USGS National Water Information System (NWIS) & Water Data for the Nation

Free public access to 135+ years of U.S. surface water, groundwater, and water quality data from over 1.5 million monitoring sites. Real-time stream flow, gauge heights, water chemistry, and downloadable datasets via web and API.

The USGS National Water Information System (NWIS) is the federal government's authoritative archive of U.S. water data, freely available to the public with no registration required. The modernized Water Data for the Nation portal at waterdata.usgs.gov provides web access to real-time and historical data from over 1.5 million monitoring sites — stream gauges, groundwater wells, water quality stations, springs, and atmospheric monitors.For watershed groups, the most-used services are real-time stream flow and gauge height (critical for flood awareness and recreation safety), historical discharge records (essential for any restoration design or hydraulic modeling), and the Water Quality Portal (a unified search across USGS, EPA, and state monitoring data). All data is downloadable in machine-readable formats and accessible through public APIs, which means it can drop into a website widget, an alert system, or a GIS layer with no licensing complications.For the Allegheny, Monongahela, Youghiogheny, and broader Ohio River basins, USGS maintains dozens of active gauges with multi-decade records. These records are the backbone of any defensible TMDL, restoration design, or impact claim.