Political boundaries usually don't mean much in the natural world. The boundaries of the state of California provide a good example: of more than 2,000 miles of state line delineating California, only about 220 coincide with a natural geographic feature -- the Colorado River's course from just north of Needles to just north of Yuma. (Though the state's 900-plus miles of coastline might seem to qualify, the state's marine boundary actually lies about three miles offshore, and is as arbitrarily chosen as the long straight lines that make up the rest of California's boundary.)
The boundaries of natural regions such as deserts and mountain ranges are somewhat arbitrary as well, but in a different way: the regions do represent something that really exists outside of cartologists' imaginations, but the actual boundaries can be a little hard to pin down. Do you follow landforms? The ranges of the region's characteristic plants and animals? The underlying bedrock or hydrological patterns? Some combination of the above? There are no easy and obvious answers.
Take the Mojave Desert. In the west and south, there are boundaries that readily suggest themselves: the neat angle between the San Gabriels and the Tehachapis that forms the Antelope Valley, for instance -- or, if you prefer, the same angle between the San Andreas and Garlock faults, which are why those ranges are there in the first place. Even as far east as Joshua Tree National Park's western end, the Mojave's boundary seems pretty sharp: you pretty much enter the Mojave -- and leave the "low desert" -- when you reach the little hill south of Morongo Valley on Route 62.
Still, it isn't as easy as it looks to figure out what point marks the beginning of the Mojave Desert even in places with big obvious boundaries. Joshua trees are often offered as an indicator species for the Mojave -- if they grow naturally in a spot, that spot's in the Mojave Desert -- but Joshua trees grow on the wrong sides of the boundary in quite a few places. Along I-5 south of Gorman, for instance. Or west of the Sierra Nevada crest near Walker Pass.
East of Morongo Valley, along the spine of the Little San Bernardinos that make up the highlands of Joshua Tree National Park, the boundary pretty much falls apart. The National Park Service offers a map of the Park with the boundary between the Mojave and the Colorado Desert -- California's portion of the Sonoran Desert -- delineated with a nice dashed line. That line does correspond pretty closely to the limits of the Joshua trees in the park, but aside from that if you stand on the line the desert looks pretty much the same for miles in either direction.
Farther east it gets even worse. Entire desert valleys sit uncomfortably in what's called a "transition zone" between the Mojave and Colorado deserts -- or at least they would be uncomfortable if they paid attention to the mapmakers, which they do not. Mojave Desert plants and animals grow right next to Sonoran Desert plants and animals in the desert valleys north of Interstate 10 in California. On the other side of the Colorado River, even the indicator species stop cooperating with the mapmakers. There are places in Arizona where saguaros -- the signature tree of the Sonoran Desert -- grow interspersed with Joshua trees, the signature tree of the Mojave.
Fuzzy boundaries like this are pretty much the only kind that exist in nature. The easier it is to draw a precise line surrounding a thing, the less likely that thing is to be real. That's not to say the fuzzy boundary doesn't exist. The Mojave Desert does have real boundaries. Barstow, Lancaster and Baker are inside those boundaries. Indio, La Cañada-Flintridge, and Bakersfield are outside them. Wrightwood, Tehachapi, and Needles? It depends who you ask.
I write here in the Back Forty about the California Desert, and I suspect most readers understand that it's a term of convenience. But it's useful every so often to remind oneself that while the various deserts that make up the eastern part of the state are real, physical things with fuzzy boundaries, the state of California is more a shared social assumption than a physical thing, and that the desert doesn't end on the other side of that "Welcome" sign with the coastal California poppies on it. People do forget that. Clark Mountain in the Mojave National Preserve tops out at 7,933 feet; a couple dozen miles north is Mount Charleston, almost 4,000 feet taller. But Mount Charleston is in Nevada, and Clark Mountain is just inside California, and so Californians often refer to Clark Mountain as the "tallest mountain in the Mojave Desert," even if they really ought to know better.
Even the "Colorado Desert," which is a California Desert pretty much by definition, includes quite a number of square miles of desert in adjacent Baja California, Arizona and Sonora. California hosts portions of three of North America's four great deserts -- the Sonoran, the Mojave, and the cold Great Basin desert to the north -- but shares each of them with other states, and in the case of the Sonoran, a number of those states are in Mexico.
As for the Mojave, a majority of it is indeed in California, and it's the Mojave that most non-Californians picture when they think of the California desert. But the Mojave Desert also occupies the southern third of the state of Nevada, a corner of Washington County in Utah, and a broad swath of northwestern Arizona. The rounded granite boulders of Joshua Tree National Park and the eroding Miocene cliffs of Red Rock Canyon State Park are defining places in the Mojave Desert, but so is the edge of the Mojave near Kingman, where the Mojave slowly fades out in the face of the Mogollon Rim, and the slopes around Beaver Dam Wash in Utah, where Joshua trees and blackbrush grow out of Colorado Plateau red rock, and the Tikaboo Valley in Central Nevada, tucked into a corner of Area 51, the one place on earth where the two subspecies of Joshua trees grow together.
There are places in the Mojave where good dirt roads wind around mountains astride a state line, or two, and you can park with your front wheels in a state governed by Mormons and your rear wheels in a state governed by Mammon, and everyone who cares about that imaginary mapmaker's line is in a state capitol far outside the Mojave's real but fuzzy boundaries. In 500 years California, Nevada, Arizona and Utah may be dimly remembered trivia, but the Mojave will still be the Mojave.
Chris Clarke is an environmental writer of two decades standing. Director of Desert Biodiversity, he writes from Joshua Tree regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.