Information on the NHD is available at the USGS Hydrography website.
The NHD User Guide, which includes interactive tutorials, feature catalog, and a video library, is located at the USGS Hydrography website.
Information on the hydrography data product and model is available at the USGS The National Map Hydrography website.
Review the Download user guide at Download and Cart.
Data is currently available in Esri File Geodatabase format (9.3.1 or 10.1) or Shapefile.
Yes, but you must specify 'Dynamic Extract' in the Hydrography product list of the USGS Available Data window. Otherwise, pre-staged NHD Subregion data are available only in Personal or File Geodatabase formats.
Yes, recent download enhancements now include Subbasin (fourth level, 8-digit hydrologic units), as USGS continues to improve packaging of NHD data for eventual distribution from the 'cloud'.
NWI data is available in the Content sub-tab under Overlays in a panel called Other Featured Data in the table of contents of The National Map viewer. NWI data layers are provided by the U.S.
Flow tables are not included in either staged State or Subregion NHD data because very large datasets may exceed an internal two GB processing limit.