How the USGS Measures Surface Water

If a low-lying area near a river near you usually gets about 50 inches of rain a year, you might think "Well, that is about 1 inch per week, so that won't cause any flooding". But, nature doesn't think the same way, and often a large percentage of a year's precipitation can fall in a major storm, in a single day. Your river might not react much to a 1 inch rain, but things might be much different...

The maximum height rivers reach during storms and floods is an important "data point" to document. In places where there are not dedicated monitoring equipment, the U.S. Geological Survey (USGS) often uses "crest stage gages" to record a one-time measurement of the flood peak.

At places where the U.S. Geological Survey (USGS) does not have real-time monitoring equipment, we use high-water marks to measure the maximum height (stream stage) of a flood or high-water event.