Abstract

Geomorphic mapping coupled with optically stimulated luminescence (OSL) dating reveal the late Pleistocene history and geomorphic development of the narrow canyon of Big Creek, a major tributary to the Middle Fork of the Salmon River in central Idaho. The most prominent feature in the region is the Soldier Bar landslide, which consists of slumped and rotated blocks of Mesoproterozoic quartzite bedrock that slid northward from an arcuate headwall, damming both the east-flowing Big Creek and Goat Creek, a south-draining tributary. Water impounded behind the dam ultimately overtopped the deposit. Overflow laterally eroded a flight of four downstream-sloping spillway terraces into the jumbled landslide deposit at elevations over a range of 450 ft (137 m). These elevations, initially estimated from 1:24,000 topographic maps with 40-foot contours, are 4,500 ft (1,372 m), 4,340 ft (1,323 m), 4,200 ft (1,280 m), and 4,050 ft (1,234 m). At the level of the highest spillway, the narrow lake extended ∼17 miles (28 km) upstream from the inferred dam at Soldier Bar, but only impounded 2 km3 of water. Assuming current river discharge and no seepage, it would have taken only six years to fill the lake to this level.

We use a simple sedimentological model based on the forced regression of fan deltas to interpret the numerous fluvio-lacustrine deposits found along Big Creek. Remnants of alluvial terraces, shoreline deposits, and deep-water lacustrine sediments enable reconstruction of the Soldier Bar landslide dam and the long, narrow Big Creek Lake. Optically stimulated luminescence (OSL) age estimates from deep water sediment just above Cabin Creek indicate a lake level well above 4,160 ft (1,268 m) at 17.1 ± 1.4 ka. Fluvial sands near Taylor Ranch at 4,124 ft (1,257 m) demonstrate that the lake had drained to this level by 11.3 ± 0.8 ka. Today, the slope of Big Creek steepens three-fold immediately downstream of the Soldier Bar landslide knickpoint, suggesting that the landslide event inhibited upstream propagation of the regional incision signal from the Middle Fork of the Salmon River into Big Creek.

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