At least eight large landslides occurred in Thompson River valley south of Ashcroft, British Columbia, between 1880 and 1982. The landslides were slow-moving translational slides and slumps, and sudden-onset, rapid flowslides. Some of the landslides disrupted trans-continental rail traffic and blocked Thompson River, creating short-lived upstream reservoirs. Most of the landslides occurred in a 10-km reach of Thompson valley, within a thick Quaternary valley fill dominated by glaciolacustrine sediments. Failure occurred on the steep walls of an inner valley that formed during the Holocene when Quaternary sediments filling the broader Thompson River valley were incised. The valley fill sequence consists dominantly of permeable sediments, the exception being a unit of rhythmically bedded silt and clay near the base of the Pleistocene sequence. Large landslides are restricted to areas where this silt-clay unit is present. Failure occurred within, or at the top of, this unit. The landslides may have been triggered by irrigation of benchlands above the river. Irrigation commenced in the 1860s, shortly before the first known large landslide occurred. Early irrigation added large volumes of water to the normally dry Quaternary sediment fill, possibly elevating pore pressures at the top of the rhythmically bedded silt-clay unit. Large landslides may occur in the future in the study area if irrigation is not carefully controlled or if climate becomes wetter for periods of years or decades. Such landslides would probably involve reactivation of nineteenth- and twentieth-century landslide deposits.

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