Extreme flooding in the Dolores River Basin, Colorado and Utah: insights from paleofloods, geochronology and hydroclimatic analysis

Abstract

The complex hydroclimatic response of the Upper Colorado River Basin (UCRB) to climate circulation patterns and their descriptive indices creates significant challenges to water managers, especially given the uncertain future of the climate. This dissertation addresses fundamental questions that surround extreme flooding in the UCRB by combining paleoflood field techniques, two analytical geochronology techniques and several numerical climate data analysis techniques. The three manuscripts included in this dissertation focus on the Dolores River Basin (DRB), a sub-basin within the UCRB in order to answer theoretical questions about the timing and climate patterns associated with extreme floods.It has become widely accepted that extreme flooding in the Lower Colorado River Basin (LCRB) is linked to a period when the frequency and intensity of El Nino periods was higher. Within the UCRB, and more specifically, the DRB, the linkages are less clear. The paleoflood chronology that we developed indicates that the peak episodes of flooding in the DRB occurred between roughly 300 A.D. and 1200 A.D. This period of flooding is out of phase with many floods in the LCRB, whose peak floods dominantly clustered in the last 700 years; a period of time coincident with the termination of large floods in the DRB. The chronology that I developed utilizes accelerator mass spectrometry radiocarbon (AMS 14C) and optically stimulated luminescence (OSL) to provide a detailed flood history, highlighting the importance of utilizing independent age control. Alternative, or less accurate chronologies would have resulted had we used AMS 14C or OSL alone, suggesting that some previously studied basins may have incorrect chronologies. The detailed flood chronology of this study has subsequently allowed us to contextualize extreme floods relative to middle--late Holocene climate variability.In an effort to provide a hydroclimatic context for flooding in the DRB, numerical analyses were applied to contemporary climate and streamflow data in order to identify the possible mechanisms that modulate precipitation and streamflow in the Western U.S. and more specifically, the DRB. Results from these techniques indicate that the DRB maintains a complex response to a major North Pacific, low-frequency circulation pattern. The North Pacific circulation modulates the low-frequency component of the DRB's precipitation and flooding, although the high frequency modulation remains very poorly characterized.