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A study of the 68-mi (109-ktn) section of the Fraser Canyon between Lytton and Hope, British Columbia, along the Canadian National Railway (CNR) was made to determine what factors controlled slope stability on a regional scale. Engineering geology aspects concerning regional faulting and related minor structure, lithology, drainage and hydrology, geomorphology, climate, river geometry, and effects of man were considered. The most significant cause of slope instability was deflecting of the Fraser River into its bank by the presence of either an alluvial fail at a tributary mouth or a river bend. This deflection allowed extensive lateral erosion and resulted in severe oversteepening, which undermined the toe of the slope. Rockfalls, rock and debris slides, and washouts have been recorded for more than 20 years by the CNR. These data indicate that about 66% of all such incidents occurred opposite alluvial fans or on outside curves of the river. The average numbers of incidents per mile occurring opposite alluvial fans and on the outside of river bends are 5.6 and 3.3 times greater, respectively, than the average number of incidents recorded for river stretches without these characteristics. Regional faulting, climatic conditions, and effects of man were also found to be important causes of slope instability.

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