<P> Sir Alexander MacKenzie (1764--March 11, 1820) became the first European to cross the Rocky Mountains in 1793 . He found the upper reaches of the Fraser River and reached the Pacific coast of what is now Canada on July 20 of that year, completing the first recorded transcontinental crossing of North America north of Mexico . He arrived at Bella Coola, British Columbia, where he first reached saltwater at South Bentinck Arm, an inlet of the Pacific Ocean . </P> <P> The Lewis and Clark Expedition (1804--1806) was the first scientific reconnaissance of the Rocky Mountains . Specimens were collected for contemporary botanists, zoologists, and geologists . The expedition was said to have paved the way to (and through) the Rocky Mountains for European - Americans from the East, although Lewis and Clark met at least 11 European - American mountain men during their travels . </P> <P> Mountain men, primarily French, Spanish, and British, roamed the Rocky Mountains from 1720 to 1800 seeking mineral deposits and furs . The fur - trading North West Company established Rocky Mountain House as a trading post in what is now the Rocky Mountain Foothills of present - day Alberta in 1799, and their business rivals the Hudson's Bay Company established Acton House nearby . These posts served as bases for most European activity in the Canadian Rockies in the early 19th century . Among the most notable are the expeditions of David Thompson (explorer), who followed the Columbia River to the Pacific Ocean . On his 1811 expedition, he camped at the junction of the Columbia River and the Snake River and erected a pole and notice claiming the area for the United Kingdom and stating the intention of the North West Company to build a fort at the site . </P> <P> By the Anglo - American Convention of 1818, which established the 49th parallel north as the international boundary west from Lake of the Woods to the "Stony Mountains"; the UK and the USA agreed to what has since been described as "joint occupancy" of lands further west to the Pacific Ocean . Resolution of the territorial and treaty issues, the Oregon dispute, was deferred until a later time . </P>

When does it snow in the rocky mountains