“The only thing that is constant is change.” ― Heraclitus

In our early pursuit of knowledge, or an understanding of the earth and our place in it, there was a belief that the earth as we know it now and the life that calls earth home has always been so. The belief was based the idea that the earth was young, that there was not enough time to create mountains and then erode them into sand. Of course, we now have a better understanding of the antiquity of the earth, and that time has not limited the great changes that have happened and continue to happen.

Sand dunes are iconic, dynamic, desert landscapes. Some deserts, like Africa’s Sahara and the Namib, are dominated by sand dunes, stretching hundreds of miles. Others, like those in the Colorado and Mojave Deserts, are isolated islands of moving sand. Perhaps surprisingly, sand dunes are born from water; rivers eroding rocks into grains of sand of just the right size to be moved by water and then to be blown into dunes by strong winds. In the Sahara, it’s the Nile. In the Namib it’s the Orange River. For most of the Mojave and Colorado Deserts it’s the Mojave and Colorado Rivers. For the Coachella Valley, it’s the Whitewater River.

To build a sand dune, ingredient number one is a sand source, lots and lots of sand, and rivers are sources, sorters, and conduits of sand. Ingredient number two is wind, lots, and lots of wind. Grains of sand that build dunes are typically 0.1-0.5 mm in diameter (mostly 0.2-0.3 mm). Any smaller and the wind blows the particles away and up into the atmosphere. Any bigger and the winds cannot move them into dunes.

If the dominant winds are mostly from just one direction, the dunes move in that direction and form crescent shapes called barchans. The points at each end of the crescent point in the direction of the dominant wind flow. If the winds shift seasonally, one direction in one season, and then the opposite direction in another season the result is more stable linear dunes, long rows of sand ridges. If the winds shift with three or more directions, the result is octopus shaped dunes called star dunes. Star dunes, like linear dunes tend to be more stable in that the broad structure as seen from far above does not move nearly as fast as barchan dunes, however their surfaces are constantly being reworked by the wind, moving from one side to the other and then back again.

The third ingredient for building a sand dune is aridity. Wet sand is hard for the wind to move and if the sand surface is wet it can foster dense plant growth that will stabilize the dunes reducing them to mere piles of vegetated sand. The reason sand dunes are iconic deserts landscapes is, well, deserts are dry. But that does not mean dunes are only in deserts. Dunes can also form along seacoasts. Most often there is a large river nearby supplying sand to build beaches, and if there is a super abundance of sand, and an abundance of wind, then the wind can dry out the sand surface and allow sand to be blown across the beach and into dune structures.

If you think about the three sand dune ingredients like variables in a mathematical equation, then sand + wind + aridity = sand dunes. If the climate is less arid, then to get sand dunes to form you need more wind and more sand. If the climate is desert-like, then with somewhat less sand and less wind, you can still build dunes. But if the desert winds are too strong, the result might be sand sheets only forming small dune structures behind shrubs where the wind velocity is slightly less, structures called dune hummocks. Only under just the right combinations of sand, wind, and aridity, the “Goldilocks zone," will large dune structures form.

Before dams were built on the Colorado River, the river periodically flooded in colossal proportions. Floods volumes were too large to be confined within the Colorado’s banks, and so water and the sand it carried spread onto the surrounding deserts. Colorado is a Spanish word for “red,” and when the Colorado was at flood stage it was carrying reddish colored sands eroded from the walls of the Grand Canyon. The flood waters and sand would fill low-lying desert basins, forming temporary lakes. When the water eventually evaporated, the dry sands were left to be carried by the wind and with enough sand and enough wind, sand dunes formed. The Algodones Dunes at the southeast end of the Salton Trough and the Gran Desierto dune fields at the northwest corner of the state of Sonora, Mexico, at the head of the Gulf or California, formed this way. The Kelso Dunes in the heart of the Mojave Desert were created by a similar process when the Mojave River flooded. Much of this dune-building occurred during the Pleistocene Epoch, wetter times, but without those dams it would still be happening today, albeit less frequently. All the sand dunes in North America were built of sands eroded from sandstone with the color of each dune system reflecting the colors of the rocks where sands originated, except one. In the Coachella Valley the flooding of the Whitewater River erodes the igneous granite batholith that is Mount San Gorgonio and the San Bernardino Mountains, sands that then form the lion’s share of the Coachella Valley sand dunes. Smaller but nevertheless significant other sand sources contributing to the development of the Coachella Valley dunes include flood events from Mission Creek, Long Canyon, and Thousand Palms Canyon, all eroding igneous rocks to create the unique Coachella Valley dunes. These sands are nearly white.

Sand dunes are beautiful. With early morning and late afternoon light the dunes take on a sensuous glow. Sand dunes are in constant motion, much like an ocean in slow motion, with their surface being reworked each night as evening winds exert their power. With stronger seasonal winds the whole dune will move. Aridity and constant motion challenge life to exist on sand dunes, but it would be the height of ignorance to then assume that dunes are lifeless. As it turns out the unique challenges that animals and plants face to exist on a sand dune result in those dunes becoming crucibles for evolution. Lizards evolving structures enabling them to run exceedingly fast on loose sand, to dive headfirst into the sand without scratching their corneas, without ingesting sand, and being able to stay there, submerged in the sand for days, weeks and months, breathing air trapped between sand grains without letting a single grain enter their lungs, is an amazing testimony to the power of evolution. More so because these kinds of adaptations have evolved independently in every dune system worldwide. Every dune system in the Mojave and Colorado Deserts has a different array of beetles, spiders, crickets, plants and lizards, and each dune system provides habitat for at least some species of plants, arthropods, and/or reptiles found only on those dunes and nowhere else on earth. Multiply that again for dunes in Africa, Australia, Asia, and South America. Animals and plants not only exist on desert sand dunes, they thrive there. Even though the surface of a due is typically bone dry (exceptions being after relatively rare rain events or in the fog deserts of Namibia), below the surface sand dunes hold on to what little rain falls each year, hydrating the roots of those plants that adapted to life on dunes and providing a world of year-round near 100% humidity for those animals that burrow down just a foot or two below the dune surface. Both conditions that are scarce in desert habitats away from dunes.

By any measure, sand dunes deserve to be protected. However, protecting a sand dune requires protecting each ingredient that is essential for their existence: sand + wind + aridity. Protecting sand means allowing rivers to flood and to erode rocks to make more sand. It means protecting corridors for sand to flow from rivers into arid plains where dunes can form, grow, and move. It also means keeping those dunes from being trampled by buggies and motorbikes. Protecting wind means not creating wind barriers, such as rows of non-native tamarisk trees or roadways with fences or other sand-stopping structures. These are all tall orders. Flooding rivers and scouring from wind-born sand grains create safety and health risks for people and their property. Yet protection can happen. Careful planning, maintaining, and protecting sand corridors as well as the dunes themselves, elevating roads that allow sand to move beneath, and educating local communities so that they understand and value the habitat dunes provide, are all parts of a sand dune protection equation.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.