The Oregon Trail was laid by fur traders and trappers from about 1811 to 1840, and was only passable on foot or by horseback. By 1836, when the first migrant wagon train was organized in Independence, Missouri, a wagon trail had been cleared to Fort Hall, Idaho. Wagon trails were cleared increasingly farther west, and eventually reached all the way to the Willamette Valley in Oregon, at which point what came to be called the Oregon Trail was complete, even as almost annual improvements were made in the form of bridges, cutoffs, ferries, and roads, which made the trip faster and safer. From various starting points in Iowa, Missouri, or Nebraska Territory, the routes converged along the lower Platte River Valley near Fort Kearny, Nebraska Territory, and led to rich farmlands west of the Rocky Mountains.
After crossing the South Platte River the Oregon Trail follows the North Platte River out of Nebraska into Wyoming. Fort Laramie, at the confluence of the Laramie and North Platte rivers, was a major stopping point. Fort Laramie was a former fur trading outpost originally named Fort John that was purchased in 1848 by the U.S. Army to protect travelers on the trails. It was the last army outpost till travelers reached the coast.
Fort Laramie was the end of most cholera outbreaks which killed thousands along the lower Platte and North Platte from 1849 to 1855. Spread by cholera bacteria in fecal contaminated water, cholera caused massive diarrhea, leading to dehydration and death. In those days its cause and treatment were unknown, and it was often fatal—up to 30 percent of infected people died. It is believed that the swifter flowing rivers in Wyoming helped prevent the germs from spreading.
After crossing the South Platte the trail continues up the North Platte River, crossing many small swift-flowing creeks. As the North Platte veers to the south, the trail crosses the North Platte to the Sweetwater River Valley, which heads almost due west. Independence Rock is on the Sweetwater River. The Sweetwater would have to be crossed up to nine times before the trail crosses over the Continental Divide at South Pass, Wyoming. From South Pass the trail continues southwest crossing Big Sandy Creek—about 10 feet (3.0 m) wide and 1 foot (0.30 m) deep—before hitting the Green River. Three to five ferries were in use on the Green during peak travel periods. The deep, wide, swift, and treacherous Green River which eventually empties into the Colorado River, was usually at high water in July and August, and it was a dangerous crossing. After crossing the Green, the main trail continued approximately southwest until the Blacks Fork of the Green River and Fort Bridger. From Fort Bridger the Mormon Trail continued southwest following the upgraded Hastings Cutoff through the Wasatch Mountains. From Fort Bridger, the main trail, comprising several variants, veered northwest over the Bear River Divide and descended to the Bear River Valley. The trail turned north following the Bear River past the terminus of the Sublette-Greenwood Cutoff at Smiths Fork and on to the Thomas Fork Valley at the present Wyoming–Idaho border.
Over time, two major heavily used cutoffs were established in Wyoming. The Sublette-Greenwood Cutoff was established in 1844 and cut about 70 miles (110 km) off the main route. It leaves the main trail about 10 miles (16 km) west of South Pass and heads almost due west crossing Big Sandy Creek and then about 45 miles (72 km) of waterless, very dusty desert before reaching the Green River near the present town of La Barge. Ferries here transferred them across the Green River. From there the Sublette-Greenwood Cutoff trail had to cross a mountain range to connect with the main trail near Cokeville in the Bear River Valley.
The Lander Road departs the main trail at Burnt Ranch near South Pass, crosses the Continental Divide north of South Pass and reaches the Green River near the present town of Big Piney, Wyoming. From there the trail followed Big Piney Creek west before passing over the 8,800 feet (2,700 m) Thompson Pass in the Wyoming Range. It then crosses over the Smith Fork of the Bear River before ascending and crossing another 8,200-foot (2,500 m) pass on the Salt River Range of mountains and then descending into Star Valley. It exited the mountains near the present Smith Fork road about 6 miles (9.7 km) south of the town of Smoot. The road continued almost due north along the present day Wyoming–Idaho western border through Star Valley. To avoid crossing the Salt River (which drains into the Snake River) which runs down Star Valley the Lander Road crossed the river when it was small and stayed west of the Salt River. After traveling down the Salt River Valley (Star Valley) about 20 miles (32 km) north the road turned almost due west near the present town of Auburn, and entered into the present state of Idaho along Stump Creek. In Idaho, it followed the Stump Creek valley northwest until it crossed the Caribou Mountains and proceeded past the south end of Grays Lake. The trail then proceeded almost due west to meet the main trail at Fort Hall; alternatively, a branch trail headed almost due south to meet the main trail near the present town of Soda Springs.