USGS Water Data for the Nation
http://waterdata.usgs.gov/nwis

USGS


This site provides access to water-resources data collected at approximately 1.5 million sites in all 50 States, the District of Columbia, Puerto Rico, the Virgin Islands, Guam, American Samoa and the Commonwealth of the Northern Mariana Islands. Online access to this data is organized around categories: Real-Time Data, Site Information, Surface water, Groundwater, Water Quality, and Mapper.

This resource is referenced here:
Subject: Geoscience:Hydrology
Resource Type: Datasets and Tools