Lassen is famous (or infamous) for establishing the Lassen Cutoff of the California Trail, which left the main trail near the modern-day Rye Patch Reservoir and crossed a desolate section of what is now northwestern Nevada, including the Black Rock Desert. The Lassen Cutoff continued to Goose Lake in northeastern California, and then followed the Pit River into California's Central Valley. Portions of this trail were particularly arduous, and many of its early travelers greatly regretted choosing it. The Applegate Trail also traveled from Rye Patch Reservoir to Goose Lake. (The Applegate Trail was intended as a safer alternative to the main route of the Oregon Trail, and it continued into Oregon's Willamette Valley.)
John D. Unruh, Jr., writes of Lassen's first attempt at using his cutoff, rescued from disaster by a group of well-supplied emigrants from Oregon who helped him reach his ranch:
Here the wily Dane orchestrated a meeting wherein the emigrants supposedly endorsed him as a guide and warmly praised his cutoffs. This deceptive endorsement was rushed eastward to be printed in newspapers and influence credulous forty-niners hell-bent for the gold fields. Planning carefully, Lassen also dispatched agents to divert forty-niners onto the cutoff and to set up trail advertisements (including a signboard at the Lassen Meadows, where the Applegate Trail branched off from the Humboldt River) with the reassuring message that the diggings were a mere 110 miles ahead... [A later] emigration trustingly followed their lead, many foolishly discarding surplus provisions on the assumption that only 110 miles remained. The suffering of those choosing the Lassen Cutoff was severe, for it proved to be some 200 miles longer than either the Carson or Truckee routes.
Lassen was granted the 22,206-acre (8,986 ha) Rancho Bosquejo Mexican land grant in Tehama County, California by Governor Manuel Micheltorena in 1844. On his third expedition to the west, John C. Fremont recorded in his journal that in the spring of 1846 he visited and stayed at the Lassen ranch, noting that Lassen had brought into cultivation wheat, grapes and cotton. In 1855 Lassen moved to the Honey Lake region, where he prospected and served as Surveyor and Governor of the unofficial Nataqua Territory.
Lassen was murdered on April 26, 1859 in Clapper Canyon (then known as Black Rock Canyon) near the Black Rock Desert as he was traveling to Virginia City, Nevada to prospect for silver. He was traveling along with Edward Clapper and Americus Wyatt; Clapper was also killed in the same incident, while Wyatt escaped. The circumstances surrounding his death remain mysterious. According to Wyatt, Lassen and Clapper were shot by an unseen sniper while breaking camp.
At the time the culprits were widely considered to be Northern Paiute, who were then in a state of unrest, which would soon lead to the Paiute War. However, Wyatt himself, Pit River Indians, and disgruntled emigrants who followed the Lassen trail, have also been suspected. In particular, an investigation at the time disclosed that none of the supplies of Lassen, Clapper or Wyatt had been taken; in the perception of the investigator, leaving the supplies was not normal conduct for a Native American raiding party at that time, and, as a result, Wyatt himself has been suspected as the murderer of Lassen and Clapper.
Peter Lassen's grave is in Susanville, California, along the eastern foothills of the Sierra Nevada.
- Unruh, The Plains Across, p. 353.
- Lassen County Historian accessed 2008-01-25.
- Egan, Sand in a whirlwind, pp. 23-24.
- Gannett, Henry (1905). The Origin of Certain Place Names in the United States. U.S. Government Printing Office. p. 182.
- Brown, Thomas P. (May 30, 1940). "Over the Sierra". Indian Valley Record. p. 3. Retrieved 7 May 2015.
- Egan, Ferol. Egan, Ferol (1985). Sand in a whirlwind: The Paiute Indian War of 1860. University of Nevada Press. ISBN 0-87417-097-4. Partial online text of book accessed 2008-01-25.
- Unruh, John D., Jr. Unruh, Jr., John D. (1993) . The Plains Across: The Overland Emigrants and the Trans-Mississippi West, 1840–1860. University of Illinois Press. ISBN 0-252-06360-0.
- Biographical statement by Lassen County historian
- Peter Lassen and His Trail at California/Nevada chapter of the Oregon-California Trails Association (CANVOCTA)
- Peter Lassen at WesternStarWiki
- Peter Lassen at Find a Grave