River Flood Levels Explained
When your local river starts rising, the NWS uses four flood stages to tell you how serious it is. Here's what each level means, how to check your river's current level, and when to start paying attention.
What Is Gage Height?
Gage height (also spelled "gauge height") is the water level measured in feet above an arbitrary reference point called the datum. It is not the depth of the water — it's a relative measurement specific to each gauge station.
For example, a gage height of 4.5 ft at one gauge has nothing to do with 4.5 ft at another gauge. Each station's datum is set when the gauge is installed, and all readings are relative to that fixed point.
USGS gauges report gage height every 15 minutes via satellite or cellular telemetry. The data is publicly available at waterdata.usgs.gov.
NWS Flood Stages
The National Weather Service defines four flood categories for many USGS gauges. These are specific water levels (in feet of gage height) where real-world impacts begin:
Action Stage
The NWS or local officials should start preparing. Water is rising and approaching levels where it could become a problem. No flooding yet, but it's time to pay attention.
Minor Flood Stage
Water is out of banks in some areas. Low-lying roads may be covered. Some yards and fields flood. Minimal property damage but travel disruptions are likely.
Moderate Flood Stage
Significant flooding. Roads closed, structures near the river threatened, evacuations possible. Property damage is occurring or imminent.
Major Flood Stage
Extensive flooding with serious threat to life and property. Major roads and bridges impassable. Widespread evacuations. This is a dangerous event.
Not every gauge has NWS flood stages defined. Many smaller creeks and streams have gauges but no official flood thresholds — you'll need to learn from experience what levels cause problems at your specific location.
What to Watch For
Rate of Rise
A river going from 3 ft to 5 ft in 2 hours is much more dangerous than the same rise over 2 days. Fast rises mean flash flooding — especially on small creeks and mountain streams.
Upstream Gauges
Check gauges upstream from your location. If the river is rising 20 miles upstream, that water is headed your way. This gives you lead time that your local gauge can't.
Crest and Recession
The "crest" is the peak water level before the river starts falling. Just because it's falling doesn't mean it's safe — moderate flood levels can persist for days after the crest.
Example: Reading a Real Gauge
Say your local gauge reads 6.2 ft and the NWS flood stages are:
- Action Stage: 7.0 ft
- Minor Flood: 9.0 ft
- Moderate Flood: 12.0 ft
- Major Flood: 16.0 ft
At 6.2 ft, you're below action stage. Normal conditions. But if it rained 3 inches overnight and the gauge was at 4.0 ft yesterday, the river has risen over 2 feet in a day and is still climbing. That trend matters more than the current number.
Monitor River Gauges on Your Phone
StormCast lets you browse over 12,700 USGS and NWS river gauges on an interactive map, color-coded by flood category. Add any gauge to your watchlist and get push notifications when it crosses into a new flood stage.
Download StormCast Free on Google Play