PRECIPITATION AND STREAMFLOW HYDROLOGY DURING THE SUMMER 2021 MONSOON FLOODING OF THE CEDAR VALLEY, UTAH

Extreme rainfall during a strong monsoon season in the summer of 2021 caused numerous localized floods within communities of the Cedar Valley watershed in southern Utah. Here we use a compilation of precipitation records, stream gaging records, public surveys of flood inundation, and GIS analyses to evaluate the hydrology of this flooding. Coal Creek, the principal stream providing surface water to the Cedar Valley, typically receives most of its annual flow from snowmelt runoff, but in 2021 had a peak snowmelt discharge 15-25% of that in average years. Following this extremely low snowmelt runoff period, more than 10 floods of Coal Creek that exceeded the snowmelt peak occurred following monsoon storms in July and August. The largest event affecting the Coal Creek watershed occurred on August 18^th, with a peak discharge of 2610 ft³/s, which is the sixth largest flood since 1916. Large wood impacted diversion structures and levees within Cedar City, resulting in levee-overtopping floods in some areas. Additionally, strong and nearly stationary thunderstorms produced rainfall rates in exceedance of the 100-year 15-minute and 1-hour peak rain events and induced more localized flooding within Cedar City (July 26^th) and Enoch (August 1^st). This flooding originated along small drainages and washes adjacent to and within urbanized areas, and was likely exacerbated by the low topography and limited drainage potential in flooded areas. While the monsoon storms brought needed moisture and helped reduced drought conditions from Exceptional to Severe, these storms also resulted in widespread flooding not typical during snowmelt runoff that is the primary source of recharge for the regional water supply in most years.