THE ELK CREEK, CO ROCK AVALANCHE – A CATASTROPHIC VALLEY WALL COLLAPSE TRIGGERED BY DEGLACIATION?

Located in southwest Colorado, Elk Creek flows through a ~1 km deep canyon incised into the core of the San Juan mountains near the Animas River. The canyon is well-known for hosting a segment of the Colorado Trail. Here we present a suite of geologic evidence pointing to a catastrophic valley-wall collapse event that filled the valley with up to 100 m of rock avalanche debris.

Part-way upstream from the steep-walled canyon’s confluence with the Animas River, the valley abruptly widens and for the next 1.5 km, the valley floor is filled with large boulders up to the size of houses. Upstream of the boulderfield, the stream lazily meanders through a broad, flat floodplain, which is uncharacteristic for this landscape position. A conspicuous scar on the northern valley wall appears to be the source area for the debris.

Through fieldwork, LiDAR analysis, in-situ 10Be exposure dating, and runout modeling with DAN3D, we reconstruct the sequence of events that produced the modern landscape. CRN ages on sampled boulders cluster around ~14 ka, which dates to the immediate deglaciation of Elk Creek (Guido et al., 2007). The event involved ~30 – 50 million m³ of mostly quartzite that appears to have collapsed from a ~2-km-long and 2-km high scar on the northern valley wall. Elk Creek was dammed, and over the ensuing millennia the new lake was alluviated while its outlet waters began incising through the debris. This event is consistent with glacial valley wall widening during the preceding glacial period(s) and the debuttressing that accompanied deglaciation. Similar-scale catastrophic hillslope failures may await us as mountain glaciers continue to retreat worldwide.