Caribbean 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 (Morelli et al. 2016). Input Data * Southeast Aquatic Resources Partnership’s Network Complexity metric The Southeast Aquatic Resources Partnership (SARP) developed metrics for their [Southeast Aquatic Barrier Prioritization Tool]. In February 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. * [Federal Emergency Management Agency (FEMA) National Flood Hazard Layer] flood zones for Puerto Rico and the U.S. Virgin Islands, accessed 10-22-2022; to download the data, visit the [FEMA Flood Map Service Center], search by jurisdiction (Puerto Rico or Virgin Islands), download all FIRM (Flood Insurance Rate Maps) panels, and locate the “S_FLD_HAZ_AR” shapefile in each download package. We used the “FLD_ZONE” attribute of the S_FLD_HAZ_AR shapefile to define an estimated floodplain depicting areas predicted to be inundated by a 100-year flood (also known as the 1% annual chance flood). To create the estimated floodplain for Puerto Rico and the U.S. Virgin Islands, we combined all areas with flood zone codes beginning with the letter “A”. These zones represent the inland (non-coastal) portions of FEMA Special Flood Hazard Areas considered at high risk of flooding. This excludes coastal areas where the high risk of flooding stems from storm waves, areas of moderate-low flood risk, and areas with possible but undetermined flood hazards where no hazard analysis has been conducted. For more details on FEMA flood zones, [read this FEMA blog] or [visit the FEMA glossary] (detailed definitions are under “Z” for “zones”). * [National Hydrography Dataset Plus High Resolution] (NHDPlus HR) National Release catchments and flowlines, accessed 11-30-2022; [download the data] 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 * [Southeast Blueprint 2023 subregions]: Caribbean * [Southeast Blueprint 2023 extent] Mapping Steps * Convert the SARP network complexity values from the NHDPlus HR flowlines to a 30 m raster. * The original NHDPlus HR catchment data was missing coverage of a small area on the west coast of Puerto Rico (just east of Parcelas Aguas Claras). Create an additional catchment polygon for this missing area so that the indicator covers the entire island of Puerto Rico. * The missing area is essentially outlined by extremely thin catchment polygons. To fill the gap, make a new rectangular feature class covering the missing area, then union it together with the original NHDPlus HR catchments. From that output, select the newly created polygon that fills in the hole. * The resulting polygon is a multipart feature, so use the explode tool to separate out just the missing catchment. Export it as a shapefile. * Union together the missing catchment with the other NHDPlus HR catchments and use that combined output as the catchment layer for the rest of the mapping steps. * Apply the network complexity values to the NHDPlus HR catchments using the ArcPy Zonal Statistic “MAJORITY” function. This results in a raster where each catchment is given 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. This creates a raster with network complexity value assigned to catchments. * Convert the FEMA floodplain polygons to a 30 m raster, giving floodplain areas a value of 1. * Extract the stream and river lines from the NHDPlus HR flowlines (ftype IN (460, 558)). Convert extracted stream and river lines to a 30 m raster. Use the ArcPy Spatial Analysis Expand function to “buffer” the streams by 1 cell. This is the method that SARP uses to create a total stream width of approximately 90 m. * Combine the FEMA floodplains and buffered flowlines using the Mosaic function to make an enhanced floodplain layer. * Clip the raster with network complexity values assigned to catchments to the enhanced floodplain layer. This limits the indicator values to the floodplain areas, where they are most relevant. * 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 explanation simplifies a complex series of analysis steps. For more specifics, please consult the code. * Assign zero values to all areas that are covered by the NHDPlus HR catchments, but that are outside the enhanced floodplain layer. Zero values are intended to help users better understand the extent of this indicator and make it perform better in online tools. * Clip to the Caribbean Blueprint 2023 subregion. * As a final step, clip to the spatial extent of Southeast Blueprint 2023. Note: For more details on the mapping steps, code used to create this layer is available [in the Southeast Blueprint 2025 Data Download] or [Caribbean-only Southeast Blueprint 2025 Data Download] under > 6_Code. Final indicator values Indicator values are assigned as follows: 4 = 4 connected stream classes 3 = 3 connected stream classes 2 = 2 connected stream classes 1 = 1 connected stream class 0 = Not identified as 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. * 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). * The NHDPlus flowlines in the headwaters could represent intermittent or ephemeral streams. They were not excluded, so the indicator could be overprioritizing headwater areas relative to second-, third-, or fourth-order streams. * The National Hydrography Dataset digitizes surface water streams that are not always connected to downstream stream segments. We assume that in these cases, the surface water streams sink into karst areas (e.g., in northwest Puerto Rico around Quebrada). We still included these stream segments in this indicator because they could provide freshwater water habitat for aquatic species. * The FEMA 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. * The National Hydrography Dataset digitizes some surface water streams that are not always connected to downstream stream segments. We assume that some of these areas have surface water streams but sink into karst areas. An example of this is in the northwest portion of Puerto Rico around Quebrada. We still included these stream segments in this indicator because they could provide freshwater water habitat for aquatic species. * NHDPlus HR contains multiple catchments that are very small. The reduced size of these catchments may result in exaggerating their values in the indicator. Other Things to Keep in Mind * We considered using different indicator values that would account for varying island sizes and the natural complexity of freshwater systems on each island. By using consistent indicator values across all islands, regardless of their maximum aquatic connectivity potential, this indicator essentially assigns lower scores to smaller islands, despite the fact that they will never reach the same level of network complexity as larger islands. Ultimately, we decided to use the same classes across all islands because more complex and connected aquatic systems on larger islands do provide more climate refugia and available habitat to aquatic species, and therefore should have higher scores. * Headwater streams are important to freshwater systems but are not always well-captured in the FEMA floodplain dataset. To better represent headwater streams in this indicator, we chose to buffer flowlines and include them as additional floodplain areas to give those areas the opportunity to be included in the Blueprint. * You may notice stream sections in this indicator that appear to be unconnected to other stream networks. These occur because the National Hydrography Dataset digitizes surface water streams that are not always connected to downstream stream segments. We assume that in these cases, the surface water streams sink into karst areas (e.g., in northwest Puerto Rico around Quebrada). We still included these stream segments in this indicator because they could provide freshwater water habitat for aquatic species. 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 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]]. Moore, R.B., McKay, L.D., Rea, A.H., Bondelid, T.R., Price, C.V., Dewald, T.G., and Johnston, C.M., 2019, User’s guide for the national hydrography dataset plus (NHDPlus) high resolution: U.S. Geological Survey Open-File Report 2019– 1096, 66 p. [[https://doi.org/10.3133/ofr20191096]]. Morelli TL, Daly C, Dobrowski SZ, Dulen DM, Ebersole JL, Jackson ST, et al. (2016) Managing Climate Change Refugia for Climate Adaptation. PLoS ONE 11(8): e0159909. [[https://doi.org/10.1371/journal.pone.0159909]]. Southeast Aquatic Barrier Prioritization Tool. Southeast Aquatic Resources Partnership (SARP). [[https://connectivity.sarpdata.com]]. U.S. Geological Survey (USGS). National Hydrography Dataset Plus High Resolution (NHDPlus HR). Accessed November 30, 2022. [[https://www.usgs.gov/national-hydrography/nhdplus-high-resolution]].