Drainage basin delineations for selected USGS streamflow-gaging stations in Virginia (Drainage_Basin)

The Drainage_Basin polygon feature class was created as a digital representation of drainage basins for more than 1,650 continuous-record streamflow-gaging stations, partial record streamflow-gaging stations of the U.S. Geological Survey (USGS), and other watercourse locations of interest. The Drainage_Basin polygon feature class replaces the drainage basins previously derived through planimetric methods on paper maps, and provides a digital representation that previously was unavailable. The dataset will be used for the update and publication of drainage areas to all USGS stations in Virginia. This dataset represents historical flow patterns and avoids anthropogenic changes to flow when possible.

The Drainage_Basin polygon feature class has a unique format with cascading, or stacked, polygons. In a coverage format, to calculate the drainage area of a large basin containing multiple sub-basins, the area from multiple polygons would be summed. However, within the Drainage_Basin polygon feature class, one polygon is used to store the entire drainage area to each USGS station. Because geodatabases and shapefiles are able to store multiple copies of the same geometry, the Drainage_Basin polygon feature class can hold polygons that are stacked. During the digitizing and editing phases topology rules were utilized to ensure that stacked linework was identical.

Each polygon in Drainage_Basin has a value within the Station_number field. Most values represent USGS station number found in the National Water Information System (NWIS) database http://waterdata.usgs.gov/nwis. The station locations stored within NWIS represent the locations of monitoring equipment maintained by the USGS. During the delineation process, these locations were used as reference points to the downstream end of a drainage basin, but usually were not the outlet point. For this reason, the point locations are not contained in this dataset. Most are available through NWIS by searching for the station number.