National Hydrography Dataset Model
The National Hydrography Dataset (NHD) is a database that interconnects and uniquely identifies the millions of stream segments or reaches that comprise the Nation’s surface water drainage system. It is based upon the USGS 1988 1:100,000-scale Digital Line Graph (DLG) hydrography dataset integrated with reach-related information from the USEPA Reach File Version 3.0-Alpha release (RF3-Alpha). The NHD provides a national framework for assigning reach addresses to water quality related entities, such as dischargers, drinking water supplies, streams effected by fish consumption advisories, wild and scenic rivers, Clean Water Act Section 305(b) and 303(d) waterbodies, Designated Uses, etc. Reach addresses establish the locations of these entities relative to one another within the NHD surface water drainage network in a manner similar to street addresses. The assignment of reach addresses is accomplished through a process known as reach indexing.
Once linked to the NHD surface water drainage network by their reach addresses, the upstream/downstream relationships of the water quality related entities and any associated information about them can be analyzed using Geographic Information Systems (GIS) software. A GIS provides a set of computerized analytical and mapping tools that operate on geographic datasets, such as the NHD, land use, soils and population, that are organized in separate layers. A GIS can also be used to combine NHD-based network analysis with these other GIS layers to help better understand and display their respective effects upon water quality.
Furthermore, since the NHD provides a nationally consistent framework for addressing and analysis, water quality information linked to reach addresses by one organization can be shared with other organizations and easily integrated into many different types of applications to the benefit of all.
The NHD is architected and maintained by the US Geological Survey . In particular their "NHDinGEO Schema" diagram is highly valuable. The EPA Reach Address Database maintains a copy of the NHD which matches the data in the RAD event program tables. This copy is periodically refreshed from the master database and event data migrated to match any changes in the NHD.