The northern Plains and Rockies had thus been entered and partially explored by 1812, and increasing numbers of trappers poured into the new Louisiana Purchase and the lands beyond. Astor stepped into the territory of the Missouri Fur Company after Lisa's death in 1820, and in 1822 the Western Division of his American Fur Company was established. Fort Union was built on the upper Missouri to serve as the headquarters of Astor's mountain realm, and steamboats served the post after 1832. A decade before this date, the Rocky Mountain Fur Company was established, and in the following year, 1823, the company's trappers began to exploit intensively the drainages of the Wind River and the upper Snake and Green rivers. The Rocky Mountain Company established a new pattern of trading. Eschewing the rigid and hierarchical organization of the British companies, it relied mainly upon the services of free trappers, who gathered once a year at agreed places to meet the company's supply trains. These gatherings, the famous trappers' rendezvous, were held in the summer at various places in Shoshone country--Pierre's Hole, Cache Valley, or the Green River. The trappers of the Rocky Mountain Company and later of the American Fur Company invested every fastness of the Shoshone hunting grounds in relentless pursuit of the beaver, the vital ingredient of the gentleman's top hat. The intense traffic in the Shoshone region was abetted by the penetration of the Snake River drainage by Donald McKenzie of the North West Company, beginning in 1818, and later by Peter Skene Ogden, under the auspices of the Hudson's Bay Company. The climax of fur trapping came in the middle of the decade of the 1830's, for at the peak of activity the commerce collapsed. Competition between the various trading companies had been ruinous, the streams had been thoroughly trapped out, and the beaver hat went out of fashion. By 1840, the fur trade in the northwest part of the United States was substantially ended.