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Science to Inform Management of Floodplain Conservation Lands on the Middle Mississippi River

December 29, 2020

Within large-river ecosystems, floodplains serve a variety of important ecological functions. A recent survey of 80 managers of floodplain conservation lands along the Upper and Middle Mississippi and Lower Missouri Rivers in the central United States found that the most critical information needed to improve floodplain management centered on metrics for characterizing depth, extent, frequency, duration, and timing of inundation. These metrics can be delivered to managers efficiently through cloud-based interactive maps. To calculate these metrics, we interpolated an existing one-dimensional HEC-RAS hydraulic model for the Middle Mississippi River, which simulated water surface elevations at cross sections spaced (less than 1 kilometer) to sufficiently characterize water surface profiles along an approximately 800 kilometer stretch upstream from the confluence with the Mississippi River over an 80-year record at a daily time step. To translate these water surface elevations to inundation depths, we subtracted a merged terrain model consisting of floodplain LIDAR and bathymetric surveys of the river channel. We completed these calculations for an 800 kilometer stretch of the Missouri River, spanning from Rulo, Nebraska to the river's confluence with the Mississippi River. Analyzed areas include the entirety of the Mississippi River floodplain, with the exception of the St. Louis metropolitan area in which analysis was constrained to currently unleveed areas only. This approach resulted in a 29,000 plus day time series of inundation depths across the floodplain using grid cells with 30 meter spatial resolution. This dataset presents 14 metrics for each of two scenarios, one using a baseline timeseries of stages from the HEC-RAS simulation and one using a timeseries of stages adjusted to account for removal of existing levees from the floodplain. These metrics are calculated on a per pixel basis and encompass a variety of temporal criteria generally relevant to flora and fauna of interest to floodplain managers, including, for example, the average number of days inundated per year within a growing season. We also include the base elevation layer that we generated to calculate depth of inundation from interpolated water-surface elevations.

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