How to read a river gauge
What a streamgauge actually measures
A US Geological Survey streamgauge continuously records the height of the water surface, then converts that to a flow volume using a rating curve developed for that site. So you get two numbers:
- Gauge height (stage), in feet — the water surface relative to a fixed local reference point. This is what flood stages are based on.
- Discharge (streamflow), in cubic feet per second — the volume of water passing the gauge each second. Better for judging whether a river is running high or low for the season.
Why gauge height is not water depth
This trips up almost everyone. The reference point — the datum — was chosen when the gauge was installed and is essentially arbitrary. It is not the riverbed and not sea level. That means two things:
First, you cannot compare gauge heights between rivers. 12 ft on one gauge may be a normal summer level while 12 ft on another is a serious flood. Always compare a reading to its own flood stage.
Second, a gauge can legitimately read a negative number. If the water surface falls below the datum, the value goes below zero. The river is low, not empty.
Comparing a reading to flood stage
Once you have the current height, the question is simply how far it sits from the thresholds set for that gauge. Below action stage, the river is normal. Between action and flood stage, it is worth watching. At or above flood stage, flooding is happening somewhere near the gauge — and the minor, moderate, and major categories describe how severe. Full detail in what does flood stage mean.
How current is the reading?
Most USGS gauges transmit every 15 to 60 minutes depending on the site; some report less often, and any gauge can go quiet during equipment or telemetry problems. IsItFlooding polls every 15 minutes, but a gauge is only as fresh as its last transmission — so every reading on this site is labelled with how long ago it was measured. If a gauge says “3 days ago,” that sensor is not reporting.
Real-time values are provisional. They have not been reviewed by the USGS and can be thrown off by ice, debris, backwater from a downstream obstruction, or a failing sensor. They are revised later after quality control.
For paddlers, anglers, and rafters
If you are reading gauges for recreation rather than flood risk, discharge in cfs is usually the more useful number — most run descriptions and fishing reports are written in terms of flow. The harder question is whether today’s flow is high or low for this time of year, which a raw cfs value cannot tell you. Our streamflow conditions map answers that directly, comparing each gauge with about ten years of its own record for the same week of the year.
Frequently asked questions
How do you read a river gauge?
Read the gauge height in feet and compare it to that gauge's published flood stage. The height alone means nothing on its own, because each gauge measures from its own local reference point. If the reading is below flood stage the river is not flooding at that location.
Why can a river gauge read a negative number?
Gauge height is measured from an arbitrary datum set when the gauge was installed, not from the riverbed or sea level. If the water surface drops below that reference point the reading goes negative. It does not mean the river is empty.
What is the difference between gauge height and discharge?
Gauge height is the water surface elevation in feet and is what flood stages are based on. Discharge is the volume of water flowing past the gauge in cubic feet per second. Discharge is the better indicator of whether flow is high or low for the season.
How often do USGS river gauges update?
Most USGS streamgauges transmit every 15 to 60 minutes depending on the site, and some report less often or go offline during outages. IsItFlooding polls every 15 minutes and shows the time of each gauge's most recent reading.
Are USGS gauge readings accurate?
Real-time readings are provisional and have not been reviewed by the USGS. They can be affected by ice, debris, backwater, equipment faults, or telemetry outages, and are subject to revision. Treat them as indicative, not authoritative.
Read a gauge now
Find gauges near you, open the national flood map, or see how flows compare to normal.
Related: what CFS means · what does flood stage mean · why is the river rising