THE LANDSLIDE DAM ON THE SADDLE RIVER NEAR RYCROFT, ALBERTA

Alberta's largest historic landslide dammed the Saddle River near Rycroft, Alberta, and thrust the bed of the river upward against the opposite valley wall forming a type 6 landslide dam. The Rycroft landslide occurred within a day or two of 17 June 1990 on the north wall of the Saddle River, 12 km northeast of Rycroft, Alberta. The triggers for the landslide are considered to be the lateral and vertical erosion of the river as a result of a severe rainstorm and the ensuing flood. The slide scarp arcs for 1 km through a cultivated field. The loss of farmed land was 13 h (32 acres); a further 10 h (25 acres) may be lost by retrogression of the scarp. The toe heaved the riverbed upward as much as 20 m and thrust it laterally 60 m to be compressed against the opposite valley wall. About 40 Mm(3) of ground moved. The rupture surface dipped steeply through postglacial lake clay and till, curved abruptly to pass on a subhorizontal surface through preglacial clay, then curved upward under the riverbed. This preglacial clay is a new factor in landslide assessment in northwestern Alberta.