Palaeohydrologic analysis of Holocene flood slack-water sediments.

Relatively narrow bedrock canyons in arid, savanna, and semi-arid regions provide excellent areas for estimating flood discharges from the elevation of slack-water deposits because: 1) relatively small discharge increases are accompanied by large increases in river stage, 2) the bedrock canyons provide stable cross-sections for slope-area calculations, and 3) the paucity of vegetation limits sediment bioturbation. Along the lower Pecos River of western Texas the slack-water sediment stratigraphies record between six and ten major flood events. This record extends back at least 2000 years in some canyons, and as far as 10 000 years at the Arenosa Shelter site. In this region we have been able to apply geological data to extend observational records of major floods to achieve flood-frequency curves over a time base in millennia, rather than in decades. The procedure allows for the realistic assessment of recurrence intervals for large-magnitude events. -from Authors