275-4 The Role of Geology in the Hydrogeomorphology of a Mountain Watershed in Southwestern Utah

Abstract:

The Parowan Creek watershed (~150 km²) is the principal water source for an endorheic extensional tectonic basin in southwestern Utah. The watershed serves as the primary water supply and aquifer recharge source for a growing rural community, has a history of wildfire and flooding, and is characterized by a diverse geology with active faults and strong structural control on topography. Parowan Creek and its tributaries are steep, gravel-bedded streams draining the Markagunt Plateau mountain block uplifted along a series of normal faults resulting in approximately 1600 m of topographic relief. These faults create horsts and grabens that influence local relief, expose different sedimentary and volcanic lithologies, affect runoff processes, and regulate sediment supply to stream channels. Hydrologically, the bulk of streamflow is derived from snowmelt, but the largest floods are caused by monsoon thunderstorms over steep terrain. Here we present a dataset from more that 20 sites within the watershed to understand the role of geology in shaping downstream changes in stream geomorphology and sediment transport capacity. Further, we explore how geologic structures affect downstream surface water-groundwater exchange via contemporaneous discharge measurements. Results show that both gaining and losing stream reaches are common, where losing reaches are generally associated with grabens underlain by Tertiary limestone, whereas gaining reaches and springs often occur where horsts expose older Cretaceous rocks. Downstream hydraulic geometry relations show significant scatter as post-fire floods caused channel widening in some reaches while other reaches remained relatively unchanged. Yet bankfull Shields stress values show a much more coherent pattern, with sediment transport capacity increasing downstream. This likely reflects increasing downstream sediment supply as streams pass through steep horst blocks near the mouth of the watershed and is supported by measurements of bed sediment lithology dominated by proximal source rocks. Despite the clear role of monsoon thunderstorms in generating sediment from steeper terrain in horst blocks, sediment transport modeling shows that long-duration snowmelt floods transport the bulk of the sediment at the basin outlet. Taken together, these results show that geology strongly influences the hydrogeomorphology of the watershed, with implications for mountain-block recharge of the downstream basin aquifer and morphological adjustment of stream channels following a fire-flood sequence.

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