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Why are there sometimes differences between USGS and National Weather Service river stages?

At some USGS streamgage installations, NOAA’s National Weather Service (NWS) maintains a separate stage sensor that is serviced by NWS technicians. Calibration of any sensing device can occasionally drift from a "true" value, so there might be differences between USGS and NWS data reports.

USGS personnel visit installations on an interval of 6 weeks or less to maintain equipment and make required adjustments. NWS technicians have a separate maintenance schedule. In either case, data retrieved from remote sensors is always considered provisional and subject to revision after quality control and analysis. Differences between remotely reported and field measured data are usually small and within tolerances required for water resources management and control.

Learn more: From the River to You: USGS Real-Time Streamflow Information