Contains all primary and secondary matches between 1:20K and 1:50K waterbody polygons. This attribute table links the new Freshwater Atlas watershed codes with the existing 1:50K Watershed Atlas watershed codes for cross reference purposes

Lookup table for watershed type codes

Links the edge type codes used in the stream network layer with an English description used to categorize stream network features

All principal and non-principal watershed boundary edges. These are the linear features that makeup the watershed polygons

All fundamental watershed polygons generated from watershed boundary lines, bank edges, delimiter edges, coastline edges, and administrative boundary edges

Links the waterbody type codes used in the waterbody layers with an English description used to categorize waterbody features

All named watershed polygons

This dataset is the digital hydrologic unit boundary layer for the 8-digit subwatershed boundaries for the conterminous United States. This dataset is intended to be used with the following two tabular dBase files: https://water.usgs.gov/lookup/getspatial?ds573_tillage_lu92e and https://water.usgs.gov/lookup/getspatial?ds573_tillage_lu01. The two tabular datasets contain the Tillage Practices in the Conterminous United States, 1989-2004---Datasets Aggregated by Watershed. This dataset and the two tabular datasets can be linked using the common attribute HUC8_N. Information about how the tabular data and geospatial data can be related are given in the data series report: https://pubs.usgs.gov/ds/ds573/ .The original dataset is the 12-digit Subwatershed boundaries (WBD_archive_17nov2009_9.2_file). The 12-digit boundaries were dissolved to 8-digit boundaries to be used with the two tabular .dbase data files containing the tillage practice data for the United States. ORIGINAL METADATA: This data set is a complete digital hydrologic unit boundary layer to the Subwatershed (12-digit) 6th level for the entire United States. This data set consists of geo-referenced digital data and associated attributes created in accordance with the "Federal Guidelines, Requirements, and Procedures for the National Watershed Boundary Dataset; Chapter 3 of Section A, Federal Standards, Book 11, Collection and Delineation of Spatial Data; Techniques and Methods 11-A3" (04/01/2009). http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/index.html . Polygons are attributed with hydrologic unit codes for 4th level sub-basins, 5th level watersheds, 6th level subwatersheds, name, size, downstream hydrologic unit, type of watershed, non-contributing areas and flow modification.

Assessment Watersheds are mesoscale aquatic units designed to replace the 3rd order 1:50K watersheds. Assessment Watersheds are based on groupings of fundamental watersheds using FWA watershed code and local code, with a target size of between 2,000ha and 10,000ha.

A crosswalk table between NHDPlus version 2.1 flowlines (using the unique field COMID) and the Watershed Boundary Dataset (WBD) 12-digit-hydrologic units (HU-12) is provided for the 48 contiguous United States. The crosswalk table provides a WBD HU-12 assignment for every networked flowline in the NHDPlus. In this way, the network developed for navigation and modeling, NHDPlus, is aligned with accounting units of the WBD HU-12s to the extent possible given the assumptions that were made in creating each. A crosswalk table for NHDPlus isolated sinks was produced by a simple overlay process where the sinks were assigned HU-12 values based on their position relative to the WBD snapshot HU-12s. This table was integrated with the flowline associations into one crosswalk table for both feature types. There is good alignment between aggregated NHDPlus catchments and WBD HU-12 units in many locations. These are areas where the flows and chemical or nutrient loads that are accumulated leave the HU-12 at a single outlet and are passed down to the next HU-12 downstream (or to the coast) as is assumed in the WBD model. A second pass through the data was made to account for any secondary outlet from each HU-12. The location of a secondary outlet with a significant drainage from that HU was identified in the alignment with NHDPlus. This resulted in a reduction in areas of mismatch as well as an improved crosswalk table. The crosswalk table should be used with caution in areas where efforts could be made to improve the data. For example, with medium-resolution NHD, a better definition of flow direction of the flowlines in Southern Florida would result in more flowlines being included in the “networked” flowlines with catchments, and that, in turn, would result in better alignment between NHDPlus catchments and HU-12s. Correcting errors in either the medium-resolution NHD or WBD would improve the alignment between the two datasets.