Overview of NHD and Reach Files
The National Hydrography Dataset is a comprehensive coverage of hydrographic data for the United States. While NHD itself can be used for some key analyses, most of the real power comes when other data is connected (reach indexed) to it. NHD should be, for most purposes, thought of as the foundation and common "language" for surface water analysis and display.
NHD is the next generation reach file developed through a cooperative effort between USGS and EPA. It was developed by combining EPA's Reach File version 3 (RF3) and the USGS Digital Line Graph (DLG) coverages. This allows NHD to have the standardized feature types and spatial accuracy of the DLG, with unique reach identifiers, hydrologic ordering and navigation which it draws from RF3. The reach codes provide a common nomenclature for Federal and State reporting of surface water conditions as required under the Clean Water Act. In addition, the hydrologic transport network defined in NHD enables the modeling of waterborne pollution associated with both point and non-point sources.
NHD is currently based on 1:100,000 scale data, but is designed so that it can incorporate higher resolution data. NHD is also designed so that improvements and corrections to the dataset by individual users can be incorporated into the national dataset.
For more information about NHD, or to download the NHD coverages, visit the NHD website at http://nhd.usgs.gov/.