge trends were dominated by emplacement of navigational dams and impoundment of slackwater pools. On all four river reaches, these results document hydrologic responses to the different engineering toolkits used on the different portions of the Mississippi River system during the past 75-150 years.

Catastrophe models often cover large geographic areas spanning multiple countries or, in the case of flood models, entire watersheds. Models must be sufficiently detailed to accurately account for hydrologic variation, which is notably challenging when the modeled region is large. This is particularly true for flood models, which require a highly detailed dataset, usually derived from a digital terrain model (DTM), for reliable floodplain mapping. For one-dimensional (1D) hydraulic models, the floodplain mapping approach tends to yield flat surfaces often resulting in artefacts and inconsistencies near river confluences. Because flood extent is limited by the length of cross-sectional lines along the floodplain, these flat surfaces tend to drop sharply when the simulation reaches a flat delta. The use of a two-dimensional (2D) model avoids these problems, but at a high computational cost, and requires high quality terrain and bathymetry data. This paper presents a new methodology for mapping floodplains using water elevation points along a river network obtained from a 1D hydraulic model and a DTM. The methodology applies kinematic and diffusion wave equations in a simplified manner, using water elevation points as internal boundary conditions. Several parameters control the expansion and smoothing algorithms that generate realistic flood extent maps for different return periods. This methodology is particularly suitable for modeling large domains since it produces accurate results but requires much less computational time than a 2D model. In addition, because the computation uses several source points per cross section, the flood extent is not limited by the cross-sectional length, making this methodology appropriate for levee breaches and in cases where river banks are not well defined and the crosssectional geometry is derived from a DTM.