Natural Landcover in Floodplains

This indicator measures the amount of natural landcover in the estimated floodplain of rivers and streams within each catchment. It assesses the stream channel and its surrounding riparian buffer, measuring the percent of unaltered habitat like forests, wetlands, or open water (rather than agriculture or development). This indicator originates from the 2019 National Land Cover Database and applies to the Environmental Protection Agency’s estimated floodplain, which spatially defines areas estimated to be inundated by a 100-year flood, also known as the 1% annual chance flood.

Reason for Selection

Habitat near rivers and streams is strongly linked to water quality and instream flow (Naiman 1997), is easy to monitor and model, and is widely used and understood by diverse partners. Intact vegetated buffers within the floodplain of rivers and streams provide aquatic habitat, improve water quality, reduce erosion and flooding, recharge groundwater, and more (WeConservePA 2014). Natural floodplain landcover provides a “front line defense” for aquatic systems.

Input Data

• Base Blueprint 2022 extent

• Estimated Floodplain Map of the Conterminous U.S. from the EPA EnviroAtlas (see this factsheet for more information; download the data)

  The EPA Estimated Floodplain Map of the Conterminous U.S. displays “…areas estimated to be inundated by a 100-year flood, also known as the 1% annual chance flood. These data are based on the Federal Emergency Management Agency (FEMA) 100-year flood inundation maps with the goal of creating a seamless floodplain map at 30-m resolution for the conterminous United States. This map identifies a given pixel’s membership in the 100-year floodplain and completes areas that FEMA has not yet mapped” (EPA 2018).

• 2019 National Land Cover Database (NLCD)

• NHDPlus Version 2.1 medium resolution catchments (V2.1 is a sub-version of the dataset generally known as NHDPlus V2; view the user guide for more information)

  NHDPlus V2.1 Medium Resolution Catchments

  A catchment is the local drainage area of a specific stream segment based on the surrounding elevation. Catchments are defined based on surface water features, watershed boundaries, and elevation data. It can be difficult to conceptualize the size of a catchment because they vary significantly in size based on the length of a particular stream segment and its surrounding topography—as well as the level of detail used to map those characteristics.

  More specifically, the NHDPlus V2.1 medium resolution catchment dataset used in this indicator incorporates snapshots of a) surface water features from the medium-resolution (1:100K scale) National Hydrography Dataset b) watershed boundaries from the Watershed Boundary Dataset, and c) the National Elevation Dataset 30 m digital elevation model.

  To learn more about catchments and how they’re defined, check out these resources:

• Basic information on NHDPlus from the EPA
• An article from USGS explaining the differences between various NHD products
• The NHDPlus V2.1 User Guide, which explains in detail how the dataset was produced
• The glossary at the bottom of this tutorial for an EPA water resources viewer, which defines some key terms

Mapping Steps

• Clip the 2019 NLCD to the EPA estimated floodplain layer.
• Reclassify the clipped 2019 NLCD to identify natural landcover using the following classes: open water, barren land, deciduous forest, evergreen forest, mixed forest, scrub/shrub, grassland/herbaceous, woody wetlands, and emergent wetlands.
• Calculate the percent of riparian natural landcover inside each NHDPlus catchment using the ArcPy Spatial Analyst Zonal Statistics function.
• Clip the resulting raster back to the EPA estimated floodplain layer (we have to do this again since the zonal statistics function outputs pixel values for the entire catchment).
• Reclassify the above raster into 5 classes, seen in the final indicator values below.
• As a final step, clip to the spatial extent of Base Blueprint 2022.

Note: For more details on the mapping steps, code used to create this layer is available in the Southeast Blueprint 2022 Data Download under BlueprintInputs > BaseBlueprint2022 > 6_Code.

Final Indicator Values

Indicator values are assigned as follows:

• 5 = >90% natural landcover within the estimated floodplain, by catchment
• 4 = >80-90%
• 3 = >70-80%
• 2 = >60-70%
• 1 = ≤60% natural landcover within the estimated floodplain, by catchment

Known Issues

• Small headwaters and creeks are not included in this indicator because the EPA estimated floodplain dataset does not include them.
• This indicator does not account for the accumulated impacts of upstream riparian buffers. Buffers at the headwaters are treated the same as those downstream.
• This indicator does not take into account the river or stream size in relation to the estimated floodplain. Aquatic habitat needs may differ based on the river size class. For example, smaller headwater streams may need more natural landcover than larger rivers to maintain aquatic health. It also does not account for variation in buffer quality within the floodplain at a scale below the catchment. This means that within the estimated floodplain, loss of natural habitat adjacent to the river is treated the same as loss farther away.
• 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 estimated floodplain dataset.
• The catchment boundaries are inconsistent in how far they extend toward the ocean. As a result, this indicator does not consistently apply to estuaries, coastal areas, and barrier islands.
• In the area just south of Guadalupe Mountains National Park in West Texas, this indicator depicts the floodplain as a series of linear lines that poorly match the actual floodplain. This is due to an error in the EPA floodplain map used in this indicator.
• The catchment boundaries cross the United States/Mexico border, but the NLCD impervious data does not; as a result, the values along the United States/Mexico border are only based on the portion of the catchment where there are NLCD impervious values.

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

EPA EnviroAtlas. 2018. Estimated Floodplain Map of the Conterminous U.S. [https://enviroatlas.epa.gov/enviroatlas/DataFactSheets/pdf/Supplemental/EstimatedFloodplains.pdf].

Naiman, Robert J., and Henri Decamps. “The Ecology of Interfaces: Riparian Zones.” Annual Review of Ecology and Systematics 28 (1997): 621–58. [http://www.nativefishlab.net/library/textpdf/19487.pdf].

U.S. Environmental Protection Agency (USEPA) and the U.S. Geological Survey (USGS). 2012. National Hydrography Dataset Plus 2. [http://www.horizon-systems.com/nhdplus/].

U.S. Geological Survey (USGS). Published June 2021. National Land Cover Database (NLCD) 2019 Land Cover Conterminous United States. Sioux Falls, SD. [https://doi.org/10.5066/P9KZCM54].

WeConservePA. 2014. ConservationTools.org: The Science Behind the Need for Riparian Buffer Protection. [https://conservationtools.org/guides/131-the-science-behind-the-need-for-riparian-buffer-protection].

Yang, L., Jin, S., Danielson, P., Homer, C., Gass, L., Case, A., Costello, C., Dewitz, J., Fry, J., Funk, M., Grannemann, B., Rigge, M. and G. Xian. 2018. A New Generation of the United States National Land Cover Database: Requirements, Research Priorities, Design, and Implementation Strategies, ISPRS Journal of Photogrammetry and Remote Sensing, 146, pp.108-123. [https://doi.org/10.1016/j.isprsjprs.2018.09.006].