Water School

Flood stage is an established gage height within a creek or river above which a rise in water surface level is described as a flood. This is the elevation at which the overflow of the natural banks of a stream or body of water begins.

The gage height is the height of the water surface above a predetermined point. Once the gage height becomes too high in the lowest bank of reach this is when flood damage begins to impact an area. The National Weather Service uses gage height numbers to inform people and communities about different levels of flooding hazards.

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Sources:
High Water Level Terminology (weather.gov)
Why We Use Gage Height - Water Data For The Nation Blog (usgs.gov)