ABSTRACT: Landslides in the Thompson River Valley between Ashcroft and Spences Bridge, British Columbia.

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Abstract

Eight large landslides have occurred along 10 kilometres of the Thompson River valley between the communities of Ashcroft and Spences Bridge in south-central British Columbia, Canada since 1880. The Thompson River flows south through the Thompson Plateau, part of the Interior Plateau of British Columbia. The river has cut down through 120 metres of glacial sediments during the Holocene. Eight units have been distinguished in these Quaternary sediments that fill the Thompson Valley. Unit 2, which may be as much as a million years old, consists of up to 45 metres of rhythmically bedded silt and clay glaciolacustrine sediments. The highly plastic, overconsolidated clays within this unit are believed to contain substantial por-tions of the rupture surfaces of seven of the landslides. These landslides are now reactivated, retrogressive, multiple, slow to extremely slow, translational earth slides. Most of the recorded slide movements happen in the late summer and early fall. The Ashcroft area, with about 200 mm precipitation per year, has a semi arid climate. In the past, irrigation of the river terraces has been thought to trigger slope movements. The move-ment rates appear to respond to drops in the level of the Thompson River after high flows and perhaps to rain-fall. This paper synthesizes previous individual site investigations into a single geographic information sys-tem. The results of current investigations are being added. This synthesis has facilitated the identification of both the triggers and stratigraphy common to the seven landslides.

Bibliography of Canadian Geomorphology