Palaeohydrologic Analysis of Holocene Flood Slack-Water Sediments

Estimates of the frequency and discharge of large floods can be refined and extended over the past 10,000 years through the study of slack-water sediments deposited in bedrock canyons and gorges. Slack-water deposits are typically fine-grained sand and silt that accumulate during major floods in protected areas where current velocity is reduced, such as in back-flooded tributary mouths, at channel expansions, and downstream from bedrock spurs. 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. The accumulation and preservation of slack-water sediment sequences appear to be controlled by tributary-mainstream junction morphology and by tributary drainage basin efficiency. Radiocarbon dating of wood, charcoal, buried soils and fine-grained organic detritus in slack-water deposits has been accomplished for our studies in central Texas, western Texas, northern and central Australia. 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.