Here is a great place to start.

The Geographic Names Information System (GNIS) is a data system developed by the U. S. Geological Survey (USGS) in cooperation with the U. S. Board on Geographic Names. The National Geographic Names Data Base is the largest of the GNIS databases, containing records on almost two million geographic feature names in the United States: from populated places, schools, reservoirs, and parks, to streams, valleys, springs, and ridges.

http://research.yale.edu/peabody/COLLECTIONS/gnis/