THE WEST SALT CREEK ROCK AVALANCHE, SAG POND, AND OUTBURST FLOOD: MORPHOLOGICAL AND SEDIMENTOLOGICAL SIGNATURES FROM A SERIES OF EXTRAORDINARY EVENTS

From May 25, 2014 to May 27, 2016, the West Salt Creek valley in western Colorado underwent a rapid morphological and sedimentological transformation from the emplacement of a fast-moving rock avalanche and the subsequent formation, and partial drainage of a 500,000 m³ sag pond. A May 25, 2014 rock avalanche filled about 5 km of the West Salt Creek valley with hummocky, shale-rich debris up to 40-m thick that blocked West Salt Creek at the head of the valley. Immediately after the rock avalanche, shale in the rock-avalanche deposit began to rapidly slake, giving the deposit a slow-moving, earth-flow like appearance. Ground-water and surface-water flow created a series of about 80 ponds on the surface of rock-avalanche deposits along the path of the former West Salt Creek. The blocked creek at the head of the valley formed a sag pond that existed until May 27, 2016, when runoff from spring snowmelt, and a landslide from the rock-avalanche headscarp, caused the pond to overtop and rapidly release about 150,000 m³ of water. The outburst flood filled the ponds on the deposit, or disconnected them from their water sources, and left a complex record of flow deposits for 13 km down the West Salt Creek and Salt Creek valleys. The outburst flood lasted about 33.5 hours and had a measured peak discharge of about 17 m³/second. Field mapping after the outburst flood revealed repeating sequences of debris flow, hyperconcentrated-flow, and water-dominated (flood) deposits. Debris flows were first in the flow sequence and were generated by extensive erosion and entrainment of rock-avalanche debris. Vertical incision in rock-avalanche deposits was as great as 32 m. When incision ceased, hyperconcentrated flows reworked earlier debris-flow deposits, and later, water–dominated flows reworked the earlier hyperconcentrated-flow deposits. This sequence occurred near the head, in the main body, and at the toe of the rock-avalanche deposit. Our work indicates that caution should be exercised when interpreting valley-fill deposits because they can be emplaced and modified in short periods of time by complex events that could easily be misinterpreted. Field-based observations and mapping are needed to understand the timing and evolution of complex valley-fill mechanisms.