How to Read Streamflow for Fly Fishing

Updated November 14, 2026. 2 min read.

CFS, not feet

Gauges report two numbers. Gauge height, in feet, is how high the water stands at that gauge. Discharge, in cubic feet per second (cfs), is how much water is moving. Anglers compare flows in cfs. A river's sweet spot is a cfs range.

Use the gauge on your stretch

A gauge 30 miles downstream, past diversions and tributaries, says almost nothing about the canyon you're fishing. Find the gauge just below the dam or closest to your stretch. For example, a Big Thompson gauge near Loveland is out on the plains and doesn't describe the canyon below Olympus Dam.

USGS vs Colorado DWR

Some Colorado gauges are run by the state's Division of Water Resources, not USGS. When a USGS page says "no data available," search DWR's station list instead. Below Olympus Dam on the Big Thompson, the station is BTBLESCO.

Compare to normal

Many gauge pages plot the typical flow for the date. At half of normal, expect low, clear water and spooky fish, so use lighter tippet and fish the deeper water. Well above normal means harder wading, and the fish push to the edges.

Watch the trend

A step up or down that morning, usually a dam release change, can put fish off for a few hours. A steady flow for several days is usually best.

A quick routine

• Open the gauge for your exact stretch the night before and again that morning.
• Compare today's cfs with that river's sweet spot and with the normal line.
• Look at the last 48 hours: steady is good, a fresh drop or spike is a warning.
• Low and clear: lighter tippet, smaller flies, deeper water. High: fish the edges, wade carefully.

Questions anglers ask

What is a good cfs for fly fishing?

It depends on the river. A small creek may fish best at 20–60 cfs, while a big tailwater might like a few hundred. Learn each river's range.

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