What does flood stage mean?
Flood stage is a threshold, not a depth
A river gauge reports gauge height (also called stage) — the height of the water surface above a fixed local reference point chosen when the gauge was installed. It is not the depth of the river, and it is not measured from sea level. That reference point is arbitrary and different at every site, which is why a reading of 15 ft can be completely routine at one gauge and dangerous at another a few miles away.
Flood stage is the number the National Weather Service assigns to each gauge to mark where water starts affecting people — the point at which low-lying roads, farmland, or buildings near that particular stretch of river begin to flood. Comparing a gauge’s current reading to its own flood stage is the only meaningful way to answer “is this river flooding?”
Action stage vs flood stage
These are often confused. Action stage sits below flood stage. It is the level at which agencies start paying attention — watching the forecast, preparing equipment, notifying staff — but no flooding is occurring yet. Flood stage is where flooding actually begins.
In practice: a river at action stage is a heads-up, not an emergency. A river at or above flood stage is already causing problems somewhere near that gauge.
The flood severity categories
Above flood stage, the National Weather Service defines three severity levels. These are the same categories used throughout IsItFlooding:
- No flooding — the river is below its action stage.
- Near flood stage (action) — water is approaching flood stage — a good time to start paying attention.
- Minor flooding — minimal property damage, but some public threat, such as low-lying roads under water.
- Moderate flooding — some inundation of roads and structures near the river, and some evacuations possible.
- Major flooding — extensive inundation, with significant evacuations and property impact.
Stage vs discharge: feet vs cfs
You will see two different numbers on most gauges. Stage is measured in feet and is what flood thresholds are based on. Discharge (or streamflow) is measured in cubic feet per second (cfs) and describes the volume of water moving past the gauge. Flooding is defined by stage, but discharge is the better measure of whether a river is running high or low compared with normal — see the streamflow conditions map.
When a gauge has no flood stage
Many streamgauges have no published flood stage at all. They may measure small creeks, canals, or research sites where no threshold has ever been defined. Those gauges still report water level and flow, but no flood category can be calculated for them — so IsItFlooding shows their readings without a flood status.
Frequently asked questions
What does flood stage mean?
Flood stage is the water level, in feet, at which a specific river gauge begins to cause flooding nearby — roads under water, property at risk. It is set individually for each gauge by the National Weather Service based on local terrain, so the same height means different things on different rivers.
What is the difference between action stage and flood stage?
Action stage is lower than flood stage. It is the level at which agencies begin preparing — monitoring the forecast and readying resources — but flooding is not yet happening. Flood stage is where flooding actually begins. A river at action stage is a warning sign, not an emergency.
What do minor, moderate, and major flooding mean?
They are National Weather Service severity categories above flood stage. Minor means minimal property damage but some public threat, such as low-lying roads flooding. Moderate means some inundation of roads and structures near the river, with possible evacuations. Major means extensive inundation with significant evacuations and property impact.
Why is flood stage different for every river?
Gauge height is measured from an arbitrary local reference point, not sea level or the riverbed, and the surrounding land differs at every site. A reading of 15 ft may be routine at one gauge and dangerous at another, so the National Weather Service sets thresholds per gauge.
What does it mean if a gauge has no flood stage?
Not every streamgauge has a published National Weather Service flood stage — many measure small streams, canals, or research sites where no flood threshold has been defined. Those gauges still report water level and flow, but no flood category can be shown for them.
Check a river now
See live river levels near you compared against each gauge’s official flood stage, or browse the national flood map.
Related: how to read a river gauge · flood watch vs warning · why is the river rising