![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
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. All NHD data held within the RAD is stored twice as both ESRI
SDE.ST_GEOMETRY
and Oracle Spatial SDO_GEOMETRY geometry storage types.
Supplemental NHD Documentation