Desert Washes

Desert washes are, of course, the drainage areas in desert terrains.  In an environment in which rainfall is rare, soils can dry up to a dense, caked surface with a packed quality, allowing little water to actually penetrate the soil surface. (Desert plants adapt by producing lateral, shallow roots that are able to catch the surface runoff.)

The surface of the desert overall may be much more consistent with loose, gravelly soils for several feet at the surface and below, but washes carry most of the water away, sometimes in rushing torrents.

The rain water that does fall has difficulty soaking in and usually gains some force flowing downhill, over dry-baked soils and sands, creating rivulets that expand with larger rainfall into rushing rivers and flash floods. They can leave behind loose, gravelly material in the dry “river” bed, or carve a deep groove in baked, compacted soils.

Eventually, runoff scours the lowest lying areas, creating dry beds of loose, sandy soil known as “desert washes.”  They form regularly on slopes in parallel, and carry the runoff to flatter ground.

Desert washes carve into the soil of the desert floor, providing an easier route for water to penetrate all the way to the aquifer below and replenish it. In many deserts, these washes are the only areas in which water is able to penetrate back to the aquifers well below the surface.

In the Mojave Desert of Southern California, Water Departments create their “sinks” in the largest washes. These are areas where water is force-collected and allowed to percolate over time back down to replenish the aquifers.

The preponderance of surface soil in some deserts is too hard and caked to allow for water penetration or even digging, in some circumstances, so rainwater just runs off, creating washes.

A common form of the hardened soil is “caliche,” a term that refers to this form of a natural cement of calcium carbonate (or in some cases, calcium sulfate), hardened over the ages, that aggregates and binds other materials such as clay, silt, gravel, and sand.  It forms when minerals leach down from upper layers or up from lower layers in the soil, begin the clump the soil together, and harden to a cement-like material when dried over time. Desert dwellers will testify to how impenetrable caliche can be.  Early native peoples built structures out of Caliche that have lasted for over a hundred years. A great example is the Casa Grande National Monument in south Central Arizona. The multi-storied structure is hundreds of years old, fabricated from the desert clays.

Normally it can be found close to the surface, but can penetrate quite deep if the conditions are right. Water Districts have found that it can take 300 years for rainwater runoff to penetrate down into the water table/aquifer. Gradually, desert washes carve crevices into the caliche, allowing water both to flow and saturate. Most of the time, washes are the only place that water can penetrate the soil.

Some plants that are able to sink a root deeper than the shallow surface “sand” can take hold and grow to respectable size in desert washes; for example, the Desert Smoke Tree, provided a wild flash flood doesn’t uproot it and carry it off downhill. They enjoy good drainage and mineral nutrients that compose the surface soil of the wash.

Never Camp in a Wash!

Play it safe and don’t camp in desert washes. It is tempting to sleep in the wash because the soil is cleared of debris enough to pitch a tent, the wash itself offers a little protection from the wind, and, because it is somewhat loose, it is easier to put a stake down.

There are several reasons why you should NOT camp in washes. Firstly, desert weather is somewhat unpredictable, and because of the inability of even meager rains to penetrate the soil, the rains quickly turn to runoff which can soon become swift torrents of water, or flash floods.  Even though the wash may be deep (like a riverbed), appear dry as a bone, and seem like it will protect you from the wind, unless you have checked the weather and are quite sure that there is no rain in the forecast for weeks to come, it is unwise to pitch a tent or lay a sleeping bag in a desert wash. Many a desert camper has drowned in unexpected flash floods.

Secondly, snakes emerge from their underground habitats in the late day to find a meal. The wash, unshielded by plants and rocks, provides a warm surface environment for them to retain heat while hunting for mice and other small critters.  In one two-mile hike down a desert wash to the main road, we once encountered five rattlesnakes.  Two were stretched out with lumps in their mid-sections (they had just eaten), while the remaining three were coiled and poised to strike, rattling at us while we passed with a wide berth.  It just isn’t worth an encounter if you can hike up out of the wash.

Desert Soils

Desert soils can be either compacted or loose, quick draining composite soils. The loose sort consists of pulverized rock (sand or small pebble-size rock) and other components that retain the fast draining characteristic.  Crumbled volcanic rock, tiny granite bits, and minerals finely ground down allow water to filter quickly through layers. The roots of desert plants have evolved to spread across the surface and quickly absorb as much water as possible.  Since desert rains can consist of quick flashes of rain lasting only minutes, it’s necessary for the roots of desert plants to be as efficient as possible.

The soil of desert washes is what most people think constitutes desert soil — loose, gravelly, and full of crumbled granitic (and other types of) rock and tiny bits of sand.  It is actually an excellent growing medium to achieve perfect drainage when not required to survive fast-moving water.  These tiny bits of granite, calcium, sands, and other loose, course materials let water flow past and then away from plant roots.

What Soil Do You Need to Grow Desert Plants?

Since plants adapted to the desert grow splendidly in desert soils, why not try to emulate what occurs naturally (that is, if you don’t already live in a desert!)?  Cactus mix is available at the plant nursery, but you can also create some great soils yourself.

Combine compost with any of these:

• lava rock
• agricultural sand (beach sand will likely carry too much salt!)
• perlite
• small pebbles (lots of them)
• crushed rock

Be sure to include lots of crushed or lava rock to keep the mixture particulate. Perlite emulates this very well and is easy to crush into smaller particles if needed for delicate roots.  Make sure that water drains quickly through your mixture. Desert plants require fast drainage!

Desert soils are typically poor in nutrients, so you need not spend a lot of effort and money enriching the soil.  Your original compost component will do just fine.

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