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Figure 4. DAN3D runout simulation
                                                                                               of the Sentinel rock avalanche. Panels
                                                                                               (A–E) show snapshots of the runout at
                                                                                               indicated times. Thickness at 200 s
                                                                                               represents final modeled values and
                                                                                               has been bulked by 28% for compari-
                                                                                               son with mapped and reconstructed
                                                                                               values (F). See the GSA Supplement
                                                                                               Data Repository (see footnote 1) for
                                                                                               animation. Shaded relief map and 30 m
                                                                                               contours show recreated sliding sur-
                                                                                               face topography. Thickness values <2 m
                                                                                               have been omitted for clarity.

GSA TODAY | JUNE 2016  At this water level, Sentinel Lake would have covered an area of ~3     Formation, the failure sent material across the canyon at speeds
                       km2, extending >7 km upstream (Fig. 2C).                                possibly reaching 90 m/s; some deposits retain portions of their
                                                                                               original stratigraphy although in a highly deformed state. Field
                         Deposition of sediment behind the rock avalanche dam began            mapping, runout modeling, and cosmogenic nuclide dating
                       to fill Sentinel Lake. Using modern sediment yields measured at         support our hypothesis of single-event, catastrophic emplacement
                       the nearby East Fork of the Virgin River (Andrews, 2000), rescaled      of rock avalanche debris. Zion Canyon was blocked over a
                       for the (nearly identical) North Fork drainage area, we calculate       distance of 3.3 km, damming the Virgin River and creating a lake.
                       that Sentinel Lake would have filled with sediment in ~600–800          At its high-stand, Sentinel Lake may have covered more than 6
                       yr. This value matches a previous estimate by Hamilton (1976),          km2, including all of Zion Canyon to The Narrows, but this water
                       which was generated from different data. Thus, from its formation       level was short-lived as incision of slide debris lowered the breach
                       at 4.8 ka, we estimate that Sentinel Lake occupied Zion Canyon          elevation to ~1345 m. There Sentinel Lake stood relatively stable
                       for ~700 yr until ca. 4.1 ka. This timing is consistent with            for ~700 yr until filling with sediment. Today, incision of the rock
                       Hamilton’s (2014) OSL age of 4.3 ka, as well as Hamilton’s (1976)       avalanche dam continues, lowering the base-level of the Virgin
                       median radiocarbon age of 4.0 cal. k.y. B.P. for post-lake deposi-      River and causing erosion of lacustrine and alluvial materials
                       tion of sand. Our results are not consistent with UGS radiocarbon       upstream in Zion Canyon.
                       ages, measured from extremely small charcoal samples in cored
                       lake sediments, which placed the date of lake formation at ca. 8 ka.      Large rock avalanches represent an infrequent but extreme-
                                                                                               magnitude hazard in Zion National Park. Our study helps address
                         Continued incision of the rock avalanche dam has resulted in          the dynamics and timing of one such event in the densely utilized
                       erosion of alluvial and lacustrine sediments deposited up to 7 km       area of Zion Canyon; however, deposits of several other large
                       upstream (Fig. 3A). In the area of Zion Lodge, the river now lies       valley-blocking landslides have been documented in the park
                       ~25 m below the highest alluvial sediments associated with the          (Hamilton, 2014). One similar event is the Hop Valley rock
                       slide. As local base level continues to drop, this material will ulti-  avalanche, with an estimated volume in the range of 50 million m3
                       mately be lost, and the canyon will return to the steep and narrow      and minimum age constrained by radiocarbon dating as ~2.6 k.y.
                       form seen in nearby tributaries. Based on observed rates of post-       B.P. The consequences of a similar event occurring today, espe-
                       slide erosion, this process is expected to require several millennia.   cially within the narrow confines of Zion Canyon, could be disas-
                       The transient disturbance to the Virgin River drainage basin            trous. Moreover, smaller recent landslides have caused notable
                       created by the Sentinel rock avalanche may thus control upstream        damage; for example, the 1995 Virgin River slide (Figs. 1A and 3A)
                       base-level and prohibit fluvial incision of bedrock for >10 ka (cf.     affected Zion Canyon’s access road, requiring extensive repairs,
                       Hewitt et al., 2011).                                                   while the 1990 Middle Fork Taylor Creek slide dammed a remote
                                                                                               canyon, which drained suddenly three years later, creating a debris
                       CONCLUSIONS                                                             flow that impacted vehicles on a nearby interstate (Lund et al.,
                                                                                               2010). Recognition of these hazards, combined with detailed field
                         The flat valley floor of modern Zion Canyon owes its origin to a      investigation, dating, and runout analysis, are the first steps
                       286 million m3 rock avalanche at 4.8 ka arising from the cata-          toward mitigating landslide risks.
                       strophic collapse of a nearly 900-m-high wall of predominantly
                       Navajo Sandstone. Aided by weak layers in the underlying Kayenta

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