Answer:
The National Hydrography Dataset (NHD) is the culmination
of recent cooperative efforts of the U.S. Environmental
Protection Agency (EPA) and U.S. Geological Survey (USGS).
It combines the best of the EPA Reach File Version 3.0
(RF3) and USGS Digital Line Graph (DLG) hydrography files:
hydrologic ordering, hydrologic navigation for modeling
applications, and a unique identifier (reach code) for
surface water features from RF3; and the spatial accuracy
and comprehensiveness of DLG hydrography.
