Landslides are mass movements of soil or rock downslope and are a major natural hazard in Canada. They have caused the deaths of a substantial number of Canadians, caused considerable damage to, and in some cases destruction of, elements of the nation's economic infrastructure. Landslides also impact on the quality of Canada's resources. They can be rapid or slow and occur in a wide variety of geologic environments including underwater. The hazard posed by landslides can be attributable to the impact of rapidly moving debris, the failure of ground directly beneath a structure, or due to secondary effects such as river damming or landslide-generated waves. Snow avalanches are mass movements of snow downslope and are a considerable hazard in the mountains of Canada particularly to those pursuing recreational activities, such as skiers and mountaineers.In the past, however, they have caused substantial death and destruction to mining and transportation facilities.

There are many types of landslides:

• Rock avalanches

  Rock avalanches involve the failure and disintegration of a large rockmass on a mountain slope and the rapid movement of this debris downslope and into a valley. Such landslides may reach very high speeds; in the 1959 Pandemonium Creek rock avalanche in British Columbia, for example, the debris reached speeds up to 360 km/hr. Rock avalanches can be very destructive when they impact on human activity. Canada's worst landslide disaster occurred at Frank, Alberta, in 1903 when 90 million tonnes of limestone detached from Turtle Mountain and buried part of the coal mining town of Frank resulting in the deaths of about 75 people. Debris avalanches are large rapid mass movements of volcanic rock originating on the slopes of volcanoes. Several have taken place in this century on the Pleistocene volcanoes of the Garibaldi Volcanic Belt, southwestern British Columbia, and are a major hazard in the development of the region. Rockslides occur when a rock mass slides on a detachment surface but does not travel a long distance. Two rockslides which occurred in the nineteenth century in Quebec City resulted in the deaths of 77 people. Rockfalls involve a smaller rockmass that disintegrates into numerous blocks that fall, bounce, and roll on steep slopes after detachment. Rockfalls are a constant problem along transportation routes through rocky terrain in Canada and, though small, have resulted in numerous deaths in this century.

• Debris flows

  Debris flows are smaller and less rapid than rock avalanches but can be very destructive. They occur when asaturated mass of surficial deposits moves down a steep stream channel. Initial failure is frequently triggered by heavy rains. Debris flows are a major problem in southern British Columbia where numerous defensive structures have been constructed to protect homes and transportation routes. They can also occur following the bursting of a natural dam formed by landslide debris, glacial moraine, or glacier ice. In Klattasine Creek, British Columbia, for example, a massive debris flow was triggered in the early 1970's when a moraine-dammed lake burst in the headwaters of the Creek.

• Slumps and slides

  Slumps and slides occur in soft rocks and sediments in many parts of Canada. Particularly prone to landslides are the Cretaceous shales of the Prairie Provinces and the sediments deposited in glacial lakes during the Pleistocene which cover many parts of Canada. Sliding in these materials may occur along a curved,curvilinear, or a multi-planar surface and are usually retrogressive. They are usually slow moving, but can damage or destroy structures founded on the moving mass. For example, a slump in Cretaceous shales in the north abutrnent destroyed the Alaska Highway suspension bridge over the Peace River, near Fort St. John,British Columbia, in 1957.

  Slumps and slides are especially common in the sensitive glaciomarine Pleistocene sediments of the St.Lawrence Lowlands of eastern Canada. The sediments are commonly called Leda Clay. Sometimes these failures develop into highly retrogressive flowslides in which the clays and silts are fluidised and flow into river channels forming landslide dams which block the flow of the river causing flooding upstream. The rapidity of the retrogression and the subsequent flow in such landslides is very destructive and has resulted inapproximately 100 deaths in Québec since 1840. In 1971, at St. Jean-Vianney, Québec, a large rapid retrogressive flowslide carried 40 homes to destruction and took the lives of 31 people. The landslide tookplace within the scar of an older landslide detectable on aerial photographs. In 1993, a large landslide took place in Leda Clay at Lemieux, Ontario and temporarily blocked the South Nation River; the hazard had been previously recognised and the village of Lemieux had been moved prior to the landslide.

• Secondary effects

  The secondary effects of landslides can also be very destructive. Waves generated by landslides entering rivers, lakes or other bodies of water have caused substantial damage in Canada. In 1908, at Notre Dame de la Salette, Québec, a landslide into the frozen Lièvre River generated a wave of water containing ice blocks that destroyed 12 houses and claimed 33 lives, eastern Canada's largest landslide disaster. Landslide dams cause damage upstream due to flooding and downstream due to a flood which may develop as a result of a suddendam break. Large landslides in glaciolacustrine sediments along the Thompson River, British Columbia have blocked the river on several occasions since the middle of the last century; in 1880 a large landslide blocked the river for 44 hours forming a lake that extended 14 km upstream.

• Underwater landslides

  Underwater landslides occur on the steep underwater slopes of deltas in lakes and the sea as well as on the slopes of the Continental Shelf. Damage may be caused to structures sited on the edge of deltas when underwater landslides involve the delta surface. At Woodfibre, British Columbia such a landslide caused damage to wharves and wharehouses in 1955. Underwater landslides may also cause landslide-generated waves. In 1975, at Kitimat, a large failure on the underwater slope of the Kitimat delta generated an 8 m wave that swept around the shores of Kitimat Arm and damaged wharf facilities.

• Earthquake-triggered landslides

  Earthquakes can be landslide triggers. The 1946 Vancouver Island earthquake, for example, triggered many rockfalls and several rock avalanches. The earthquake triggered a rock avalanche from Mount Colonel Fosterwhich entered Landslide Lake generating a displacement wave that destroyed mature forest up to 3 km from the lake. In eastern Canada, the 1663 earthquake in the St. Lawrence region triggered many landslides in Leda Clay. A huge underwater landslide, triggered by the 1929 Grand Banks earthquake, severed submarine cables on the sea floor and caused a tsunami which killed 27 people when it struck the Burin Peninsula of Newfoundland.

• Snow avalanches

  Snow avalanches occur widely in the mountains of western Canada where they threaten transportation routes in the winter. The worst disaster occurred in 1910 when 62 workmen were killed in a snow avalanche at Roger's Pass, British Columbia, when trying to remove snow from an earlier snow avalanche that covered the tracks of the Canadian Pacific Railroad. Defensive works, route realigmnent, and artillery control have reduced thehazard, however. Avalanches, have on occasion caused much destruction at the sites of settlements. In 1965, for example, 26 workers were killed when a large snow avalanche destroyed part of a camp facility at the Granduc Mine, near Stewart, British Columbia.

In summary, Canada has a wide range of landslide types that pose hazards to the people of Canada and the nation's economic infrastructure in many parts of the country. Snow avalanches are a significant hazard to those pursuing recreational activities in the mountainous regions of western Canada. Avalanche control programs along major transportation routes, however, have significantly reduced the high degree of hazard that previously existed. Nevertheless, historical records indicate that landslides and snow avalanches have resultedin over 600 deaths in Canada since 1850 and have caused billions of dollars of direct and indirect damage to Canada's economic infrastructure.