The Bear River Watershed covers 7,500 square miles of mountain and valley land in parts of Idaho, Utah, and Wyoming. Bear River, the main tributary to the Great Salt Lake, is the longest stream in the western hemisphere that does not empty into an ocean, crossing state boundaries five times along its 500-mile course within a mountain-enclosed basin with no drainage outlets. The basin lies within an overthrust belt where older rock was pushed over younger rock, and over millions of years a closed depression formed the watershed. The river and its tributaries water thousands of acres of wetlands that provide critical habitat for wildlife, including more than 75 species of mammals, reptiles, and amphibians and more than 225 bird species. The basin has also been an important resource: since the early 1900s the river has been used for power generation and agricultural irrigation, and more recently for recreation such as hiking, camping, boating, fishing, and wildlife viewing. In earlier times the Shoshone, Bannock, and Ute inhabited the basin, Hudson's Bay Company fur trappers arrived as early as 1812, more than 500,000 emigrants passed westward through the region between 1840 and 1870, and logs cut in the Uinta Mountains were milled into ties and floated down the Bear River for construction of the Union Pacific Railroad.