High-tech instrumentation is not the only tool used to analyze flood-flow information. Something a simple as a "seed line" can help determine how high the water was (and the subsequent stream stage) during a flood at Peachtree Creek. Instruments that read stream stage can indicate how high the stream stage was during a storm, but cannot show how high water got in the surrounding land and how much land will flood when Peachtree Creek overflows its banks (unless detailed surveys are conducted). One way to determine this is to use high-water marks that occur during floods. If hydrologists can find a high-water mark on a tree or mailbox after a storm, then that information combined with the stream-stage data (from stream-stage records) can be used to estimate how much flooding will occur at different stream stages.
Take our quiz:Here are a couple of pictures of Peachtree Creek at the Northside Drive bridge (notice the pipes in the background of the top picture). Both pictures were taken on the river bank next to Peachtree Creek a few days after high water occurred during a storm.
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.
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.
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U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/highwatermarks.html
Page Contact Information: Howard Perlman
Page Last Modified: Friday, 07-Nov-2008 15:44:52 EST
