A stream gauge measures two things: stage — the height of the water surface above a fixed reference, in feet or meters — and often discharge, the volume of water flowing past per second. Stage is what FloodWatch shows because flood thresholds are defined in terms of it.
Each gauge has official flood categories tied to local impacts. Action stage means take preparatory action; minor flooding brings limited road and low-lying impacts; moderate flooding inundates structures and roads and may require evacuations; major flooding causes extensive, dangerous inundation. FloodWatch colors each gauge by the category it has reached.
Rate of rise matters as much as height. A gauge climbing rapidly toward flood stage can be more urgent than one sitting just above it and falling. The same crest is far more dangerous when it arrives in an hour than over a day.
Small does not mean safe. Flash floods on small, steep streams rise in minutes and produce fast, debris-laden water — the deadliest kind. Large rivers usually flood more slowly, giving more warning but covering vast areas.