Wiktionary
n. The elevation at which water overflows the natural banks of a river or other body of water in a given portion of the body of water.
Wikipedia
Flood stage is the level at which a body of water's surface has risen to a sufficient level to cause sufficient inundation of areas that are not normally covered by water, causing an inconvenience or a threat to life and/or property. When a body of water rises to this level, it is considered a flood event. Flood stage does not apply to areal flooding. Because areal flooding occurs, by definition, over areas not normally covered by water, any water at all creates a flood. Usually, Moderate and Major stages are not defined for areal floodplains.
Usage examples of "flood stage".
Day and night, night and day, the immeasurable power of the canyon- cutting river drove the cataract southward toward the break at the intake through which, by this time, the entire Colorado at its highest flood stage was turned.
It was not the ocean, but an arm of the Tennessee River, backed up behind the Appalachian range, maintained at super-flood stage by the continuing runoff from the mountains.
Her lacrimal sacs emptied into the lacrimal lakes, which reached flood stage, and two more droplets sluiced a little misery out of her eyes.
These rivers raged at flood stage, pushed and prodded by the Old Ones in their circles.
The river's water flushed into the sewers a kilometer and a half from Haven's border via a waterfall high enough that even at flood stage the sewers' water couldn't back into the river.
We heard a roar, went to the mouth of the tunnel and looked out, and Avalanche Gulch looked more like the rapids of the Grand Canyon when the Colorado's in flood stage.
But the fair itself, deployed between two rivers on a narrow plain under the bluffs and below the town, on rocky land that appeared to have been raised artificially above flood stage, was most impressive.