USGS StreamStats

StreamStats delineates the drainage basin above any point on a US stream, computes basin characteristics, and applies the USGS regional regression equations to estimate peak-flow frequencies and other streamflow statistics — the standard first stop for ungauged-site design flows in the United States.

Output is report-ready and citable, and underlying services are accessible programmatically. Coverage and available statistics vary by state (each state's regressions were developed separately), so check the documentation for your study area. Outside the US, national equivalents differ; the regression-on-basin-characteristics pattern is universal.