Network Complexity (Southeast Blueprint Indicator)

Reason for Selection River networks with a variety of connected stream size classes are more likely to have a wide range of available habitat to support a greater number of species. This will help retain aquatic biodiversity in a changing climate by allowing species to access climate refugia and move between habitats. Input Data * [Base Blueprint 2022 extent] * [Southeast Blueprint 2024 extent] * Southeast Aquatic Resources Partnership’s Network Complexity metric The Southeast Aquatic Resources Partnership (SARP) developed metrics for their [Southeast Aquatic Barrier Prioritization Tool]. On June 7, 2023, Brendan Ward with [Astute Spruce] (software developer working on behalf of SARP) shared high resolution NHDPlus flowlines with attributes depicting the network complexity attribute for each functional network (see definition of “functional network” below). The network complexity attribute calculates the total number of different stream size classes within each functional network. SARP assigned stream and river reaches to size classes based on total drainage area: * 1a: Headwaters (<3.861 sq mi) * 1b: Creeks (≥3.861 and <8.61 sq mi) * 2: Small Rivers (≥38.61 and <200 sq mi) * 3a: Medium Tributary Rivers (≥200 and <1,000 sq mi) * 3b: Medium Mainstem Rivers (≥1,000 and <3,861 sq mi) * 4: Large Rivers (≥3,861 and <9,653 sq mi) * 5: Great Rivers (≥9,653 sq mi) Functional Network SARP compiles the Southeast Aquatic Barrier Inventory from national, regional, state, and local partner databases across the Southeast region. These include the National Inventory of Dams (2018), National Anthropogenic Barrier Dataset (2012), databases from state dam safety programs and other state agencies, information from local partners, and dam locations estimated by SARP. Waterfalls are compiled from national datasets and local partners. Dams and waterfalls are snapped to hydrologic networks extracted from the National Hydrography Dataset (NHD) - High Resolution Beta version. All dams and waterfalls are treated as “hard” barriers for network connectivity analysis. Aquatic networks are cut at the location of each barrier. All network “loops” (non-primary flowlines) are omitted from the analysis. An upstream functional network is constructed by traversing upstream from each barrier through all tributaries to the upstream-most origination point or upstream barrier, whichever comes first. Additional functional networks are defined from downstream-most non-barrier termination points, such as marine areas or other downstream termination points. The total length of all network segments within a functional network is summed to calculate the total network length of each functional network. Each flowline segment within the NHD is assigned to a size class based on total drainage area. This was used to calculate the number of unique size classes per functional network. * 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] 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). * [National Hydrography Dataset Plus High Resolution] (NHDPlus HR) National Release catchments, accessed 11-30-2022; [download the data]; [view the user guide] * [NHDPlus Version 2.1 medium resolution catchments ](note: V2.1 is just the current sub-version of the dataset generally called NHDPlusV2); [view the user guide] 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. To learn more about catchments and how they’re defined, check out these resources: * [An article from USGS explaining the differences between various NHD products] * The glossary at the bottom of [this tutorial for an EPA water resources viewer], which defines some key terms Mapping Steps * Merge the functional network lines from the 11 subregions delivered by SARP into one feature class. * Convert the combined SARP network complexity values from the high resolution NHDPlus flowlines to a 30 m raster. * Clip to the Base Blueprint 2022 extent. * Apply the network complexity values to the NHDPlus HR catchments using the ArcPy Zonal Statistics “MAJORITY” function. This results in a raster where each catchment is assigned the majority network complexity value that intersects the catchment. Most catchments have only one intersecting line, but for catchments with interior dams, the analysis uses the majority network complexity value. * To define the analysis extent of the indicator, make a copy of the NHDPlus HR catchments and convert it to raster, assigning it a value of 1. * Clip the network complexity raster to the EPA floodplain layer. During this step, assign a value of 0 to areas outside the EPA floodplain. Zero values are intended to help users better understand the extent of this indicator and make it perform better in online tools. * Some areas of the floodplain are not scored in the resulting layer because they are missing SARP network complexity values. This is due to the fact that some small reaches, such as braids and loops in the stream network, are not assigned a network complexity value. SARP has to remove loops and braided streams in order to calculate network complexity because the analysis can only accommodate a one-way flow of water. Identify these holes in the floodplain and fill them in by looking at the network complexity value of the surrounding pixels and assigning the maximum value to the missing catchments in the floodplain. Note: This simplifies a complex series of analysis steps. For more specifics, please consult the code. * Clip the network complexity raster to the NHDPlus V2.1 medium resolution catchments. This removes estuarine areas that are outside the intended scope of this indicator, particularly on the NC coast. * As a final step, clip to the spatial extent of Southeast Blueprint 2024. Note: For more details on the mapping steps, code used to create this layer is available in the [Southeast Blueprint Data Download] under > 6_Code. Final indicator values Indicator values are assigned as follows: 7 = 7 connected stream classes 6 = 6 connected stream classes 5 = 5 connected stream classes 4 = 4 connected stream classes 3 = 3 connected stream classes 2 = 2 connected stream classes 1 = 1 connected stream class 0 = Not identified as a floodplain Known Issues * This indicator does not include other smaller scale attributes of complexity (e.g., sinuosity, mixtures of riffles/pools/runs) that influence the habitat quality of the connections. * The EPA Estimated Floodplain layer sometimes misses the small, linear connections made by artificial canals, especially when they go through areas that wouldn’t naturally be part of the floodplain. As a result, some areas (like lakes) that are connected via canals may appear to be disconnected, but still receive high scores. * Small headwaters and creeks are not included in this indicator because the EPA estimated floodplain dataset does not include them. * 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. * This indicator likely overestimates the number of connected stream classes in some areas due to missing barriers in the inventory, such as smaller dams or road-stream crossings. It could also underestimate the number of connected stream classes, given the extensive ongoing restoration work to improve aquatic connectivity across the SECAS geography. If you identify a missing barrier or a removed barrier, please let SARP know by emailing Kat Hoenke at [kat@southeastaquatics.net]. You can learn more about the current inventory of dams and road-stream crossings by visiting [https://connectivity.sarpdata.com/]. * SARP did a lot of work to snap the dam locations to the line network, but there are likely still dams (including some large ones) that didn’t get snapped correctly due to the large distance between the centerpoint of the dam and the nearest flowline. If you see any of these cases when reviewing the data, please let SARP know (the giveaway is networks that look longer than they should on a map). * 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 floodplain map used in this indicator. * Due to issues with the national NHDPlus HR catchments layer, there are a handful of missing catchments (e.g., northwest TX, coastal LA, and eastern NC). These places receive a value of NoData in the indicator and are therefore underprioritized. We are investigating ways to resolve this in future updates. * This indicator may slightly overvalue network complexity in WV compared to other Southeast states because the coverage of dams and barriers data is not as comprehensive. While the dams and barriers data coverage improved sufficiently for us to use the network complexity indicator across the entire state of WV in 2023 for the first time (whereas it was only used in the southern part of the state in 2022), there is still room for improvement and we anticipate significant progress in the next Blueprint update. 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]]. Martin, E. H, Hoenke, K., Granstaff, E., Barnett, A., Kauffman, J., Robinson, S. and Apse, C.D. 2014. SEACAP: Southeast Aquatic Connectivity Assessment Project: Assessing the ecological impact of dams on Southeastern rivers. The Nature Conservancy, Eastern Division Conservation Science, Southeast Aquatic Resources Partnership. [[https://secassoutheast.org/pdf/SEACAP_Report.pdf]]. Southeast Aquatic Barrier Prioritization Tool. Southeast Aquatic Resources Partnership (SARP). [[https://connectivity.sarpdata.com]/].