Abstract

At least two rock avalanches have occurred during Holocene time at the confluence of Texas Creek and Fraser River, about 300 km upstream from Vancouver. The morphological features of the avalanche debris are well preserved, and its stratigraphic relations are well exposed. Failures occurred in fractured argillites adjacent to a small pluton at a site where a steep slope is maintained as a result of undercutting by Fraser River. The first and largest (45 × 106 m3) avalanche occurred during middle to late Holocene time. Debris infilled the postglacial gorges of the streams and spread across a partly dissected alluvial fan. An anomalous ridge on the avalanche deposit reflects the influence of a buried scarp on debris motion and failure in underlying Pleistocene silts. A sharp contact between debris of differing lithologies suggests that the avalanche moved as a coherent mass. A second, smaller (about 7.2 × 106 m3) avalanche occurred about 120014C years ago.Archaeological and geological evidence from sites upstream suggests that the second avalanche impounded Fraser River, causing backwater sedimentation at Lillooet and blocking the migration of salmon. Another avalanche could occur at this site, with similar effects; sudden failure of the landslide dam could generate a catastrophic flood downstream.

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