Holocene flood histories in south-western USA

River basins in south-western USA are some of the most extensively studied arid land fluvial systems in the world. Since the early 1960s their hydro-climatic histories have been reconstructed from the analysis of alluvial cut-and-fill cycles, while from the late 1970s there have been investigations of slackwater deposits and palaeostage indicators for large floods in stable-boundary bedrock reaches. However, no studies have regionally integrated Holocene fluvial histories from these two different types of fluvial environments. The current study combines the alluvial archive with flood records from bedrock reaches to generate a probability-based 12,000 year record of flooding in south-western USA. Using more than 700 ¹⁴C-dated fluvial units, the analysis produces a high resolution (centennial) flood record. Seven episodes of increased flooding occurred at 11,250-10,400, 8800-8350, 8230-7600, 6700-5700, 5600-4820, 4550-3320 and 2000-0 cal. BP. Bedrock reaches are found to record more frequent floods during the middle to late Holocene, while in alluvial rivers more flood units are dated to the early and middle Holocene. These differences are primarily the result of selective preservation with alluvial reaches tending to erode during periods characterised by very large floods. Episodes of major Holocene flooding recorded in slackwater deposits within bedrock systems correspond with periods of increased precipitation in the region and lower temperatures. In contrast, within alluvial rivers above-average flooding probabilities, as well as regionally extensive channel entrenchment episodes, match with reduced annual pre-cipitation and lower temperatures. The results of this study clearly demonstrate the value of the Holocene fluvial archive for reconstructing regional, short-term hydro-climatic change in south-western USA.