The Siskiyou Trail stretched from California's Central Valley to Oregon's Willamette Valley; modern-day Interstate 5 follows this pioneer path. Originally based on existing Native American foot trails winding their way through river valleys, the Siskiyou Trail provided the shortest practical travel path between early settlements in California and Oregon.
In 1834, Ewing Young brought a herd of horses and mules over the Siskiyou Trail from missions in California for sale at British and American settlements in Oregon. Although this initial effort was met with suspicion by Hudson's Bay Company officials in Oregon, Young returned to California in 1837, where he purchased 700 head of cattle which he drove over the Siskiyou Trail to Oregon. This monumental task, requiring nearly three months, helped widen and establish the trail thereby solidifying the new American settlements in Oregon.
In 1841 an overland party of the United States Exploring Expedition came down the Siskiyou Trail with the first scientists and cartographers in the region.
The California Gold Rush, beginning in 1848, ushered in dramatically increased use of the Siskiyou Trail. The discovery of gold in Siskiyou County and especially at Yreka, California, brought thousands of Forty-Niners along the trail in search of riches. The terrain was so rugged over the mountains of the trail, that travel was restricted to mule trains and horses. Early travelers were able to travel perhaps 20 miles (32 km) in a day, stopping at wayside inns and hostels, such as at Portuguese Flat, Upper Soda Springs and Sisson, in Northern California. It was not until the 1860s that toll roads usable by stagecoaches were finally carved through the mountains of Northern California, permitting uninterrupted stagecoach travel for the length of the Siskiyou Trail.
The first telegraph line connected early towns along the trail in 1864. Development accelerated with the arrival of the Central Pacific Railroad track completed in 1887, which followed the path of the Siskiyou Trail.
The historic route of the Siskiyou Trail extended from the Columbia District headquarters of the Hudson's Bay Company, at Fort Vancouver in southern Washington, to the San Francisco Bay Area. In California the trail went through or near modern-day Redding, Dunsmuir and Yreka. In Oregon the route went through or near modern-day Ashland, Grants Pass, Eugene, Salem and Portland.
The trail used the valleys of the Willamette, Umpqua, Rogue, Klamath, Shasta and Sacramento rivers to make the connection between Oregon and California, and to traverse the rugged mountains of Northern California and Southern Oregon (Siskiyou Mountains). The trail crested at the Siskiyou Summit (elevation 4,310 ft (1,310 m)) just north of the Oregon-California border, and went past or near landmarks such as Mount Shasta, Upper Soda Springs, Castle Crags and Sutter Buttes.