A PRELIMINARY LOOK AT GEOMORPHIC IMPACTS AND TIMING OF TWO LARGE, DRAINAGE-DAMMING LANDSLIDES IN THE CENTRAL WASATCH

Mass wasting plays a major role in the evolution of watersheds in tectonically active mountain ranges. Large stream-damming landslides can create flood risks, alter the water supply to communities downstream, form landslide-dammed floodplains (LDFs), as well as alter canyon geomorphology, sediment transport, and local ecology. Large landslide triggers include earthquakes and weather events. We seek to better understand the impacts, frequency, and triggers of large drainage-damming landslides in Utah’s Wasatch Mountains by studying two large, possibly coseismic, landslides: the White Pine landslide in Little Cottonwood Canyon and the Grandview Peak landslide in City Creek Canyon. These Holocene events affected rivers that supply drinking water to Salt Lake City.

The White Pine landslide dam consists of at least 250,000 (Cardoso, 2002) to > 1,000,000 m³ of quartz monzonite broken into ~2m – scale boulders, and a ~89,000 m² LDF has formed behind it. Preliminary seismic refraction and tile probe penetration tests suggest the LDF is underlain by up to 7.7 meters of impounded sediment. We hypothesize the presence of a pronounced pre-landslide bedrock knickpoint beneath the landslide deposit, because a linear pre-landslide stream profile would require much more than 7.7 m of sediment beneath the LDF, and a large tributary joins the White Pine trunk stream at the base of the landslide dam.

The remote Grandview Peak landslide dam is composed of limestone and quartzite and extends ~1.5 km down the canyon. The dam is at the confluence of City Creek and a tributary, and an LDF has formed in each stream. Preliminary mapping indicates a total LDF area of 89,400 m². We are constructing 5 cm DEMs of the Grandview Peak site from low altitude aerial photographs taken from a quadcopter UAV and processed with Structure from Motion software. The DEMs are being used to measure post-landslide fluvial erosion of the landslide dam and as an aid for high resolution mapping of the landslide and LDF. Combined with tile probing and a planned seismic refraction survey to estimate sediment thickness in the LDFs, these data will allow us to estimate net influx of sediment into the City Creek watershed from the landslide. In ongoing work, we are using ¹⁰Be surface age dating methodology on both landslides to test the hypothesis that they were seismically triggered.