Sequoia and Kings Canyon National Parks form a contiguous reserve of 349,525 ha located on the western slope of the south-central Sierra Nevada of California. This reserve ranges in elevation from 400 m in the low western foothills to 4,418 m on the crest of the Sierra, and is composed largely of rugged, mountainous terrain; over 70% of park lands are above 2,500 m elevation. Three major river systems -- the Kings, Kern, and Kaweah -- originate within and drain the reserve; additionally, the northernmost boundary of Kings Canyon National Park includes a portion of the headwaters of the south fork of the San Joaquin River.

Yosemite National Park is a 302,768 ha reserve located in the central Sierra Nevada. It ranges in elevation from 640 m in the Merced River Canyon to 3,997 m atop Mt. Lyell. The park is comprised of two major watersheds, the Merced River in the south and the Tuolumne River in the north, and completely encompasses the headwaters of each. Two-thirds of Yosemite lies between 2,100 and 3,050 m, resulting in a higher proportion of forested terrain than the southern parks, and a lower proportion of alpine.

The vegetation of all three parks is quite varied, reflecting the substantial environmental heterogeneity created by a large elevation gradient. Vegetation types include chaparral, oak woodland, upland hardwood forest, conifer forest and woodland, meadows, and alpine plant communities. The parks support rich and diverse vascular floras with 1,365 taxa in Sequoia and Kings Canyon and over 1,400 in Yosemite. Of these, 179 (7.5 %) are introduced in Sequoia and Kings Canyon and 130 (11%) in Yosemite with approximately 46% overlap between the two noncontiguous areas.