- hydrology
- data-source
The United States Geological Survey is the federal earth-science agency. For this record its role is water and terrain: it runs the streamgage network served through NWIS, defines the watershed addressing of the hydrologic unit code, and publishes the elevation and geologic layers that underpin drainage and siting work.
USGS data is authoritative reference in the platform's evidence grammar — a gauge reading or a mapped watershed boundary is a [reference] fact from a neutral federal source, not an agency's contested position. That neutrality is why a design low-flow statistic like the 7Q10 — the one that applies varies with the pollutant, receiving water, jurisdiction, and permit method — is anchored to the USGS record: it lets the water-quality math start from ground the parties don't dispute.