How to Read a USGS Flow Chart Before You Fish

Every river page on RiverUpdate is built on the same public data: the U.S. Geological Survey's streamflow and water-temperature gauges. Once you know how to read that chart, you can tell whether a river is fishable before you ever leave the driveway.

CFS is the headline number

Flow is reported in cubic feet per second (CFS) — literally how much water is moving past the gauge each second. A small freestone creek might run 80 CFS; a big tailwater can push several thousand. There is no universal "good" number, because what matters is the flow relative to normal for that river at that time of year.

The historical median band is the context

That is why the flow chart shows a shaded band behind the current line: the historical median and the typical high-low range for the date. When the current flow sits inside the band, the river is running about normal. Well above it usually means high, off-color water; well below it can mean warm, skinny, stressed water in summer.

Rising or falling matters as much as the number

A river dropping back into shape after a runoff spike often fishes better than the same flow on the way up. Look at the slope of the last day or two of the line, not just today's reading. A steady or gently falling trend is the reassuring one.

Water temperature is the other half

Trout are cold-water fish. The same gauge network reports water temperature, and it drives everything from which hatches come off to whether you should be fishing at all. Sustained water temperatures pushing past the high 60s Fahrenheit are the signal to give the fish a rest — a topic worth its own post.

Put together — flow versus the normal band, the recent trend, and water temp — you have the same picture our plain-English verdict is built from. If you want to see it applied to a specific river, the Madison River page is a good place to start.