How to Read a USGS Stream Gauge

Every USGS stream gauge reports two fundamental measurements: gage height (also called stage) and discharge. Understanding the difference — and how they relate — is the key to interpreting river conditions.

Gage Height vs. Discharge

Gage height is measured in feet above a local datum — a fixed reference point at the gauge site. Think of it like a ruler stuck in the riverbed: the gauge reads how high the water surface sits on that ruler. When gage height is at 5.5 ft, it doesn't mean the river is 5.5 feet deep. It means the water surface is 5.5 feet above the gauge's reference mark.

Discharge is the actual flow — the volume of water moving past the gauge per second, measured in cubic feet per second (CFS). This is computed from the gage height using a site-specific rating curve, a mathematical relationship built from actual field measurements.

The Rating Curve

USGS hydrologists periodically visit each gauge and measure the actual discharge using acoustic Doppler instruments or traditional current meters. By measuring discharge at many different water levels, they build a rating curve that converts any gage height into a discharge estimate. This is why two gauges with the same gage height can have very different discharge — the channel geometry is different at each site.

Reading the Numbers

When you visit a station page on StreamFlowData.com, you'll see both numbers. For flood warnings, gage height matters most — flood stage thresholds are set in feet of gage height. For fishing, paddling, or hydrology, discharge in CFS is more meaningful because it's comparable across rivers.

The percentile rank tells you whether the current discharge is normal for this time of year. A reading at the 50th percentile is perfectly median; below the 10th percentile is Very Low; above the 90th is Very High.