The USGS Water Science School
The U.S. Geological Survey (USGS) has thousands of real-time water-monitoring sites throughout the Nation that record stream stage (height of a river) and streamflow (how much water is flowing at that instant) continuously. This network of gages is invaluable during floods and high water to allow the general public and emergency responders to know where dangerous flooding might be occuring. Knowing the peak height of a flood got is important information for the historical records of streams and to planning for future development alongside rivers.
In this picture, these young ladies would be able to offer USGS valuable information as to the peak height that water got during a flood in Venice in 2009. But, no, the USGS is not able to hire young people to sit in choice locations and tell us how high water got in a flood. At places where the USGS does not have real-time monitoring equipment we have to either be at the site at the exact river peak (unlikely) or visit the site afterwards and look for visual clues as to the peak height a stream reached. Then, using surveying equipment, we can "run a level" to a known USGS gage-height altitude point and come up with the peak gage height the stream reached during the storm. Once that is know, the USGS can estimate the maximum streamflow during the flood and know the recurrence interval of the flood event.
Hydrologists often visit streams after a large storm to check for high-water marks. If, for example, records show that stream stage reached 17 feet during a storm, a high-water mark will show the hydrologist what a stage of 17 feet means in terms of how high the water was on the riverbanks and surrounding land, which helps to estimate how much land alongside a stream will be inundated at that stream stage. This kind of information is valuable in developing maps and information concerning the impact of floods on the adjacent landscape, structures, and people living there.
Below are a couple of pictures of Peachtree Creek in Atlanta, Georgia. The pictures were taken on the river bank next to Peachtree Creek a few days after high water occurred during a storm. USGS hydrologists visited the site to try to find out the peak river height during the storm. High-water marks are visible in both pictures, although a hydrologist would only regard one of the marks as being reliable.
The pictures below are close-ups of the high-water indicators in the top pictures. Were you able to spot them?
The left picture shows a poison ivy vine with the bottom leaves covered in dried mud. The line where the mud stops indicated where muddy stormwater was flowing, and, thus, how high Peachtree Creek got. The right-side picture shows a limb that hangs over Peachtree Creek. During a flood the rapidly-moving water carries leaves and straw (along with whole trees!). These wet leaves get stuck on limbs that are partially submerged in the stream. When the stream recedes the leaves stay stuck on the limbs. The top of the leaves and pine straw indicate how high Peachtree Creek was during the last storm.
The mud on the poison ivy vine is a much better high-water mark than the tree limb, though. During high water, part of the tree limb will be submerged in the fast-moving water which will cause it to move up and down. Hydrologists would not use this type of high-water mark to estimate peak stream stage during a flood.