The NHD Viewer is an Instance of The National Map (TNM) Viewer. The NHD Viewer is meant for viewing and downloading hydrography data. There are several basemap options in the viewer including an NHD Basemap, TNM (The National Map), Imagery, and Blank. The NHD Basemap opens to the view of the Watershed Boundary Dataset. As you zoom to larger scales more NHD layers become visible. Other TNM data themes are also accesible through the NHD Viewer. To Learn More about the NHD Viewer, it's basemaps, and navigating the Viewer, Please see the Interactive Tutorial called "Navigating the NHD Viewer".