Measurement & units
Streamflow basics: what cfs actually means
cfs stands for cubic feet per second, the standard measure of how much water is moving past a point in a river. One cubic foot is about a basketball of water, and 1 cfs flowing for a full day adds up to about 1.98 acre-feet.
Updated Aug 20, 2026
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CFS stands for cubic feet per second: how much water moves past a single point in a river every second, and the standard US unit for river flow and streamflow. One cubic foot is roughly a basketball-sized cube of water, so 1 cfs is one of those passing every second. A small creek might run a few cfs, a major river tens of thousands. It measures a rate of movement rather than a stored volume, which is what separates it from the acre-feet a reservoir is counted in.
The conversion worth memorizing
Streamflow and storage speak different units, flow versus volume, and one constant bridges them: 1 cfs flowing for 24 hours delivers about 1.98 acre-feet. Call it roughly 2 acre-feet per cfs per day. That is the number that lets you turn a river reading into reservoir terms, and it does not change, so it is worth keeping in your head.
Why percent of median is the signal
A raw cfs number means little on its own, because rivers are wildly different sizes and every river swings across the seasons. The useful read is percent of median for the date: today's flow against the typical flow for this same day of the year. That is how you tell a genuinely low river from one that is simply in its normal late-summer recession. This site reports gauges that way for exactly that reason.
The hydrograph has a shape
Snowmelt rivers are not random. Through the year their flow traces a predictable curve, the hydrograph: low and steady in winter, a steep rise as the mountain snowpack melts in spring, a peak in late spring or early summer, then a long taper. The timing of that peak carries information. An early peak often means the snowpack melted out fast, which can leave less water for late summer even if the spring looked strong.
Worked example: turning a gauge into acre-feet
- Discharge
Take the Colorado River at Lees Ferry, charted above, the official divide between the upper and lower Colorado basins. Read today's flow in cfs off the chart, then apply the constant: multiply by 1.98 to get acre-feet moving past that point per day. A flow of 10,000 cfs, for instance, is about 19,800 acre-feet in a single day, enough to supply a good-sized city for a year, passing one point in 24 hours. That is the scale a big western river moves at, and why the same gauge tells you both about today's weather and about the reservoirs downstream.
Now look at the live data
Common questions
What does CFS stand for?
Cubic feet per second. It is the volume of water passing a fixed point in a river each second, and the unit every US streamflow gage reports in. One cubic foot per second is about 7.48 gallons per second.
Is a high CFS reading good or bad?
Neither on its own, because rivers differ by orders of magnitude and every river swings across the seasons. The reading that carries information is percent of median for the date: today's flow against the typical flow for the same day of the year. A river at 40% of median in July is running genuinely low; the same cfs number in a different month or a different river might be entirely normal. The streamflow tracker reports every gage that way.
How is CFS measured at a gage?
A USGS gaging station measures the river's height, called stage, continuously, usually every 15 minutes. Field crews separately measure actual discharge across the channel at a range of river heights and build a rating curve, the relationship between stage and flow at that specific site. The gage then converts each stage reading to a cfs value through that curve. Readings are provisional until USGS reviews them, and a rating curve gets revised when the channel itself shifts.
How do you convert CFS to acre-feet per day?
Multiply by 1.98. One cfs sustained for 24 hours delivers about 1.98 acre-feet, so a river running 10,000 cfs moves roughly 19,800 acre-feet in a day. That constant is the bridge between the flow units rivers are measured in and the volume units reservoirs are measured in. See what is an acre-foot for the other half of the conversion.
Is CFS the same as cusecs?
Yes. Cusec is a contraction of cubic feet per second and means exactly the same rate. The term is common in India, Pakistan and parts of the UK; US agencies write cfs. Neither is the same as cumecs, which is cubic metres per second and equals about 35.3 cfs.
What is a normal CFS for a river?
There is no single normal, because it scales with the size of the drainage the river collects. A mountain creek can run under 10 cfs, a mid-size western river a few thousand, and the Colorado at Lees Ferry tens of thousands. The comparison that works is a river against its own history for the same date, which is why every gage on this site is shown as a percent of its own median.