Late Holocene flooding on the Escalante River, south-central Utah

The late Holocene flood history and associated channel changes were reconstructed for the Escalante River in south-central Utah. Analyses of flood deposits at 8 sites in the bedrock canyon indicate that the frequency of large floods was at a maximum 1100 to 900 yrs BP and in historic times in a 2000 year record. The largest flood occurred approximately 900 to 1000 yrs BP and was 7 times the largest flood recorded at a gaging station. The paleoflood discharges were close to the "maximum expected flood" derived from a regional flood envelope curve, and the 100-yr flood was increased 220% to 800 cubic meters per second (cms) with the addition of four historic flood discharges. Possible nonstationarity in the distribution due to channel changes and climatic shifts reduced the reliability of statistical flood-frequency analyses. The additional parameters of the "largest recorded flood" in 2000 years of paleoflood record -- 720 ems -- and the "maximum expected flood" -- 1180 cms -- were added to the flood-frequency summary. Channel changes in the upstream alluvial channel were related to flood-frequency changes. Valley-margin stratigraphy representing 1600 years of deposition indicated that after 1100 yrs BP, a time of increased frequency of large floods, a marshy floodplain was converted to a dry, fire-swept meadow and an arroyo 24-m wide and 2.5-m deep formed. This arroyo quickly filled with sediments between 500 and 400 yrs BP and a smaller channel then formed and persisted until settlement of the basin. Floods between 1909 and 1940 transformed the small channel into an arroyo up to 100-m wide and 17-m deep. The cause for flood-frequency and consequent channel changes on the Escalante River is complicated. Land-use practices caused pronounced changes in watershed and floodplain conditions. A subtle shift in climate increased the amount of summer precipitation and intensity of storms. The inability to test either the land-use practices or climatic shift hypotheses independently precludes the determination of a regional cause for arroyos.hydrology collectio