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<br />draft 3120/98 <br /> <br />Reliabiiity of Paleostage Indicators for Paleoflood Studies <br /> <br />Robert D Jarrett, US Geological Survey, Box 25046, MS 412, Denver, Colorado 80225 <br /> <br /> <br />(rjarrett@usgs,gov) 80225; John, F. England, Jr., Bureau of Reclamation, Denver, Colorado; and <br /> <br /> <br />Dorothea Jarrett, Volunteer for Science, US Geological Survey, Denver, Colorado <br /> <br />Abstract <br /> <br />Paleoflood hydrology is the study of past or ancient floods during thousands of years and <br /> <br /> <br />supplement relatively short streamflow gaged records that often do not contain adequate samples of <br /> <br /> <br />extreme floods to draw robust frequency conclusions. Paleoflood hydrology has become accepted in <br /> <br /> <br />the scientific, engineering, and regulatory communities and increasingly is being used to complement <br /> <br /> <br />engineering hydrology for a variety of water-resources investigations. A critical issue pertains to the <br /> <br /> <br />accuracy of discharge estimates made from paleostage indicators (PSIs) most commonly flood- <br /> <br /> <br />deposited sediments comprised of sand, gravel, and boulders. This paper documents the first <br /> <br /> <br />systematic assessment of the relation of PSIs and the peak stage of floods responsible for their <br /> <br /> <br />emplacement. Recent large floods in the western United States allowed for a comprehensive <br /> <br /> <br />evaluation of the relation between flood-transported sediments were deposited as flood bars and <br /> <br /> <br />slack-water deposits (new PSIs) and flood high-water marks (HWMs). Surveys of flood-deposited <br /> <br /> <br />sediments, HWMs, and channel geometry were made for streams in the western United States. <br /> <br /> <br />Analysis of the differences in PSIs and HWMs indicates that the elevation at the top of the flood <br /> <br /> <br />sediments (PSIs) average +0.06 m (+5 percent of flood depth) of the flood HWMs. Thus, use of the <br /> <br /> <br />top of flood-deposited sediments (PSIs) may provide a reliable estimate of the maximum paleoflood <br /> <br /> <br />depth that is used to reconstruct the discharge of paleofloods. <br /> <br />1 <br /> <br />~-------~- <br />