Streamflow basics: what cfs actually means

cfs means cubic feet per second, the rate at which water flows past a gauge in a river. 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 is the standard unit for streamflow because it measures a rate of movement, not a stored volume.

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

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.