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.
Flood stage is the water level, as read by a stream gauge or tide gauge, for a body of water at 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 [body of water]", or simply "stage") is the level of the water surface above an established zero datum at a given location. The zero level can be arbitrary, but it is usually close to the bottom of the stream or river or at 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 many of these kinds of measurements. The flood stage measurements are given as a height above or below the zero level. Levels below zero are 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, lake houses, 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 (3.7 m)).
In the United States during flood events, the National Weather Service will issue flood warnings 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 gauges, 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.
In the United States, there are five levels of flooding.