Flood stage is the water level at a specific river gauge above which a further rise begins to create a hazard to lives, property or commerce. The National Weather Service (NWS) sets it for each forecast location in coordination with local officials, so the number has meaning only at that site.
The ladder of stages
Below flood stage sits action stage, the level where the NWS or a partner needs to start taking mitigation steps in preparation for possible significant high water.
Above flood stage, river forecast points use three flood categories defined by impacts on property and the public. Minor flooding means minimal or no property damage but possibly some public threat. Moderate flooding brings some inundation of structures and roads near the stream, and some evacuations may be needed.
Major flooding means extensive inundation of structures and roads, with significant evacuations of people or moving property to higher ground.
Why every gauge is different
Stage is measured above a local reference point, called a datum, so the same reading at two gauges describes different rivers and different impacts. Flood stage is also not necessarily the same as bankfull stage, the level where water starts spilling over the lowest natural bank somewhere in the reach.
Under NWS hydrologic directives, flood stage can change after channel changes, floodplain development or a gauge move, with regional NWS approval. NWS Caribou, Maine, adjusted flood stages at two gauges in coordination with local emergency managers, one after the gauge was moved to a different slope of the riverbed and one after reviewing a December 2023 flood and flood inundation mapping.
How it drives warnings
River forecasts are compared against these levels at each forecast point. The NWS office in Jackson, Mississippi, describes issuing a Flood Warning once a river is forecast to go above its designated flood stage at the forecast point, with Flood Statements used to update or end it.
What it does not tell you
Flood stage is a threshold at one point, not a map of what will flood. The categories describe typical impacts in the stream segment near the gauge, so a low crossing, a tributary or a neighborhood with poor drainage can behave differently.
Flood stage also applies to rivers with gauges and forecasts. Fast-rising small streams and dry channels are handled through flash flood products, which key on rainfall and rapid rises rather than a set stage.