Is the river flooding? How to read a river gauge

A river gauge tells you one thing that matters: how high the water is at that spot, compared with the levels where that same spot floods. Read the gauge's height against its own flood stage, not against any other river and not as a depth. Then check the forecast, because what matters most is where the river is heading.

Stage is the water's height, not its depth

A gauge's main number is its stage, also called gauge height: how high the water surface is, in feet, above a fixed reference point chosen for that gauge. It isn't how deep the river is. A river at 4 feet may be far deeper than 4 feet. A few gauges, mostly on lakes and reservoirs, measure height above sea level, so a reading in the thousands of feet can be normal there. The USGS's own page can also show a different number for the same gauge than the NWS does; Atmostorm shows the NWS's, the one its flood levels use. That's why a stage only means something next to that gauge's own flood levels. A river page shows both side by side: "The river is 4.6 ft below flood stage (87 ft)."

Action stage and the three flood levels

The National Weather Service sets levels for each gauge it forecasts:

• Action stage ("Near flood stage" on Atmostorm's river pages): the river is getting high. People who manage the river start preparing. Nothing floods yet.
• Minor flooding (flood stage): water starts reaching low land, some roads and parks. Little or no damage to property.
• Moderate flooding: some buildings and roads near the river are flooded, and some people may need to move out.
• Major flooding: widespread flooding of buildings and roads, and significant evacuations.

Each level is set by what floods at that particular place. "Two feet above flood stage" can mean wet parking lots on one river and flooded homes on another.

Flow: how much water is moving

Many gauges also show flow, or discharge, in cubic feet per second: how much water passes the gauge each second. The U.S. Geological Survey, which runs most gauges, works it out from the stage. Flow is hard to picture, so next to the number Atmostorm compares it with the same date in past years: "much higher than normal for May 3", along with the usual range for that date.

The forecast and the crest

At gauges with an NWS forecast, the river page shows where the level is expected to go and, when it's rising, when and how high it's expected to crest, meaning peak. Rivers crest later downstream, sometimes days after the rain stops on large rivers, so a river can still be rising under a clear sky. Forecasts change as rain falls, or as the rain forecast changes. Check again rather than relying on yesterday's number.

When to act

• A Flood Warning means flooding is happening or expected. Never drive or walk into floodwater: turn around, don't drown. Unlike a flash flood, a river flood usually builds over hours or days. Watch the gauge's forecast, follow local officials, and move to higher ground if they tell you to or water rises near you.
• Why "turn around, don't drown". Six inches of fast-moving water can knock over an adult. A foot of rushing water can carry away most cars, and two feet can carry away SUVs and trucks. Over half of flood drownings happen when someone drives into floodwater (NWS, citing the CDC).
• A flash flood is different: small streams and city streets can flood within minutes to hours of heavy rain, with no river gauge nearby to watch. Flash flood warnings are their own alert.

Follow a river on Atmostorm

Each river page shows the level now and, where they exist, its flood levels, the NWS forecast and flow against normal for the date. Some also show a camera. On any gauge with NWS flood levels you can get flood alerts for that river by email, with no account: one confirmation link, and a stop link in every email. Where your browser supports it, you can get a notification on your device instead. See all river gauges or find yours on the map.