How to Read a Texas River Gauge: CFS, Gauge Height, and When to Stay Home

If you've spent five minutes on the conditions page you've seen the two numbers that show up on every gauge: a height in feet and a flow in CFS. Most folks I've floated with can tell you what those numbers were last weekend, but couldn't tell you what they mean. Worth fixing, because reading them right is the difference between a great day and a story that ends in a helicopter.

CFS: how much water is moving past you

CFS is cubic feet per second. One cubic foot is about seven and a half gallons, so a thousand CFS is seventy-five hundred gallons every second sliding past your tube. It's a measure of how much water is in the river, not how high it is.

For a rough Hill Country sense:

• Under a hundred CFS: slow, lazy, you might walk a stretch.
• A hundred to four hundred: classic float water.
• Four hundred to a thousand: fast, fun, swiftwater starting to matter.
• Over a thousand: you should know what you're doing.
• Over two thousand: most outfitters are closed and so should you be.

These are rough. The Comal at two hundred CFS feels different than the Guadalupe at two hundred CFS because the channel is narrower. Local context wins.

Gauge height: how far the water is climbing

Gauge height is the depth at the gauge, in feet. It's measured from a fixed point under the gauge, not from the riverbed, so the absolute number isn't what matters. What matters is the trend.

A gauge at six feet that's been at six feet for a week is a quiet river. A gauge at six feet that was at four feet two hours ago is a river that's coming up, and it's coming up faster than you can outrun on foot.

The flood-stage numbers on each gauge are set by the National Weather Service. When you see "action stage" or "minor flood" on the conditions page, those aren't suggestions.

Why both matter

CFS tells you what kind of float you're going to have. Gauge height, especially the trend, tells you whether the river is safe to be on at all. A river can be at a fine CFS for tubing and still be climbing fast because heavy rain ten miles upstream is on its way down. By the time the height starts climbing where you are, your decision time is gone.

This is especially true on the Edwards Plateau. The limestone country doesn't soak water up. Six inches of rain on a thin-soil watershed becomes runoff in an hour, and the runoff funnels into narrow channels that go from two feet to twenty in a hurry.

How to use the dashboard

Three-step routine before you put in:

1. Open the conditions page and look at the gauge for your river. Is the CFS in the float-friendly range?
2. Look at the trend. Has it been steady, climbing, or falling? A climbing river is a red flag, especially after rain upstream.
3. Look at the gauge upstream of where you are, not just at your spot. The map at the explorer shows them all. If the one upstream is climbing, you have time to leave. If yours is climbing, you don't.

When to stay home

A few simple rules from somebody who has been wrong before:

• Heavy rain in the upper watershed within twelve hours? Stay home.
• Gauge climbing more than half a foot an hour? Get off the river.
• "Flood watch" or "flash flood watch" anywhere in the basin? Stay home.
• Brown water where it's usually green? Get off the river.

The river will be there next weekend. Most of the people who get hurt in flash floods made the call to keep going when something looked off.

Read the numbers. Trust the trend. When in doubt, drive home.