Rivers & gauges

Gauge Height vs Streamflow: What's the Difference?

By the Water Level Atlas team · Last reviewed Oct 21, 2026

Gauge height (also called stage) is how high the water surface is above a fixed reference point at a gauge, measured in feet. Streamflow (also called discharge) is how much water is moving past that point each second, measured in cubic feet per second (cfs). USGS measures gauge height continuously and calculates streamflow from it using a site-specific "rating curve".

What is gauge height?

USGS defines gage height as the distance of the water surface above the gage datum. The gage datum is a fixed reference level chosen for each site. USGS also uses the names stage, river height, river level, stream height and water height for the same thing.

Gauge height is the number USGS reports continuously, because it is much easier to measure all the time than flow. It is also the default value shown on many USGS gauge pages, because people find it easy to picture: "the river is up 2 feet".

Is gauge height the same as water depth?

No. This is the most common misunderstanding.

The datum is not the river bed. USGS deliberately sets each gauge's datum below the base of the streambed, because stream beds erode and fill in over time and a reference point must stay fixed. Near the coast, a datum may be set at sea level instead.

So a reading of 6 ft does not mean the river is 6 ft deep. It means the water surface is 6 ft above that site's reference point. Two gauges on the same river can read very different heights for the same depth of water, because each has its own datum. You cannot compare gauge heights between sites, only changes at the same site.

If you need to know how deep the water is at a boat ramp or wading spot, the gauge height will not tell you directly. Local knowledge and official boating information matter more. See low-water boating hazards.

What is streamflow?

Streamflow is the volume of water moving past a point per unit of time. In the US it is usually given in cubic feet per second. See what is cfs for how to picture that.

Streamflow cannot be read from a sensor the way height can. USGS field crews measure it by hand on site visits: they work out the area of the river's cross-section and the speed of the water, and multiply the two.

What is a rating curve?

Each time a crew measures flow, they also note the gauge height at that moment. Plot all those pairs on a graph and you get a curve: as the water rises, flow increases. This is the stage-discharge relation, or rating curve.

USGS computers apply the rating curve to every gauge height reading to produce a continuous flow record. Because the curve depends on the shape, size, slope and roughness of the channel at that spot, every gauge has its own rating curve. USGS describes each one as unique to that site and a particular period of time.

Why can the same height mean a different flow?

Rivers change. Floods scour the bed or drop sand and gravel. Weeds grow in summer. Ice and debris can back water up. Any of these can change how much water passes at a given height.

When a new field measurement shows the river no longer matches the rating curve, USGS applies a correction. USGS calls these corrections "shifts": temporary changes in the rating, often due to a change in the streambed, that are applied mathematically to the defined rating. If the channel changes for good, a new rating curve is built.

For this reason, the same gauge height of, say, 5 ft (an example, not a real site) might match one flow this spring and a somewhat different flow next year. USGS says online rating files should be considered provisional and subject to change. It is also one reason recent flows can be revised. See provisional vs approved data.

Which number should I watch?

It depends on the question.

• Flood risk: official flood stages from the National Weather Service are set in gauge height. See flood stages explained.
• Paddling, fishing, how much water is moving: many people use flow, because it describes the amount of water rather than its height above a local reference point. See fishing and water levels.
• Comparing past years at the same gauge: either can work, but old gauge heights may have been recorded on a different datum, so they may not line up with today's readings. See vertical datums.

Using this on Water Level Atlas

Our river pages show both numbers from the same USGS station: flow in cfs and gauge height in feet, with a "normal for the date" comparison from USGS statistics where USGS publishes one. NOAA flood stages and historic crests are given in gauge height, so compare them with the gauge height figure, not the flow. More on how we use each number is on our methodology page.