This data release contains a crosswalk between subwatersheds (12-digit hydrologic unit codes; hereafter, HUC12s) and hydrologic regions (sometimes called "Van Metre regions"). This crosswalk allows for data at the HUC12 scale to be summarized regionally. Hydrologic regions are boundaries of hydrologically distinct areas modified from hydrologic subregions (4-digit Hydrologic units; HUC4s) defined by Qi and Mason (2023; https://doi.org/10.5066/P98194QR) for use in Van Meter et al. (2020; https://doi.org/10.1007/s10661-020-08403-1). These hydrologic regions should not be confused with 2-digit hydrologic unit codes (HUC2 or HU2), also referred to as "hydroregions" or "HydroRegions." Although they are similar in number and size, they represent different concepts: HUC2s denote drainage basins of major rivers, while the hydrologic regions defined by Van Metre et al. (2020) are areas with similar hydrology and water availability concerns that were originally developed to help inform selection of basins for more in-depth sampling, analysis, and modeling. For comparative purposes, we further grouped the hydrologic regions into four CONUS aggregated hydrologic regions based on location and shared water-availability characteristics and challenges (Northeast through Midwest, Southeast, High Plains, and Western). The HUC12 boundaries used are those made available in the Mainstems data release (https://doi.org/10.5066/P92U7ZUT), which are modified from the stable NHDPlusV2 snapshot of the Watershed Boundary Dataset.