USGS NWIS — Streamflow Instantaneous Values

Sub-hourly streamflow + stage + water quality observations from ~10,000 USGS gauges across the US, plus historical archives back to 1900s. Authoritative ground-truth source for flood forecast validation and parametric flood trigger derivation.

Provider	USGS Water Mission Area
Delivery mode	stream
CNR score	82
Cadence	PT15M
Origin	https://waterdata.usgs.gov/nwis/rt