What a river flood stage actually tells you

“The river is really high” is not enough information to decide anything. Forecast offices do not describe rivers with adjectives: they describe them with measured thresholds, gauge by gauge, and with a prediction of how high the water will go and when.

The four thresholds

Stage	What happens on the ground
Action stage	The river is nearing the point where mitigation should begin. No damage yet
Minor flooding	Minimal damage; low ground, parks and some roads by the channel start to be affected
Moderate flooding	Water enters some structures and closes roads near the river; some evacuations
Major flooding	Extensive inundation of buildings and roads, with significant evacuations

Why the crest matters more than the rise

Watching a river rise produces reaction; knowing where it will stop allows a decision. A river forecast gives two things: the expected peak level and the time it will arrive.

With those two numbers the decisions order themselves:

• If the forecast crest stays below your threshold, your job is to watch in case the forecast is revised upward.
• If the crest crosses it, you have a countdown with a specific hour, and everything that needs moving has to be upstairs before then.
• If the crest arrives overnight, your useful window ends the previous afternoon. Nobody moves a house in the dark with water in the street.

How to read a river forecast

A typical bulletin gives the current level, the trend, the forecast crest and its estimated time. Three common mistakes when reading it:

Confusing stage with discharge. Stage is the height of the water at the gauge; discharge is the volume passing through. Flood thresholds are set in stage, and stage is what you compare.

Assuming there is only one crest. If it keeps raining upstream, the forecast gets revised upward. A long event can have two peaks.

Believing downstream is later but equal. In a large basin the crest propagates and can gain volume from tributaries. Later does not mean smaller.

Where to check the river

Forecast offices publish the level of each gauge and its predicted crest. The ZonaProtec monitor also shows the forecast flow of the nearest channel to a point compared with its own record over the last 92 days, so you can see whether it is running unusually high, and links to official warnings where they exist.