Water in a Dry Land: Joshua Tree National Park, and Getting Tanked

A lake in the desert? That can't be right...

Joshua Tree National Park has a bit of a schizophrenic aspect. Geologicially, the park is divided into vast tracts of metamorphic rock separated from large areas of granitic rock. Topographically, the park is divided into the higher Mojave Desert and the lower Colorado Desert (the High Desert and Low Desert of SoCal weather reports). The elevation and rock differences result in large contrasts in water availability, and thus vegetation; the iconic Joshua Tree is not found in the lower parts of the park, for instance.

The higher parts of the park exceed 3,500 feet, and the climate is cooler and wetter ("wetter" being a relative term), and this higher elevation desert even receives the occasional snow flurry in winter. My vivid memory of January camping at Joshua Tree as a Boy Scout was that of coldness, absolute extreme cold (probably merely close to freezing, but I was a southern Californian; we panic when the temperature drops below 50 degrees).

There's more water in the high desert, enough that the landscape could almost be classed as savanna. These higher elevations supported enough grass that a few ranchers scratched out a living in the region. Cattle need water, but the water doesn't flow at the surface much of anywhere in the park. The ranchers built a few small dams in scattered drainages to catch the runoff from rare rainstorms. These so-called tanks were of varying quality, and some have fallen into such disrepair that they no longer hold water, but others survive. One of the nicer (and popular) short hikes in the park visits Barker Dam in the Wonderland of Rocks area. We had a pleasant stroll there last week, on a sparkling clear day.

With a bit of revegetation effort and pathway control, one can see how grasses could almost support a cow, although the measure would be acres per cow rather than cows per acre. The grazing no doubt put severe pressure on the local population of bighorn sheep, who are occasionally seen in the vicinity of the reservoir.

The trail winds past some unique desert oak trees and a variety of yucca called Parry Nolina, which blooms into the showy stalks seen in the picture above. The bedrock in the area is entirely composed of a variety of granite called monzogranite, which formed deep underground as magma, and cooled slowly to produce the crystal-rich rock. As the granitic rock was exposed at the surface, expansion and stresses caused the rock to fracture along joints which became the focal point of weathering and erosion. The boulders weather more readily at corners and edges, giving them a rounded aspect. The process is called spheroidal weathering.

Closer to the dam, the rock is less jointed, and breaks mostly parallel to the surface, forming a dome-like structure, which is a relatively rare sight at Joshua Tree, and more familiar in places like the Sierra Nevada. This is the process of exfoliation (below).

The little reservoir captures the rare runoff from thunderstorms, and the resulting lake attracts all manner of birds, insects, mammals and other creatures, including tourists.

A look at the dam reveals what probably amounts to a lack of sophisticated dam engineering, but it has lasted for more than sixty years.

The area downstream of the dam is a beautiful landscape of rock and vegetation. A number of petroglyphs and pictographs (some unfortunately recent) can be found hidden in these rocks. The ranchers had probably built on top of a natural reservoir in the rock, and the area has been a source of water for a long time.

So what about these Joshua Trees? Is there anything geological about them? That will be next time...

Water in a Dry Land: Joshua Tree National Park, and why Twentynine Palms is missing a few

It is, of course, a cliche to say that water is life. I can barely imagine a more terrifying thought than to be trapped in the open in a place like the Pinto Basin in Joshua Tree National Park (above) on a scorching hot summer day without a car, without a phone (no coverage anyway), and without water. The Pinto Basin is the rarely visited wilderness end of Joshua Tree National Park, a place far removed from the famous granite boulders of Wonderland of Rocks or Hidden Valley. The Pinto Basin is a vast alluvial plain crossed by two old dirt tracks leading to abandoned mines, and nothing else.

The Pinto Basin was never developed like other desert basins in California because of a lack of available water resources and the arrival of the Great Depression just as speculators were moving into the region. The establishment of the monument sealed the deal and protected the wide basin from any more development.

It wasn't always like this. Studies at Joshua Tree have established that the region was more tropical for millions of years, and that during the Ice Ages a stream flowed through the valley. It was a place where humans could thrive instead of simply surviving. Archaeological work has established the presence of a people (the Pinto Culture) that lived in the basin from around 9,000 to 2,000 years ago.
Water is precious in the desert, and there are precious few sources there today. Ironically, the source of the water that is there is the same as the water the Pinto People used thousands of years ago. The gravels of desert basins like the Pinto act like giant sponges, storing water in the spaces between sand grains in the thousands of feet of loose sediment filling the valleys. Billions of gallons of Ice Age water remain hidden deep in the sediments, revealed only when geological conditions force water to the surface.

California has her faults, and faults are one of the ways that water can be forced to the surface. The grinding along fault zones produces a clay-like material called gouge, which is impermeable to water. Water flowing slowly underground can be dammed by fault gouge and forced to the surface. I visited two of those spots last week.

The first is Cottonwood Springs at the less-visited south end of the park (picture above). Water still flows there, but only a trickle. During the mining era, some 3,000 gallons a day provided the only secure water source for many miles around. The palms were planted in the 1920s (though they are a native species). The cottonwoods were present earlier. Water has been present here for thousands of years, indicated not only by Native American grinding holes in the rock, but also from travertine-like rocks in the area, deposited by water over tens of thousands of years. Needless to say (but I always say such things anyway), the water attracts a wide variety of animal and insect life.

The second "spring" we visited was the Oasis of Mara in an outlier of the park that serves as park headquarters in Twentynine Palms. Early miners called the oasis "Twentynine Palms Oasis", but the present name is a derivation of the Native American name for the spot. The oasis is marked by a line of fan palms along a recently formed scarp formed by the active Pinto Mountain fault (visible in the picture above). I was fascinated by the beautiful little pond in the midst of the healthiest looking fan palms, but I soon realized it was lined with plastic. A bit of research (like reading signs on the nature trail!) revealed that the springs of the Oasis of Mara have been dry for decades. It's not too hard to figure out why. The presence of water led to the establishment of the town of Twentynine Palms, and the town residents started drilling wells right away, drawing down the groundwater table and and drying up the springs. I'm not sure there are actually 29 living palms left today.

The last source of water we saw was a lake, of all things, hidden among the granite boulders. That's a unique Joshua Tree story for another post.

I've used three sources of information about the park beyond my own observations:

Cates, Robert, 1995, Joshua Tree National Park: A Visitor's Guide, Live Oak Press, 100 pages.

Trent, D. and Hazlett, R., 2002, Joshua Tree National Park Geology, Joshua Tree National Park Association, 64 pages.

Eggers, M, editor, 2004, Mining History and Geology of National Park, San Diego Association of Geologists, c/o Sunbelt Publications, 119 pages.