The National Hydrography Dataset (NHD) and Watershed Boundary Dataset (WBD)
are used to portray surface water on The National Map. The NHD represents the
drainage network with features such as rivers, streams, canals, lakes, ponds,
coastline, dams, and streamgages. The WBD represents drainage basins as enclosed
areas in eight different size categories. Both datasets represent the real world
at a nominal scale of 1:24,000-scale, which means that one inch of The National Map
data equals 2,000 feet on the ground. To maintain mapping clarity not all water features
are represented and those that are use a moderate level of detail.
The NHD and WBD are digital vector datasets used by geographic information systems (GIS).
These data are designed to be used in general mapping and in the analysis of surface water
systems. In order to make a map these data must be used by a GIS to render the data and
then print a map or make an image. The NHD is portrayed on the
US Topo map product produced
by the USGS and the NHD and WBD can be viewed on the
or the general mapping oriented
The National Map Viewer.
In mapping, the NHD and WBD are used with other data themes such as elevation, boundaries,
transportation, and structures to produce general reference maps. The NHD and WBD are often
used by scientists using GIS. GIS technologies take advantage of a rich set of attributes
imbedded in the data to generate specialized information. These analyses are possible because
the NHD contains a flow network that allows for tracing water downstream or upstream. The NHD
and WBD use an addressing system based on reach codes and linear referencing to link specific
information about the water such as water discharge rates, water quality, and fish population.
The WBD exists in six levels of a nested hierarchy permitting the analysis to determine which
drainage basin a particular location is enclosed in. This makes it possible to determine which
rivers and lakes could be affected by an event such as a toxic spill. Using basic NHD features
like flow network, linked information, and other characteristics, along with one of the six
levels of WBD areas, it is possible to study cause and effect relationships, such as how a source
of poor water quality upstream might affect a fish population downstream.