 The first glowing reports of country rich with thousands of game animals came in 1806 when Meriwether Lewis and William Clark returned from their two @-@ year expedition . Their journals described lands rich with thousands of buffalo , beaver , and river otter ; and also an abundant population of sea otters on the Pacific Northwest coast . In 1807 , explorer Manuel Lisa organized an expedition which would lead to the explosive growth of the fur trade in the upper Missouri River country . Lisa and his crew traveled up the Missouri and Yellowstone Rivers , trading manufactured items in return for furs from local Native American tribes , and established a fort at the confluence of the Yellowstone and a tributary , the Bighorn , in southern Montana . Although the business started small , it quickly grew into a thriving trade . 
