CLIMATIC AND GEOLOGIC CONTROLS ON STREAMFLOW HYDROLOGY IN SOUTHWESTERN UTAH

Understanding the hydrology of streams in southwestern Utah is vital to sustainable water management and flood risk mitigation in an area of rapid development. Floods in this region are generated from three primary mechanisms: winter to early spring rain, spring to summer snowmelt, and summer to early fall monsoon thunderstorms. These different flood-producing mechanisms complicate the understanding of regional flood hydrology, because most statistical approaches to flood recurrence do not account for these mixed-population floods. Furthermore, baseflow discharge is fed by groundwater inputs from aquifer systems and may change longitudinally due to the permeable and faulted sedimentary rocks that dominate the landscape. Here we use historical streamflow records to evaluate the spatial variability in the dominant flood hydrology of southwestern Utah, and a case study from the Parowan Valley watershed using hundreds of streamflow measurements to illustrate potential geologic controls on gaining and losing streamflow. Results show that in lower elevation watersheds, the largest snowmelt flood is a common rain-induced flood, and thus snowmelt is unlikely to cause flooding because the channel is adjusted to convey these larger floods. By contrast, in higher elevation basins, discharge may exceed the flood stage during snowmelt for extended periods and monsoon floods are rare. The dominant flood-producing mechanism can differ even where watersheds share a drainage divide, highlighting the need to assess flood hazard on an individual watershed basis. Within the Parowan Valley watershed, our results show that the major streams are all gaining near the mountain-valley interface, but interchange between gaining and losing as the streams cross between horsts and grabens that alter the dominant lithology exposed at river level within the mountain block. Taken together, these results have implications for flood risk mitigation and the coupling of surface water-groundwater systems in a rapidly growing region where groundwater levels are declining.