9.6: Flood Stage and Flooding

Flood Stage and Flooding

Flooding is a common and a serious problem on and along our nation’s waterways. Flood stage Links to an external site. is reached when the water level in a stream overflows its banks. Floodplains Links to an external site. are popular sites for development but are best left for playgrounds, golf courses, and the like.

Have you ever heard someone say, “that flood was a 1 in 100-year flood”? What does this mean? Does it mean that a flood will only occur every 100 years, and that we are safe the other 99? The short answer is No; on average, we can expect a flood of this size or greater to occur within any 100-year period. However, we cannot predict whether it will occur in any particular year, only that each year has a 1 in 100 (1%) chance of occurring in any year. The timing between these major flooding events, or any major geologic event (drought, volcanic eruption, earthquake, tsunami, etc.), is referred to as a recurrence interval Links to an external site.. This is the average time period within which a given flood event will be equaled or exceeded once.

In order to better understand stream behavior, the USGS Links to an external site. has installed thousands of stream gauges Links to an external site. throughout the country, locations with a permanent water level indicator and recorder. Data from these stations can be used to make flood frequency curves, which are useful in making flood control decisions.