Reading River Flows for Fly Fishing

What CFS means, how to read a USGS gauge, and why a good flow belongs to that river.

From the book Fly Fishing for Nerds — $9.74 →

A CFS remembered from a forum has no river attached. Four hundred cubic feet per second can be a flood on a small creek and an ordinary day on a large river. Read the gauge on the reach you will fish, compare it with that gauge’s own history for this time of year, and write down whether the line is rising.

1. CFS is a volume per second

CFS means cubic feet per second. The U.S. Geological Survey calls that quantity discharge: the volume of water moving down a stream per unit of time. USGS also publishes the same idea as gallons per day. A streamgage does not tip a bucket every second. It measures how high the water is, then converts that height into discharge.

The conversion is the part worth understanding. Discharge equals the area of water in a channel cross section times the average velocity through that cross section. More water in the cross section, or faster water, and the CFS rises. The USGS account of how streamflow is measured is the full method: stage, a periodic discharge measurement, and the curve that connects them.

2. Gage height is a local ruler

Gage height, which USGS also calls stage, is the height of the water surface, in feet, above a zero point chosen for that gauge. The zero is arbitrary. It is often near the streambed, and it is not a fishability score. A staff gage, the painted ruler on some bridge piers, is the same measurement you can read with your eyes. USGS streamgages record stage on a short interval, usually every 15 minutes.

Discharge comes from a rating curve, the relationship at that one site between stage and discharge. USGS builds the curve by measuring the river at many heights. It depends on the shape, size, slope, and roughness of the channel, so every gauge has its own. Sediment, vegetation, debris, and ice move the relationship, and the rating is revised when new measurements show that it has moved. The USGS note on rating curves is that mechanism in one page.

Read cubic feet per second when you are deciding whether to drive. Use gage height to see whether this river is up or down relative to itself. Four feet on one gauge page and four feet on another are two different rulers.

3. The number on your phone is provisional

Real-time values are provisional until USGS reviews them. They can change. Backwater from ice or debris, algal growth, sediment movement, or a recorder that quit can all throw the stage-discharge relationship off. USGS treats the station record as provisional until the data are published, usually within six months after the water year ends on September 30. The provisional-data note is the caution in USGS’s own words.

Provisional data are still the right figure for a trip decision this morning. They are a planning figure USGS may revise after review. If the graph is flatlined, spiked, or nothing like the river in front of you, believe the river and pick another plan.

4. A good flow is a percentile on that gauge

USGS compares a day with the approved history for that same calendar day at that gauge. The statistics documentation is the rulebook. A duration graph draws recent discharge over those lines: the highest and lowest daily mean for that date, the 90th percentile, the 10th percentile, and the band between the 25th and 75th percentiles. On the accumulation graph, USGS calls that 25th-to-75th span the estimated normal range. Inside the band, the river is having an ordinary day for the date. A discharge above the 90th-percentile line is higher than 90 percent of the approved daily means for that date. Below the 10th-percentile line, it is lower than 90 percent of them.

If the band is missing, this gauge does not have the 20 years a discharge duration graph requires, and a lone CFS still has nothing to be good relative to. The historical lines are computed from approved data only. Today’s dot on the graph can still be provisional. The USGS note on the graphs is where those plots sit on a monitoring-location page.

That ordinary band is as close as the record comes to a good flow, and it still does not promise a fish. Four hundred CFS inside the band on a large river is a different day from four hundred CFS above the 90th percentile on a creek. If your river is missing from Water Data for the Nation, a state water agency often posts its own gauge. Read that page the same way: discharge, the recent trend, and whatever it says about the number still being provisional.

5. Rising, falling, and blown out

When discharge rises, more volume is passing the gauge. Unless the channel has suddenly gotten much wider, average velocity in the main lane goes up with it. A trout that was holding in a moderate current may not be able to hold there. The slower water is usually along the bank, in an eddy, or on the soft side of a seam. That is the hypothesis. Test the soft edge for a handful of drifts before you change flies and call the river dead.

A falling graph is the other half of the same record. The lane that looked right at the peak can be too shallow by the time you wade in. Walk the reach again with the reading-water guide and mark the features that are actually there today.

Blown out is angler language. USGS does not publish a status by that name. Anglers mean the river is well above its usual range for the date and too colored to read. On the duration graph, that is often a discharge above the 90th-percentile line, in water you cannot see into. If you cannot see the seam, you cannot tell whether the drift worked. Fish a slow edge you can still see, or choose another river. High water is also a wading problem. Footing and flow are the first item on the bank checklist.

Rain is the usual reason a freestone does this. The after-rain guide is the rise and the drop, the color, how long to wait, and what to fish while the river is still off color.

A streamer along the bank is one way to test the soft edge when the river is up. The streamer guide is the retrieve, and the flows where a swing is the wrong cast.

6. Check the gauge before you leave

Open Water Data for the Nation and search the river. The monitoring-location page is the one you want.

• Pick the gauge on your reach. A gauge below a large tributary is measuring a different mix of water than the bridge you park at.
• Read discharge in cubic feet per second. If the duration graph is there, see whether today sits inside the 25th-to-75th band, above the 90th-percentile line, or below the 10th. Glance at gage height only as this river’s own ruler.
• Look at the last day or two. Rising, falling, or flat. The direction over that stretch tells you more than the latest dot alone.
• Read the provisional label so you know the figure can still be revised.
• If the page shows water temperature, write that number down too. The water-temperature guide is what to do with it, including when to stay home.

On the bank, look at the water you can see. A gauge a few miles upstream can disagree with the pool at your feet after a local storm. One line in your phone is enough: CFS, where it sat against the band (or “no band yet”), rising or falling, and whether you could see the bottom.

PingWhen (paid, from $9 a month) can email you when a USGS gauge crosses a flow or gage-height level you set.

Flow, water temperature, and the other conditions you can check before a cast get more room in Fly Fishing for Nerds: A Data-Driven Guide to Finding, Hooking, and Landing Trout. Readers can take 25% off ($9.74) with code NERDS25 at https://leanpub.com/flyfishingfornerdsadata-drivenguidetofindingfishfly-fishing-for-nerds/c/NERDS25.

The Riverbank Checklist is the one-page routine for the bank once you are there: safety first, then one hypothesis and a short test.