From the NWIS Mapper help page: The National Water Information System (NWIS) provides access to data at over 1.5 million sites. There are 56 possible site types used in the NWIS data base, including 14 primary types and 42 secondary types. To simplify mapping of the site locations for most practical applications, the site types are displayed in 5 groups that include the following primary and secondary types. The full list of site types can be found here.
Summary
From the NWIS Mapper help page:
The National Water Information System (NWIS) provides access to data at over 1.5 million sites. There are 56 possible site types used in the NWIS data base, including 14 primary types and 42 secondary types. To simplify mapping of the site locations for most practical applications, the site types are displayed in 5 groups that include the following primary and secondary types. The full list of site types can be found here.