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Figure 3 - face of rock topple event

Rock Topple Event at Lake Powell, Memorial Day Weekend 2022

/ 3 July 2022 / 0 Comments

On 31 May 2022, a massive block of Jurassic Navajo Sandstone toppling into Lake Powell threatened a family boating off the northwest point of Wild Horse Mesa, roughly 0.1 mile south of the Arizona-Utah border. While steering the boat into the open lake to avoid the swell, the boaters captured video, still images, and rough coordinates (36°59'56"N 111°24'29"W ) of the event (Figure 1). The accuracy of the coordinates is poor because the boat was racing away from the swell when the single data point was captured.

Location Map
Figure 1. Location of Lake Powell rock topple of Memorial Day weekend 2022 (yellow dot). Trace of the Smoky Mountain Anticline crops out just east of the rock topple site.

Water level at Lake Powell fluctuates annually, but has declined dramatically since the onset of two-decade long drought beginning in 1998-1999. Lake level on 31 May 2022 was 3,531.7 feet above MSL; nearly 170 feet below the full-pool level of 3,700 feet MSL. Rock falls, rock topples and landslides are common along steep slopes of reservoirs during dramatic drawdown of water level. Drawdown decreases lateral confining pressure on rock units or sediments along the rim of the reservoir, while the shear strength of the rock is reduced due to high pore-water and seepage pressures (Schuster, 1979).

Falling rock slabs of Navajo and Wingate Sandstone occur along near-vertical joints, and pose a life-threatening risk to boaters and campers on Lake Powell (Schuster, 1979). One of the largest falls on record involved a block of sandstone 290 feet long and 145 feet high (Brokaw, 1974). Besides the threat of being struck by falling rocks, large, fast-moving waves on the lake may result from rockfall, and these may overwhelm boaters and threaten people on nearby shores (Petley, 2022). Swells may increase in height rushing into narrow side canyons.

Vertical or near-vertical joints near the site of Memorial Day’s rock topple trend north to northeasterly (Billingsley and Priest; 2013). The topple site is on the western limb of the north-south trending Smoky Mountain anticline, approximately 1.7 miles west of the axial trace (Knudsen and others, 2020). Figure 2 captures the rock topple sequentially from shortly after onset, when the block interior begins to fracture (A), to toppling into Lake Powell (B & C), to the moment when a large swell forms (D).  

rock topple event
Figure 2. Rock topple collapse sequence captured in four photographs; from A (earliest) to D (latest). (Photo courtesy of S. Carter.)

Vertical fractures in the rock face adjacent to the rock topple zone are illustrated in Figure 3 (cover illustration). The fractured rock above the topple zone hosts an overhang of massive and unstable rock. To the lower right, and near the base of the topple, a large block of sandstone is undermined. The fractured topple zone and surrounding rock face appear unstable and could produce additional rock fall events in the very near future.

Table 1 models the mass of the rock topple. Block lengths are estimates based on a measurement of approximately 200 feet (60 m) (from Google Earth Pro) for the distinctive red outcrop resting above the topple zone (Figure 4). Sandstone densities range broadly; I applied a mid-range value of 2,300 kg/m3. Block thickness block is poorly constrained, so I modeled thicknesses of 15 feet (5 m), 25 feet (7.5 m), and 30 feet (9 m). These values appear to be reasonable estimates of block thickness as observed in Figure 2A and B. The resulting mass ranges from about 4,700 metric tons to 13,400 metric tons.

Measured rock figure 4
Figure 4.  Estimated dimensions of the rock topple zone of Memorial Day weekend 2022. The prominent red-colored outcrop perched on the cliff measures about 200 feet long on Google Earth. That length is the basis for evaluating the approximate width and height of the topple zone. Assuming a density of 2,300 kg/m3  (Manger, 1963) for the Navajo Sandstone and a block thickness of 5, 7.5, or 9 m, the mass of the block ranges from about ~4,700 metric tons to ~13,400 metric tons.

Table 1. Modelling mass of the topple block using estimates of average width and height and thickness ranging from 5 m to 9 m.
Table 1

The Lake Powell topple event is eerily similar to that of the January 2022 Canyon de Furnas’ rock topple behind Furnas Dam on Brazil’s Rio Grande. The Furnas event killed 10 people and injured 27. Petley (2022) described it as a classic flexural topple that occurs on vertical or near vertical structures (e.g., joints).

The rock face surrounding and including the rock topple is badly fractured and unstable. Boaters should be wary of approaching the face too closely.

Acknowledgments
I thank Steve and Mila Carter for sharing their photographs, site coordinates, and video of the rock topple event. Dave Petley graciously recommended literature on the causes of mass wasting in reservoirs. Phil Pearthree provided an editorial review that improved this paper.

Posted by FM Conway

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landslide
rock topple
Lake Powell
Navajo Sandstone
geohazard
geologic hazard

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