The National Weather Service measures floods using stream gauges in key locations across the country. During times of flooding, they measure how high the water has risen above the gauges to determine flood levels.
There are several types of stream gauges used to measure floods. As of 2014, the most common device is a stilling well, which consists of a hole dug next to a river, and the hole has a float in it. The float rises naturally as rainfall or river water fills the hole. The resulting height of the float is used to determine the height of the flood. Though the measuring system is much the same in every location, different cities consider the state of flooding to begin at different water levels, depending on their elevation and how well they are equipped to withstand floods,