Flood stage and crest have fairly simple definitions.
The flood stage is the height of a river above which damage begins to occur, typically because the river begins to overflow its banks. The crest is the highest level that a river reaches.
Prediction is a more complex matter, involving thousands of readings of river stages and water flows and computer models of likely outcomes. The predictive models are based on experience and must be updated with changes such as each new rainfall.
The U.S. Geological Survey monitors flood conditions in cooperation with the National Weather Service.
A river’s stage, whether it is flooding or not, is the height of the water surface above a predetermined reference elevation. River stages are continually monitored by the Geological Survey, typically with automatic measuring devices linked to satellite transmitters, so that the data can be obtained even in extreme weather.
If the stage of the streambed is known and is subtracted from the water-surface stage, then the result is the depth of water in the stream.
Monitoring devices called stilling wells are sheltered tubes linked to the water with several inlets. They “still” the waters so that momentary surface fluctuations such as waves and surges are smoothed out. Inside, a float goes up and down with the water level, moving a pulley that drives a recording device.
Other devices, called bubblers, measure changes in water level indirectly, by bubbling gas out a tube below the water surface. Changes in the pressure on the gas reflect changes in the depth of the water over the outlet.
Measuring water volume, or discharge, involves physical measurement of the cross-sectional area of the stream and the velocity of the stream. The discharge in cubic feet per second is determined as the product of the area times the velocity.
Velocity is measured by using a current meter with a propeller that is rotated by the action of flowing water. The rotation depends on the velocity of the water passing by. With each complete rotation, an electrical circuit is completed and recorded.
A stream is marked off into sections and separate measurements are taken for each segment of width, depth, and average velocity of flow. The total of these measurements is the discharge of the river.