The Hydrologic Units are determined using national standards and are based solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects, nor particular program or agency. At a minimum, the hydrologic units dataset is delineated and georeferenced to the USGS 1:24,000 scale topographic base map meeting National Map Accuracy Standards (NMAS).The dataset is primarily used by government agencies as an accounting unit; by local watershed groups to define area of interest or extent of coordination; and by researchers as a unit of spatial analysis. Additional information fully documenting the history and development of the WBD can be found at: http://nhd.usgs.gov/wbd.html
The Hydrologic Units dataset doesn’t change frequently. It will change when better source data and staff resources are available to improve the precision of the data. It may also change if the outflow of a unit changes (i.e. landscape change resulting from a high water year). Currently, there is one USDA NRCS staff member (the theme lead) working about 30% time on stewardship tasks specifically for the dataset. Stewardship for this dataset involves being an advocate for data development, actually doing data editing, and occasional outreach to users.