River forecasts are issued in feet of stage, yet the same stage means something different on every river. The translation from a measured height to a statement about risk involves several steps worth separating.
What a gauge actually measures
A stream gauge records water surface elevation relative to a fixed local reference point, which is the only quantity that can be measured continuously and reliably.
The reference is arbitrary and specific to that station, so a stage of a given height carries no meaning without knowing which gauge produced it.
Measurement is typically continuous through pressure sensors or float systems in a stilling well, transmitted at short intervals by satellite or radio.
Why discharge has to be calculated
The quantity that determines what a river does downstream is discharge, meaning the volume of water passing per unit of time, not the height at one point.
Discharge cannot be measured continuously, so it is derived from stage using a relationship established by direct measurements taken across a range of flows.
That relationship is specific to the channel geometry at the gauge, and it shifts when a flood scours the bed or deposits sediment, which is why it is periodically remeasured.
How flood stages are defined
Each gauge carries locally defined categories describing what happens at particular heights, based on observed impacts during past floods.
The lowest category marks the point at which water begins affecting property or roads, and successive categories describe progressively wider inundation.
Because the definitions come from local experience rather than from a formula, the same numeric stage can be routine at one gauge and severe at another.
Why the record is essential and imperfect
Estimating how often a given flood level should occur requires a long record at that station, and the estimate is only as good as the length of that record.
Short records produce wide uncertainty, and a single extreme event can noticeably change the estimate derived from a brief series.
Development upstream, dam operation and channel modification all change the relationship between rainfall and flow over time, so older observations may not describe the current river.
Why gauges fail when they are needed most
Extreme floods can damage or destroy the structures gauges are mounted on, and the highest readings are sometimes the least reliable.
Debris jams, ice and backwater from a downstream confluence can raise stage without a corresponding increase in flow, distorting the reading.
Forecasters cross-check against nearby stations and field observations for this reason, and indirect estimates made afterward often revise the peak recorded during an event.