Modern-day Interstate-5 follows a pioneer route known as the Siskiyou Trail—which stretched from California's San Francisco Bay Area through Tauhindauli Park, to southern Washington. Originally based on existing Native American footpaths winding their way through river valleys, the Siskiyou Trail provided the shortest practical travel route between early settlements in California and the Pacific Northwest.

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The earliest European or American visitors along the Siskiyou Trail were likely hunters and trappers connected with the Hudson's Bay Company who, in the 1820’s, began to travel the rivers of Southern Oregon and Northern California in search of fur and pelts.

The Hudson's Bay Company had established itself in 1824 at Fort Vancouver (in today's Washington State, just north of today's Portland) as its regional headquarters. Hudson's Bay Company trapping parties began to explore south toward California in 1825. A Hudson’s Bay Company trapper named Peter Skene Ogden led exploration and trapping parties south beginning in 1826, reaching the Klamath River in 1827.  A trapping party led by Alexander Roderick McLeod reached the upper Sacramento River in 1828, and in 1829, he led the first Hudson's Bay Company trapping expedition to the Sacramento Valley, which allowed later expeditions to reach as far south as today's French Camp near today's Stockton, California.

These exploring and trapping expeditions were a first use of the Siskiyou Trail by non-Indigenous people.  The Trail linked Fort Vancouver with the Sacramento Valley, and at first the Trail was known by names such as the California Brigade Trail and the Southern Party Trail (it has also been known as the "Oregon-to-California Trail").

McLeod and other members of his parties reported that the Native Americans south of the Umpqua River, along the Klamath and Siuslaw Rivers, had never seen white men before. Although the 42nd parallel (today the boundary between California and Oregon) marked the northern border of Mexican California, the Mexicans knew little about California’s interior—the Spanish and later the Mexicans had limited themselves to missions and settlements along the Pacific coast. As a result, although the Hudson's Bay Company trappers were entering California without permission, they were careful to avoid the coast and ranged south into the Sacramento Valley at will, avoiding the Mexican authorities.

Other Hudson's Bay Company trappers who made early use of the Siskiyou Trail included Michel Laframboise.

In 1834, an American resident in Oregon named Ewing Young came south into California and drove back to Oregon a herd of horses and mules over the Siskiyou Trail from missions in California to sell 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 again drove north over the Siskiyou Trail to Oregon. This monumental task, requiring nearly three months, helped widen and establish the Trail and helped solidify the new American settlements in Oregon.

In 1841, an overland party of the famous United States Exploring Expedition came down the Siskiyou Trail with the first scientists and cartographers in the region.  Their official report notes a soda springs in the vicinity of Upper Soda Springs, and may be the first official mention of Upper Soda Springs.

The California Gold Rush, beginning in 1848, ushered in dramatically increased use of the Siskiyou Trail. The discovery of gold in Trinity County in 1850 and in Siskiyou County in 1851 (especially at Yreka, California) brought thousands more miners along the Trail in search of riches. The terrain was so rugged over the mountains of the Trail that travel was restricted at first to mules and horses. Early travelers were able to travel perhaps 20 miles in a day, stopping at wayside inns and hostels, such as at Portuguese Flat (today's Pollard Flat), here at Upper Soda Springs, and at Sisson (today's Mount Shasta City). It was not until the 1860’s that toll roads usable by stagecoaches were finally carved through the mountains of Northern California, permitting uninterrupted stagecoach travel along the length 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 State, to the San Francisco Bay Area. In northern California, the Trail went through or near modern-day Yreka, Dunsmuir, Redding, Sacramento, and Stockton. In Oregon, the route went through or near modern-day Ashland, Grants Pass, Roseburg, Eugene, Salem, and Portland.

The Trail used the valleys of the Willamette, Umpqua, Rogue, Klamath, Shasta and Sacramento rivers to make the connection between the Pacific Northwest and California, and to traverse the rugged Siskiyou Mountains of Northern California and Southern Oregon. The Trail went past or near landmarks such as Mount Hood, Mount Shasta, Black Butte, Upper Soda Springs, Castle Crags, and Sutter Buttes.

The first telegraph line connected early towns along the Trail in 1864. Between 1869 and 1887, the Oregon & California Railroad Company built a railroad along the southern portion of this route, crossing Siskiyou Summit in 1887--this is the railroad route operated for many decades by the Southern Pacific Company, and in more recent years by the Union Pacific Railroad. 

In the mid-1910’s, the pioneering Pacific Highway, later numbered as U.S. Route 99, provided the first easy automobile access along the path of the Trail. Interstate 5 was built in the 1960's along the route of the original Siskiyou Trail. About four miles north of the California border, and just south of Ashland, Oregon, Interstate 5 crosses Siskiyou Summit, the highest point on I-5 (elevation 4,310 ft). The railroad and interstate highway deviated from the original Siskiyou Trail in small ways according to the needs and engineering available to their builders.