EVALUATING THE EFFECTIVENESS AND LONG-TERM SUSTAINABILITY OF EXPERIMENTAL FLOODS ON THE COLORADO RIVER IN THE MARBLE CANYON REACH OF GRAND CANYON, AZ

The combination of severely reduced sediment supply and thirty years of a highly altered flow regime caused evacuation of fine sediment from the river corridor downstream of Glen Canyon Dam on the Colorado River through Grand Canyon. Experimental floods (high flow events or HFEs) are designed to mimic annual pre-dam snowmelt floods and rebuild sandbars by utilizing tributary sediment inputs. For Marble Canyon (first 60 river miles of Grand Canyon below the Paria River confluence), the Paria River is estimated to supply approximately 90% of the total available fine sediment via monsoon-triggered flash floods; HFEs are intended to mobilize and distribute this sand specifically. While previous studies confirm that Paria River sediment accumulates on the channel bed following monsoon floods and the ability of HFEs to increase the overall sand storage and sandbar volume in Marble Canyon, evaluating the long-term sustainability and effectiveness of HFEs requires confirmation that Paria sediment, versus relict, pre-dam channel sediment, is used to build bars and increase sand storage. This study aims to quantify the contribution of Paria-derived sediment to Marble Canyon sandbars using a geochemically based sand signature. X-ray fluorescence testing yields elemental concentration data that is used to differentiate sediment from the two endmember source populations for Marble Canyon: the Paria River and the main stem of the Colorado River above the Paria confluence (used as a proxy for pre-dam main stem sand). For each of five grainsize fractions, the combination of the strongest discriminating intermineralic element ratios are used in a composite fingerprint applied in a mixing model to individual HFE deposits in Marble Canyon. Based on concentrations of Sr, K, Fe, Ca, and P, the majority of HFE deposit sediment appears to be Paria-derived. Bar- and reach-scale estimates of Paria contribution are weighted based on discriminating power of each XRF element ratio and grain size distribution within each sandbar. High density sampling of two years of HFE deposits (2013, 2014) in Marble Canyon will quantify the extent of Paria sediment dispersal downstream, and comparisons between the upper and lower part of HFE deposits should elucidate whether Paria sediment proportion diminishes during HFEs as it is excavated from the bed.