NHD Initial Release and Flow Validation

The initial release of the NHD is an excellent source for creating map products, assigning reach events and, to a lesser extent, supporting network analyses since it contains known problems with flow (network) relationships. Editing of these relationships, performed centrally by the USGS, will continue subsequent to the initial release. This flow validation process will focus initially on the portions of the drainage network that interconnect the hydrologic subbasins across the country. The collection of rivers that flow between and, thus, link together subbasins form the backbone of the country's surface water drainage network. Once network errors along this backbone are identified and corrected, editing will continue on flow problems detected in other significant parts of the network as time permits. These updates, which will be described in the metadata (under process description) as "flow validation has been performed", may include steps such as minor re-delineation of features, re-ordering of coordinates and associated measures, and migration of reaches between adjacent subbasins; however, these steps will not be described individually in the metadata. The metadata can be queried to identify and retrieve the desired set of updated subbasins.

The NHD has been designed to accommodate the incorporation of user-supplied updates as well. Users submitting updates must also provide metadata documenting their changes to the data. The status and plans for accepting user-supplied updates is discussed under NHD Data Maintenance.