- hydrology
- data-source
The National Water Information System is the USGS database and web service behind the nation's streamgages. It publishes discharge (flow) and stage, plus water temperature and quality where instrumented, as both a live feed and a long historical record, each gauge addressed by a site number.
NWIS is the live-flow spine of the water balance. The platform's hydrology connectors pull gauge records from NWIS to establish a reach's actual flow regime, and the long record is what a design low flow like the 7Q10 is computed from. Reading flow from a real, dated NWIS gauge — rather than estimating it — is what lets a receiving-water value be tagged [verified] and joined by HUC to the facility under study.