How to Read River Levels: Gage Height, Streamflow and Flood Risk

Rivers are the part of the weather system that's easiest to forget about — until water is coming up the street. The river layer puts thousands of live gages on the map, each reporting how the water is behaving right now. But the readings only help if you know what they mean, so let's unpack the two numbers every gage gives you and how to turn them into a sense of flood risk.

Two numbers: height and flow

Every streamgage reports up to two live values, and they're not the same thing:

• Gage height (also called stage) is how high the water surface sits, in feet, above a fixed local reference point. Think of it as the water's depth at the sensor. It's the number that most directly tracks "is the river rising or falling."
• Streamflow (or discharge) is how much water is actually moving past the gage, in cubic feet per second. A small creek in flood might read a few hundred; the Mississippi at St. Louis moves hundreds of thousands.

The two usually rise and fall together, but they tell you different things. Height answers "how deep" and is what matters for whether water tops a bank. Flow answers "how much" and is what matters for the sheer power and volume heading downstream. LEV sizes each river dot by flow magnitude — so the big rivers read big — and shows the live gage height inside the popup.

Why a high reading isn't automatically a flood

This is the single most important thing to understand about river data: a number on its own is not a flood verdict.

Every river has its own personality. Each gage has a normal range and an official flood stage — the specific height at which water starts causing problems at that location. Those thresholds vary enormously. Fifteen feet might be an ordinary Tuesday on a large river and a disaster on a small one. A reading that looks alarming in isolation may be completely routine for that river, and vice versa.

That's why LEV labels the river layer plainly as a live reading, not a flood alert. To judge whether a given height means trouble, you compare it to that river's own flood stage — which the National Weather Service publishes for its forecast points. (For official flood warnings, the Official Alerts layer is the authoritative source; the river layer is the raw measurement underneath.)

Where the data comes from

The readings come from the U.S. Geological Survey's streamgage network — thousands of automated sensors on rivers nationwide, many running for decades. They measure water height (and frequently flow) and transmit new readings every 15 to 60 minutes. Because the record is so long, hydrologists can say not just "the river is at 12 feet" but "that's higher than 90% of days on record" — context that turns a raw number into meaning. LEV refreshes the layer every 15 minutes to stay close to the gages' own cadence.

Reading it on the map

The river layer is most powerful when you watch the water and the weather feeding it:

• Add Precipitation Radar. Heavy rain falling upstream of gages that are already running high is the textbook flood setup — the watershed simply can't absorb more.
• Use the Flash Flood fusion. We bundle rain radar + river levels into one tap, so you can jump straight to "is heavy rain landing on already-swollen rivers."
• Watch the trend, not just the value. A gage climbing fast after upstream rain is a stronger signal than a single high reading. Click a gage to open its full USGS page, where you can see the rising or falling trend over hours and days.

Used this way, the river layer becomes an early-warning lens: rain on the radar is the cause, rising gages downstream are the effect, and watching them line up is watching a flood try to form — well before the water reaches anyone's street.