Floodplain Inundation (Southeast Blueprint Indicator)

Reason for Selection “Although floodplains support high levels of biodiversity and some of the most productive ecosystems on Earth, they are also among the most converted and threatened ecosystems and therefore have recently become the focus of conservation and restoration programs across the United States and globally” (Opperman et al. 2010). The various aquatic indicators in the Blueprint depict different aspects of healthy, functional, and connected aquatic ecosystems, including landcover, barriers to fish passage, and more. This indicator measures the frequency of inundation in order to assess hydrologic function within the floodplain. The timing, extent, and duration of floodplain inundation—in other words, when, where, and how often flooding occurs, and how long it lasts—has a significant impact on the quality of fish and wildlife habitat, the diversity of aquatic species, and the supply of ecosystem services (Allen 2016). In a well-functioning floodplain system, water can move from lakes and mainstem rivers out onto the adjacent floodplain. Variation in high and low flow drives natural variation in the parts of the floodplain that are wet or dry at any given time. These fluctuations influence the availability and quality of food, spawning, and nursery habitat for aquatic species; the structure, composition, and productivity of wetlands and swamp forests; the availability of nutrients such as nitrogen; and the downstream impacts of flooding on natural and human communities (Allen 2016). Regular, but not persistent, inundation is essential for many floodplain-dependent species, including numerous fish (e.g., alligator gar), migratory waterfowl, and more. When portions of the floodplain remain constantly wet or constantly dry, this often signals a loss of hydrologic function from human alteration. Outside of lakes and mainstem rivers, which naturally experience persistent inundation, constant flooding can result from dams, dikes, levees, and stream incision due to erosion. Restricting water to certain parts of the floodplain leaves other parts constantly dry, disrupting seasonal variability in the ebb and flow of water. Input Data * [Southeast Floodplain Inundation Frequency] dataset developed by Yvonne Allen with the U.S. Fish and Wildlife Service (last updated 4-2-2025) This Southeast-wide map of floodplain inundation frequency was created following the methods described in Allen 2016, then modified to use Google Earth Engine (Gorelick et. al 2017) and Sentinel-2 data from 2017 to present instead of Landsat data from 1980s to present. Sentinel-2 offers higher resolution and higher imaging frequency. This dataset measures the relative frequency of floodplain inundation on a scale from 0-100. The inundation frequency score is an index reflecting the number of satellite scenes in which a given location appeared to be flooded, out of the total number of available images for that site. Places that are constantly flooded, such as main river channels, receive a score of 90-100, while places that never flood receive a score of 0. Intermediate scores depict varying levels of periodic to frequent flood inundation. * Estimated Floodplain Map of the Conterminous U.S. from the [Environmental Protection Agency’s (EPA) EnviroAtlas]; [see this factsheet] for more information; [download the data] (Estimated_floodplain_CONUS.zip) * [Southeast Blueprint 2025 extent] Mapping Steps * Reclassify the inundation frequency data into the following bins: * 0 = inundation index of 0 (no observed inundation) * 1 = inundation index of 91-100 (persistent inundation) * 2 = inundation index of 2-5 (occasional inundation) * 3 = inundation index of 6-20 (regular inundation) * 4 = inundation index of 21-90 (frequent inundation) * Reclassify the EPA floodplain layer to assign areas within the 100-year floodplain a value of 1, and areas outside the floodplain a value of zero. * Combine the reclassified inundation frequency score and the reclassified EPA floodplain layer. Add a value of 1 to the categorized inundation frequency value, then multiply by the reclassified floodplain raster. This assigns a value of zero to all areas outside the floodplain, while all areas within the floodplain receive their previous score + 1. * As a final step, clip to the spatial extent of Southeast Blueprint 2025. Note: For more details on the mapping steps, code used to create this layer is available [in the Southeast Blueprint 2025 Data Download] under > 6_Code. Final indicator values Indicator values are assigned as follows: 5 = Frequent inundation within the floodplain (flooded in 21-90% of days with available data) 4 = Regular inundation within the floodplain (flooded in 6-20% of days with available data) 3 = Occasional inundation within the floodplain (flooded in 2-5% of days with available data) 2 = Persistent inundation within the floodplain (flooded in 91-100% of days with available data 1 = No detected inundation within the floodplain (flooded in 0% of days with available data) 0 = Not identified as a floodplain Known Issues * When the Sentinel-2 satellite images were mosaiced together using Google Earth Engine, an error occurred where not all available images were used for all sites. This primarily affects two areas around 100 km2 in size near Georgia and Florida (e.g., near Augusta, GA). This error has been reported to Google Earth Engine and will hopefully be rectified for future updates to this indicator. Patching the missing scenes using Landsat imagery would be another option to address this in subsequent revisions. * This indicator takes an ecological view of the floodplain, which does not encompass permanent waterbodies, but rather focuses on the adjacent areas that are regularly flooded and dried. In a well-functioning hydrologic system, it is natural for some waterbodies, like mainstem river channels, to remain constantly flooded. However, persistent inundation can also signal hydrologically altered areas like impoundments. Given that other Blueprint aquatic indicators prioritize the open water areas of rivers and lakes, we erred on the side of not overprioritizing impoundments and focused this indicator on the parts of the floodplain that experience regular, but not constant, flooding. As a result, this indicator undervalues the importance of the open water of unaltered rivers and natural lakes. * The Sentinel-2 imagery only goes back to 2017, which may not be long enough to see the full range of flood conditions that may occur. Between Sentinel-2 A and B, the same location is imaged every 5 days (each of the two satellites covers the same site every 10 days). However, this is an improvement over Landsat, which flies the same location every 16 days. Combining Landsat and Sentinel-2 imagery in the future could be a future option to explore help extend the covered hydroperiod further back to the 1980’s, but Landsat does not provide the same resolution as the Sentinel-2 imagery. * In the area just south of Guadalupe Mountains National Park in West Texas, this indicator depicts the floodplain as a series of straight lines that poorly match the actual floodplain. This is due to an error in the EPA 100-year floodplain map used in this indicator. * While this indicator generally includes the open water area of reservoirs, some open water portions of reservoirs (e.g., Kerr Lake in NC/VA) are missing from the EPA estimated 100-year floodplain dataset. * Due to the extent of the source data, this indicator includes some estuaries and parts of the nearshore marine environment that are not typically considered as part of a functional floodplain system. However, these areas receive a relatively low score of 2, as they are constantly inundated, so should not overly influence the Blueprint priorities. * This indicator does not account for some types of human alteration to hydrology, including rice fields, some modifications to natural flow regimes due to dam releases, and others. Disclaimer: Comparing with Older Indicator Versions There are numerous problems with using Southeast Blueprint indicators for change analysis. Please consult Blueprint staff if you would like to do this (email [hilary_morris@fws.gov]). Literature Cited Allen, Y. “Landscape Scale Assessment of Floodplain Inundation Frequency Using Landsat Imagery,” River Research and Applications 32, no. 7 (September 1, 2016): 1609–20. [[https://doi.org/10.1002/rra.2987]]. Gorelick, Noel & Matt Hancher, Mike Dixon, Simon Ilyushchenko, David Thau, Rebecca Moore, Google Earth Engine: Planetary-scale geospatial analysis for everyone, Remote Sensing of Environment, Volume 202, 2017, Pages 18-27, ISSN 0034-4257. [[https://doi.org/10.1016/j.rse.2017.06.031]]. Opperman, J.J., Luster, R., McKenney, B.A., Roberts, M. and Meadows, A.W. (2010), Ecologically Functional Floodplains: Connectivity, Flow Regime, and Scale. JAWRA Journal of the American Water Resources Association, 46: 211-226. [[https://doi.org/10.1111/j.1752-1688.2010.00426.x]].