FloodWatch — glossary
Plain-language definitions of the terms used across FloodWatch (GDACS + NOAA NCEI + USGS Water Services).
Measurement
- Stage
- The height of the water surface at a gauge above a fixed reference (datum), usually in feet or meters. The value flood thresholds are based on.
- Discharge
- The volume of water flowing past a gauge per unit time (e.g. cubic feet or meters per second) — a measure of how much water is moving.
- Crest
- The highest stage a river reaches in a flood event before it begins to fall. Forecast crests drive evacuation timing on large rivers.
- Datum
- The fixed reference elevation a gauge measures stage against, so readings are comparable over time.
Flood categories
- Action stage
- The level at which agencies begin preparing — mitigation and monitoring — before any flooding occurs.
- Minor flooding
- Limited inundation of low-lying areas and roads with minimal property threat.
- Moderate flooding
- Inundation of structures and roads near the waterway; some evacuations may be needed.
- Major flooding
- Extensive, dangerous inundation of structures and roads, often with significant evacuations.
Flood types & alerts
- Flash flood
- Rapid flooding within hours of heavy rain or a dam/levee failure, on small or steep watersheds. The deadliest flood type.
- Floodplain
- The low, flat land beside a river that naturally floods. Mapped flood zones describe how often an area is expected to flood.
- Watch vs. warning
- A watch means flooding is possible — be prepared; a warning means flooding is imminent or occurring — act now.
Intensity scale (ft (gauge height))
- Low
- 0 ft (gauge height)
- Normal
- 8 ft (gauge height)
- Elevated
- 16 ft (gauge height)
- High
- 24 ft (gauge height)
- Flood
- 32 ft (gauge height)