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Figure 3. (A) Long-profile of the Virgin River (V.R.) from The Narrows to the    input of slide path topography, source thickness, and material         GSA TODAY | www.geosociety.org/gsatoday
town of Springdale, with extrapolated pre-slide profile (black dashed line).     properties. We used a modified version of DAN3D that allows
Reconstructed top of rock avalanche deposits is compared to modeled profile.     initial rigid-body motion (Aaron and Hungr, 2014). We selected a
Inset shows simplified schematic of the UGS core. Clay lake bed deposits are     Voellmy rheology for basal shear resistance and assigned uniform
projected onto the long-profile. 36× vertical exaggeration. (B) Cross section    material properties across the runout path. The best-fitting input
through the Sentinel rock avalanche deposit showing reconstructed base of        parameters were (1) Voellmy friction coefficient f = 0.27 and
slide, top of slide, and original valley profile, compared to model results and  turbulence parameter � = 200; (2) unit weight � = 20 kN/m3; and
modern topography. Qts—sandy talus cone at base of source; Jts—Temple Cap        (3) an internal friction angle �i = 35°. The relatively high Voellmy
Formation; Jnw—Navajo Sandstone, white; Jnp—Navajo Sandstone, pink;              friction coefficient for the Sentinel rock avalanche (cf. Hungr and
Jnb—Navajo Sandstone, brown; Jkt—Kayenta Formation, Tenney Canyon                Evans, 1996) indicates comparably low mobility, which is due in
Tongue; Jnl—Navajo Sandstone, Lamb Point Tongue; Jk—Kayenta Formation;           part to the cross-valley flow orientation and likely dry rock
Jms—Moenave Formation, Springdale Sandstone; no vertical exaggeration.           avalanche and substrate materials.

measurement uncertainty due to high boron content; Z4, with                        Runout modeling results for the Sentinel rock avalanche are
likely inheritance from pre-slide exposure; and Z8, which was                    shown in Figure 4 (see also animation in the Supplemental Data
highly weathered with possible inheritance. The remaining nine                   Repository [see footnote 1]). After 4.5 s of initial rigid-body
ages are consistent, indicating a mean age of 4.8 ± 0.4 ka (error:               displacement (~20 m southeast), the mass collapses, fluidizes, and
±1�). No evidence for a concurrent paleo-seismic event in the                    rapidly accelerates. The ensuing rock avalanche travels predomi-
area has been identified, which might have triggered the rock                    nantly southeast, crossing Zion Canyon in less than 20 s with
avalanche (Stenner et al., 1999).                                                maximum velocity of 90 m/s and energetic runup on the opposite
                                                                                 valley wall. Debris then spreads laterally up and down canyon.
Runout Modeling                                                                  After ~50 s, the distal ends of the deposit have been reached and
                                                                                 most of the material has come to rest, traveling a maximum
  We performed numerical runout simulation of the Sentinel                       source-toe path distance of 2.4 km at an average velocity of ~50
rock avalanche using the equivalent-fluid code DAN3D                             m/s. The modeled deposit boundaries match well with those
(McDougall and Hungr, 2004). Our goal was to reproduce the                       mapped in the field (Figs. 4E and 4F), although our simulated
observed deposit extents and thickness, and in doing so validate                 deposit exceeds the mapped boundaries in areas to the east and
the hypothesized single-event failure scenario. DAN3D allows                     southwest. Excessive material spreading in the initial part of our
modeling of rock avalanche motion over 3D terrain based on                       simulation may account for some of this discrepancy. The thick-
                                                                                 ness and overall distribution of mass within the simulated deposit
                                                                                 also match well with our topographic reconstruction.
                                                                                 Independent velocity constraint can be obtained from estimated
                                                                                 runup (h) on the opposing valley wall as: V = (2ghmax)0.5 (Jibson
                                                                                 et al., 2006), where g is gravity and hmax = 200 m, indicating a
                                                                                 minimum velocity needed to achieve this runup of 62 m/s (value
                                                                                 does not account for basal shear resistance).

                                                                                 SENTINEL LAKE

                                                                                   The Sentinel rock avalanche blocked Zion Canyon over a
                                                                                 distance of more than 3 km, damming the Virgin River and
                                                                                 creating a lake. We estimate an initial breach elevation of the land-
                                                                                 slide dam of 1420 m, which is lower than the maximum height of
                                                                                 ~1445 m and corresponds to a topographic low in the northern-
                                                                                 central region of the deposit. At this initial level, Sentinel Lake
                                                                                 would have covered an area of ~6.2 km2 (Fig. 2C). Assuming that
                                                                                 discharge of the Virgin River was similar to modern mean values
                                                                                 (USGS, 2014), we estimate that ~5–10 yr were required to fill the
                                                                                 lake to this high-stand.

                                                                                   Preserved clay beds and mollusks indicate that a calm, deep-
                                                                                 water environment was sustained by Sentinel Lake for several
                                                                                 centuries (Hamilton, 1976, 2014). Lake-bed deposits are found as
                                                                                 low as 1285 m (UGS core log data) and as high as 1315 m (Fig. 3),
                                                                                 representing at least 30 m maximum thickness. Core and surface
                                                                                 observations show that these clay beds are gray and yellow in color
                                                                                 and layered at the centimeter scale. We estimate an approximate
                                                                                 stable water elevation for Sentinel Lake of 1345 m based on the
                                                                                 maximum elevation of identified clay deposits and morphology of
                                                                                 the landslide dam. Field evidence of debris slumps and clay-filled
                                                                                 channels indicates that this elevation varied slightly over time.

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