One of the most familiar figures in the history of the West was the explorer and politician John Charles Frémont (1813-90). At the age of 25 he was commissioned as a lieutenant in the U.S. Army to accompany an expedition exploring and mapping the region between the upper Mississippi and Missouri Rivers. Frémont went on to lead five well-publicized expeditions during the 1840s and early 1850s exploring the lands stretching from the western Great Plains, across the Rocky Mountains, and to the Pacific Ocean. The published accounts of his expeditions, written by Frémont and his wife, Jessie Benton Frémont, were engagingly written, and did much to stimulate American settlement in these lands, both before and after their acquisition by the United States. (The Annexation of Texas in 1845 was followed by the Oregon settlement with Great Britain in 1846, the Mexican Cession in 1848, and the Gadsden Purchase in 1853. These annexations comprised all of the modern states of Washington, Oregon, Idaho, California, Nevada, Utah, Arizona, New Mexico, and Texas, and major portions of Montana, Wyoming, Colorado, Kansas, and Oklahoma.)

The status of the lands north of the Mexican territories of California and New Mexico, known as the Oregon country, had long been a source of friction between the United States and its traditional enemy, Great Britain. Valued for its furs, timber, and other natural resources, Oregon was also seen as an ideal location for agricultural development. British fur trading in the area under the auspices of the Hudson's Bay Company, secured by a handful of small forts, had been going on since the eighteenth century. The Lewis and Clark Expedition of 1804-06 pioneered the American presence in the area, and John Jacob Astor's American Fur Company established its own outpost, Astoria, at the mouth of the Columbia River in 1811. Increasing familiarity with the region stoked interest in the well-watered, fertile, and temperate valleys of the Columbia River basin in the 1820s, and by the mid-1830s the Hudson's Bay Company had settled about 1,000 non-Indians (mostly French Canadians) to cultivate Oregon's Willamette Valley. American settlers soon poured into the region, overwhelming the numbers of British settlers by the end of the 1830s, and creating a diplomatic crisis that was to fester until resolved by a partition of the region between an American Oregon Territory and a British Columbia was achieved in 1846.

Frémont's reconnaissance of the "Road to Oregon," achieved in two stages, on his first (1842) and second (1843-44), was organized in part to help secure American control and settlement of the Oregon territory. Though strongly associated with the more famous Frémont, the map was actually compiled and drawn by the surveyor and topographer, Charles Preuss, who accompanied Frémont both in 1842 and 1843-44. It was published in 1846, while Frémont was off on yet another expedition. Preuss's road map of the trail was the first large-scale description of the route. Congress published the map in large numbers-perhaps anticipating the need for an accurate guide to the trail. Yet, strangely none of the many accounts of travel on the Oregon Trail mention its actual use as a guide map. Its seven large sheets, drawn at a scale of ten inches to the mile, nevertheless provide an exquisitely detailed impression of the landscape of the American northwest and of early non-Indian impressions of it.

The trail itself had many branches, cutoffs, and alternatives, but nearly every traveler to Oregon followed the roughly 280 miles of this portion of the route essentially as they are laid out in Preuss's Sheet 6. At the eastern (right) end of the sheet is Fort Hall (near modern Pocatello), established in 1834 by the American fur trader Nathaniel Wyeth and a major layover and safe haven for the emigrants. At the western end of the sheet is Fort Boise, an outpost of the Hudson's Bay Company also established in 1834 in response to the threat to its interests posed by Fort Hall. In between, the Snake River (known then as the Lewis Fork) provided a natural, if treacherous passage through a landscape of gulleys and deserts that remains to this day the most important route through southern Idaho.

Traders, mountain men, and missionaries had used this portion of the Oregon Trail route since the Astorians went through here in 1811-12. Migration by larger numbers of American settlers began in earnest in 1841, growing to an annual total of 4,000 in 1847. In all about 12,000 settlers are believed to have used the route to Oregon in the 1840s. The relative importance of migration to Oregon was eclipsed by the rush to California after gold was discovered there in 1848. The California Trail was virtually identical to the Oregon Trail, however, until it crossed the Raft River, which appears on our map where Fremont camped on September 26-27, 1843. From here, they would head south, towards northern Nevada and the Humboldt River Valley, which would lead them to Sierra Nevada Mountains and California. As many as 50,000 are thought to have traveled along the route between Fort Hall and the Raft River for various points west in the summer of 1850 alone.

According to Preuss's explanatory note at lower left, "This was the most trying section for the traveller on the whole route. Water, though good and plenty, is difficult to reach, as the river is hemmed in by high and vertical rocks." Grass, a necessity for feeding pack animals, was also difficult to find here, and game nonexistent, though salmon could readily be acquired from local Indians just downstream from (west of) Fishing Falls, which lies at the center of the map. "Lucky that by all these hardships," Preuss writes, "the traveller is not harrassed by the Indians, who are peac[e]able & harmless." Indeed, the early history of Indian and white relations along this stretch of the trail was generally one of peace. This changed as the threat to the Indian's security and livelihood increased. Two notorious skirmishes between emigrants and Indians occurred between Forts Hall and Boise. The first, known as the Ward Massacre, occurred in 1854 on the portion of the trail that follows the Boise River, near Frémont's camping place of October 8-9, 1843. Eighteen of twenty emigrants on a single wagon train were killed, resulting in bloody reprisals. The second occurred in August 1862 near the mouth of Rock Creek, below American Falls, resulting in the death of five emigrants at what is now known as Massacre Rocks.

Preuss's expressive use of hachures gives us some appreciation of the features on either side of the river-broken plains and deep ravines on the south and higher cliffs, rising to mountains on the north. Small drawings of tents mark the location and date of the nightly encampments of Frémont's party through this region. We learn, for example, that the party rested at Fort Hall for three nights (September 18 - 22), having already logged 1,180 miles from the trail's origin at Westport Landing, Missouri (in modern Independence).

Several excerpts from Frémont's official report on the expedition appear on the map commenting on the landforms, vegetation, geology, soil, weather, and Native Americans they encountered. A meteorological chart records the daily temperature readings and weather observations, so that a fairly complete picture of the party's experiences on this stage of their journey can be gathered from the map. Drawings prepared on the spot were attached to Frémont's report. (These may be viewed by clicking on the appropriate places on the core map.)

The Hot Springs Frémont mentioned in his entry of October 5, 1843, do not flow anymore during the summer months because the demand for irrigation water has depleted their source. The springs are a few miles due east of the town of Mountain Home, Idaho.

Fort Boise (Boisée) guarded an important but difficult fording place on the Snake River as the trail it crosses from the modern state of Idaho into the modern state of Oregon. This fort should not be confused with another Fort Boise, established in 1863 by the U.S. Army to guard the new settlement of Boise, future capital of Idaho. This second Fort Boise lay some 40 miles east of the old one, approximately where Frémont camped on October 7-8, 1843.