The Colorado Desert

Only one million years ago - a blink of an eye in geologic time - the desert lands of Anza Borrego State Park were a cool temperate land of open woodlands, riparian waterways and lakes of varying sizes. During that time, now-extinct species of mammoths and camels grazed the open woodlands and a host of migratory birds flocked the waterways. In the riparian corridors, the vegetation was lush with buckeyes, willows and Washingtonia palms while oaks, laurels and bunch grasses grew on the drier elevated landforms. One million years ago, it was a pluvial period in the time of the Pleistocene. 

​During the Pleistocene (approximately 2.5 mya to 11,700 mya), the climate cycled between different periods of cold-wet and warm-dry. During cold-wet pluvial periods, glacial ice covered portions of the Sierra Nevada as well as other scattered peaks in northern California. Rain was also abundant throughout California, including southern California. The abundant rainfall, in conjunction with extensive runoff, provided southern California with a network of Pleistocene era rivers and lakes. 

If you were to look at a satellite image of the Mojave and Colorado Deserts, you would see the dried lakebeds of these former Pleistocene era lakes. Death Valley, often considered the hottest and driest region of California, is a famous example. During the Pleistocene, Lake Manly covered Death Valley to a depth of 275 meters. In fact, many desert valleys throughout the Mojave and western North American deserts are sites of now dried Pleistocene era lakes.

If you were to look at a satellite image of southeastern California today you would also notice a very large prominent lake body still persists. The Salton Sea is California's largest lake, covering approximately 525 square miles in the Coachella and Imperial Valleys of southeastern California. But the Salton Sea is not a Pleistocene era lake. Rather, it is an accident. The Salton Sea was created in 1905 by poor management of the irrigation canals that were diverting water from the Colorado River for agriculture in the Imperial Valley. In the spring of 1905, a combination of sediment choked irrigation canals, heavy rainfall, and vast snowmelt within the Colorado River Watershed caused the Colorado River to spill over into the dry arroyos and washes that lined the river. Utilizing these paths of least resistance, the Colorado River began re-directing its flows into the lower lying Imperial and Coachella Valleys. This eventually formed the Salton Sea. The irony of this hydraulic accident is that it was not the first time the Colorado River had re-directed its flows to create an inland lake.

The Salton Sea was preceded by Lake Cahuilla. Lake Cahuilla was actually a Pleistocene era lake that once occurred in the Coachella and Imperial Valleys of southeastern California. It too was very large. Lake Cahuilla stretched from the Coachella Valley at its northern shore, down through the Imperial Valley and terminated in the very northern portion of Mexico. It roughly covered an area of 2,200 square miles. Today, the Salton Sea occurs within the footprint of former Lake Cahuilla.

​Much of the water that filled Lake Cahuilla didn’t necessarily come from the surrounding hills and mountains. While several local streams and rivers supplied Lake Cahuilla with water during those pluvial periods, much of its water came from an outflow of the Colorado River. Over the course of its life, the Colorado River has naturally meandered back and forth as its flows varied. These variations in river flow also resulted in different rates of sedimentation within the Colorado River Delta. Consequently, the Colorado River was pushed around typical to how many river deltas function. At various points, the Colorado River re-oriented itself northward where it drained into the low-lying Coachella and Imperial Valleys, eventually leading to the formation of Lake Cahuilla.

​Lake Cahuilla disappeared about 500 years ago. Had it not been for the meandering flows of the Colorado River, Lake Cahuilla would have likely dried up many thousands of years prior. Regardless, from the inception of Lake Cahuilla to the present day, the climate of southeastern California has changed dramatically. Today, southeastern California is one of the most arid portions of California. Where lake bodies and lush riparian corridors once existed during the Pleistocene, today they are the dry arroyos and washes that traverse a desert landscape. Badlands trace where water once flowed and alkali sinks mark where lakes once developed. While the landscape that is visible today may not look like the landscape it once was, it looks as it does precisely because of the past. The present is only an accumulation of that past.

The area covering present day southeastern California, including the Borrego, Coachella and Imperial Valleys, is the home to the Colorado Desert. The full expanse of the Colorado Desert is bound by the Peninsular Ranges on the West, the Colorado River and Sonoran Desert on the East, and the Mojave Desert on the North. Technically, the Colorado Desert is a subset of the greater Sonoran Desert. What distinguishes the Colorado Desert from the adjacent Mojave and Sonoran Deserts is its low elevation status, also making it one of the hotter desert regions in California.

The arid deserts of southern California, including the Colorado and Mojave Deserts, are relatively new ecoregions in the grand evolution of California. However, the genesis of these ecoregions still trace back millions of years. Back in the Miocene (approximately 20 mya) California's climate was warmer and wetter. During this time, California received ample summertime moisture with no real occurrence of winter frost. These warm and wet conditions supported various species of Oaks (Quercus spp.), Pines (Pinus spp.), Cypresses (Cupressus spp.) and Buckeyes (Aesculus spp.). Also growing during this time were the thorn-scrub species, which had small, finely divided leaflets and prominent thorns. Many of these thorn-scrub species occurred on the dry, rocky, and low-lying slopes. Those thorn-scrub species included various species of Acacia, Bursera and Cercidium.

Following the end of the Miocene and onward into the Pliocene (approximately 5 mya to 2.5 mya), California's climate transitioned to a cooler and drier climate regime. This was largely due to a developing cold water current off California's west coast. As such, precipitation, especially warm season precipitation, began to decrease. During this time much of the warm and wet adapted vegetation had to retreat south towards the subtropics. However, a number of moderately drought tolerant species, notably the thorn-scrub species, remained in southeastern California. Those drought tolerant species were well adapted and able to sustain themselves under the drier climate. And then came the Pleistocene.

The Pleistocene (2.5 mya to 11,700 years ago) cycled between periods of cold-wet and warm-dry. Additionally, uplift of the Sierra Nevada, Transverse and Peninsular mountains began to create a rain shadow over eastern and southeastern California. The developing rain shadow only exacerbated the drier times. It also meant southeastern California still remained "relatively" dry, even during those cold-wet pluvial periods. During cold-wet pluvial periods, the relatively dry conditions over southeastern California proved essential for retaining a number of dry adapted plant species. Thus, southeastern California remained a refuge of sorts. For those dry adapted plants that could tolerate colder conditions, they found themselves well suited for the arid environment that would later develop.

Since the end of the Pleistocene, the climatic conditions throughout southeastern California have only become hotter and drier. This increasingly hot and dry climate eliminated some plant species altogether. Other species were forced to find ecological niches that augmented the changing conditions.

Species like California Fan Palm (Washingtonia filifera), California’s only native palm tree, is now entirely restricted to perennial water sources within the California deserts. These perennial water sources occur where geologic fault lines allow subsurface ground water to seep to the earth’s surface. California Fan Palm, likely once widespread throughout southeastern California, would have quickly perished if it were not for these seeps. On the other hand, plant genera like Eriogonum and members of the Asteraceae family have embraced the new arid conditions. Various Eriogonum species and members of the Asteraceae family employ genetic traits that are exceptionally tailored to a life under aridity. Their genetic fitness has not only sustained their existence, but has also triggered speciation events and helped them expand under the growing aridity. And then there are species like Elephant Tree (Bursera microphylla) and Octotillo (Fouquieria splendens) who spend months on end waiting for rain. These species have sophistically calibrated metabolic systems that enable them to withstand months of desiccating heat and evapotranspiration. They bank on the prospects that monsoonal rains will come again. And when such rains do return, they quickly flush themselves in green and resume photosynthetic growth.    

All in all, the flora of the Colorado Desert is a rich flora precisely because of past climatic shifts. It is likely that California’s climate will only get hotter and drier under the Anthropocene. And how will this landscape and its flora respond? It's a fascinating thought to ponder when stumbling across lush springs or a palm oasis that verdantly contrasts with the rocky, dry terrain. A great reminder that the present never is without the past.

Additional Reading

The Jepson Desert Manual: Vascular Plants of Southeastern California by Bruce G. Baldwin, Steve Boyd, Barbara Ertter, Robert Patterson, Thomas J. Rosatti, Dieter Wilken, & Margriet Wetherwax

Thornhill, A.H., Baldwin, B.G., Freyman, W.A. et al. Spatial phylogenetics of the native California flora. BMC Biol 15, 96 (2017).

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