Abstract
Paleoflood hydrology is the study of past or ancient flood events. The most accurate technique involves the analysis of slackwater deposits and paleostage indicators (SWD-PSI). Slackwater deposits are sand and silt emplaced from suspension in exceptionally deep, high-velocity floods that characterize narrow, deep canyons in resistant geologic materials. These and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events. Favorable sites for slackwater deposition include channel-margin areas where stresses and velocities are reduced below critical values necessary to maintain fine-grained bedload material (sand and silt) in suspension. Computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections. The flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers. Correlation of multiple SWD-PSI sites along a river reach is used to identify the maximum paleostage achieved by a given flood. Advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years. The major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits. An important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14C. Tiny blebs of charcoal, seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance. Since SWD-PSI studies yield very accurate determinations of paleoflood ages and magnitudes, there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood-frequency analysis. Nevertheless, SWD-PSI paleoflood hydrology has moved beyond the research phase; its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites.
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