of large deposits of glacial silt, sand, and gravel in the lower Thompson River valley, large landslides are common. The area downstream from the town of Ashcroft is prone to landslide events; eight major events between 1880 and 1982 have been recorded. Several of them have obstructed the river, and caused large, temporary lakes. An 1880 slide caused the formation of a short-lived lake over 14 kilometers long with a maximum depth of 18 meters. These slides have caused major damage to the rail lines and farming operations in the river valley. Heavy irrigation has been blamed for some of the events. History Aboriginal peoples The Interior region of British Columbia was first populated after the retreat of the continental ice sheets of the last ice age. The ice moved out of the Thompson River region approximately 11,000 BCE, and migration by the ancestors of the Nlaka'pamux and Secwepemc people is thought to have occurred soon after. Some of the older archaeological sites on the lower Thompson include the Drynoch Slide site, near Spences Bridge, with artifacts dating to about 7350 BCE, and the Landels site, near Ashcroft, which dates to older than 8000 BCE. Archaeologists theorize early settlers lived in small groups, beginning with nomadic bands hunting ungulates on the plateaus around the river, who then established more permanent dwellings along the river benches as their fishing techniques developed. The South Thompson has the watershed's oldest dated evidence of human habitation, at the Gore site near Pritchard. The human remains date to 8250 BCE, and bone analysis suggests the person was a hunter with small amounts of his protein coming from salmon. Archaeological investigation in the North Thompson has been sparse, but artifacts near Bridge Lake to the west of the river have been dated to 3000 BCE, while pieces found near the tributary Clearwater River are possibly as old as 6000 to 7000 BCE. European exploration and settlement Explorer of the Fraser River and North West Company employee Simon Fraser named the river, after passing its mouth on the Fraser in 1808. He named the river after his colleague, David Thompson, who had mapped much of western Canada and was at the time exploring the Columbia River basin to the east. Thompson never visited the river that bears his name. The first documented traverse of the Thompson from Kamloops to Lytton was by Hudson's Bay Company governor George