Flood Stage - Definition

Definition

Flood stage is the water level as read by a gauge for a body of water a particular location, measured from the level at which a body of water threatens lives, property, commerce, or travel. The term "at flood stage" is commonly used to describe the point at which this occurs. "gauge height" (also referred to as "stream stage", "stage of the ", or simply "stage") is the level of the water surface above an established zero level at a given location. The zero level can be arbitrary, but is usually close to the bottom of the stream or river, or the average level of standing bodies of water. Stage was traditionally measured visually using a staff gauge, which is a fixed ruler marked in 1/100 and 1/10 foot intervals, however electronic sensors that transmit real-time information to the internet are now used for a vast majority of measurements. The flood stage measurements are given as a height above or below the zero level. A level below zero is reported as a negative value.

While usually the flood stage is set at the elevation of the floodplain, it can be higher (if there are no structures, roads, or farming areas immediately on the floodplain) or lower (if there are structures such as marinas, lakehouses, or docks low on the banks or shores of the body of water) depending on the location. Because flood stage is defined by impacts to people, as opposed to the natural topography of the area, flood stages are usually only calculated for bodies of water near communities.

The flood stage can be listed for an entire community, in which case it is often set to the lowest man-made structure or road in the area, the lowest farming field in the area, or the floodplain. It can also be set for a specific location ("flood stage is 12 feet on Maple Street at First Avenue" means that the specified intersection will begin to flood when the stage reaches 12 feet).

In the United States during flood events, the National Weather Service will issue flood warnings or watches that list the current and predicted stages for affected communities as well as the local flood stage. Current stage data is collected by the USGS using a network of gages, over 9000 of which transmit real time data via satellite, radio, or telephone. Many communities have inundation maps that provide information on which areas will flood at which stages.