Environmental Flows and Bottomland Hardwood Habitat in the Southeastern U.S.

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Environmental flows describes the quantity, quality, and timing of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems.

Individual polygons of bottomland hardwood habitat identified from the National Wetlands Inventory color-coded by elevation

Individual polygons of bottomland hardwood habitat identified from the National Wetlands Inventory color-coded by elevation above the Trinity River near Liberty, TX: Low (dark blue), Mid (light blue), High (white)

The Science Issue and Relevance: Environmental flows describes the quantity, quality, and timing of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems. Environmental flows can be defined as the streamflow needed to sustain ecosystems while continuing to meet human needs. Typically, environmental flow criteria are applied to riparian corridors downstream from a dam. In addition to dams, other factors related to human activity that can contribute to freshwater ecosystem degradation are water diversions, channel revetment, timber harvest, wetland draining, groundwater extraction, invasive species, and gravel extraction. Bottomland hardwood (BLH) habitats are particularly vulnerable because they are perched above the baseline flow of stream channels, yet rely on seasonal flooding from peak flows to maintain typical BLH species composition and healthy growth and productivity of those species. It is these peak flows which are most often modified by human activity. Flow releases from an upstream dam can be managed during times of adequate reservoir storage capacity to restore and enhance downstream riparian habitats for instream aquatic flora and fauna as well as adjacent intermittent or seasonally flooded wetlands.

Methodologies for Addressing the Issue: Satellite derived inundation maps are combined with existing data of river hydrographs, habitat maps, National Wetlands Inventory Maps, and LIDAR to identify BLH habitats and calculate historical inundation frequencies by their topographic positon in the landscape. A vulnerability assessment of individual BLH tracts and associated floodplain habitats within riparian corridors is carried out based on historical hydrology, classifying tracts by height above base flow, and identifying areas particularly vulnerable to future changes in hydrology. The following may be quantified: sensitivity, exposure, and adaptive capacity according to the species present and other abiotic factors specific to each site. Permanent forest plots for long term growth and productivity may also be installed. Tree cores may be extracted for dendrochronological analyses of pre- and post-disturbance correlations with hydrologic regimes.

Future Steps: Ongoing studies are being carried out in Texas by USGS on the Trinity River National Wildlife Refuge below Lake Livingston Dam and on Big Cypress Bayou between Lake O’ the Pines dam and Caddo Lake. The Trinity River Project, funded by the U.S. Fish and Wildlife Service, is a vulnerability assessment to prioritize and choose sites for further study. A proposal will be submitted for funding in FY16-FY17 to install long-term forest plots. The U.S. Army Corps of Engineers funded the installation of forest plots on Big Cypress Bayou. Potential future funding from the USACE will be used to re-measure the forest plots and take tree cores.

Related Project(s) and Product(s): 

Environmental Conservation Alliance. 2013. Installation of the Big Cypress Bayou Monitoring Network: Groundwater, Soil Moisture, and Long-Term Vegetation Plots 2012-2013. Project Report prepared for the Caddo Lake Institute, Austin, TX.