Hydrology

Central to watershed-scale planning for water quality enhancement is an understanding of hydrologic relationships within and across watersheds. DU compiled a series of useful datasets to help users better understand the hydrology of these systems. USGS 8 digit HUC's were incorporated to provide a region-wide coverage of drainage basins. 11 digit HUC equivalents were acquired for Mississippi and Louisiana. http://water.usgs.gov/GIS/huc.html National Hydrography Dataset water bodies were included for reference. NHD stream reaches were incorporated to elucidate drainage networks. http://nhd.usgs.gov/

Relative flood probability

Hydrologic functionality, namely the flooding regime, stands as likely the single most important factor in the ecological integrity of wetland ecosystems (Mitsch and Gosselink, 2000). Though the majority of the LMAV historically functioned as a floodplain system, the widespread hydrologic engineering of the 20th Century drastically altered natural hydrology, thereby producing a much smaller active floodplain at present day. Because understanding hydroperiod for sites throughout the region is paramount to the potential effectiveness of wetland restoration/ rehabilitation activities and water quality planning, the need for spatially explicit, medium to high resolution flood probability data became apparent. Revegetation of denuded flood zones contributes substantially to slowing down flood waters, thereby causing sediment to fall out and be retained. Vegetation can take up excess nutrient loads, and root structures help stabilize floodplain soils, thereby preventing additional erosion. DU GIS specialists, with support from the USDA Forest Service and the LMVJV office, completed a Relative Flood Probability Dataset, which provides an analytical tool to understand the current extent and relative frequency of the high frequency floodplain. The Relative Flood Frequency dataset arose as a synthesis of river gage data analysis and the classification of satellite imagery. The gage data were used to determine appropriate dates for each individual stream segment at flood stages of interest. Dates were correlated with available Landsat satellite imagery, which was used to document flood extents. Through the combined efforts of Ducks Unlimited and the Lower Mississippi Valley Joint Venture Office, Period of Record (POR) river gage data were acquired from the various US Army Corps of Engineers districts that comprise the Lower Mississippi Alluvial Valley: New Orleans, Vicksburg, Memphis, and Little Rock. Satellite scenes were selected ranging back to 1982 based upon a frequency analysis of flood events from the river gage data. Water features were extracted from the imagery and compiled to reflect the percentage of times out of all scenes that each pixel was observed inundated. The analysis revealed that despite the high level of hydrologic engineering within the LMAV, there still exists a significant active floodplain. Of the 23 years worth of flood events captured via satellite imagery, approximately 11.9 million acres or 48% of the LMAV was observed to flood on at least one point in time or another. Of those 11.9 million acres, approximately 2.1 million were observed to flood during every satellite observation, suggesting permanent winter water (see the discussion under Managed Water 87-05). Separating out these annually present winter water acres leaves nearly 9.8 million observed acres, or nearly 40% of the region, that are assumed to be intermittently flooded.