December Ephemeral Drainage Flow

A dry wash the morning after a flash flood came though.

The mid-December rain is the most reliable rainfall we receive here in the Sonoran Desert.  This rainstorm is almost like clockwork.  Every December, usually around the 15th or so, a strong Pacific frontal storm system brings rainfall in from the northwest.  One-half to one inch of rain pretty much falls across the entire desert with higher totals in the mountains.  Of the past ten Decembers, only one failed to produce any rainfall and that was during one of the driest winters on record in Arizona.  This year was picture perfect with one-half to one inch of rain falling in the Sonoran Desert between December 13th and 15th.  With this rain being almost like clockwork, the flow of the normally dry washes also flow during this rain almost like clockwork.  This year was a little odd in that the rain was spread out over a three day period making flows a little weaker than normal.  Typically, dry washes require a significant amount of rain over a short period of time in order to generate enough runoff to supply a flow.  A lot of drainages did flow at least a little though. 

Of course, a lot of rain over a short period of time helps these washes to flow in the desert, but there are other factors involved also.  Geology, or geomorphology, are probably the most important factors in determining flow.  Geomorphology is simply a scientific term that describes how landforms came about and how they function.  One of the functions of geology and geomorphology in the landscape is to determine how and where water flows.  For example, shallow unbroken bedrock is going to prevent water from seeping down into the soil and therefore will result in greater amounts of runoff.  Type of soil also matters in the amount of runoff produced.  Rain seeps very slowly into clay soils so a lot of runoff can be generated.  Sandy soils however can quickly absorb a lot of rain so not much will runoff.  Number of rocks also makes a difference.  Soils with fewer rocks have more runoff than soils with more rocks.  Rocks on a soil surface slow the speed of runoff and with slow speeds of runoff the water has more time to be absorbed into the soil.  Size of the dry wash also makes a difference with smaller washes flowing more frequently than larger washes.  However, larger washes tend to run longer than small washes when they do flow.  Larger washes simply need a lot more water to flow. Depending on the combination of these factors some washes will flow a few times annually while others will only flow a few times a decade.

All of these things factors also determine what lives where along a dry wash.  Flow is normally very short in duration in a wash.  This is simply because flowing water quickly is lost as it is absorbed into the sediments of the stream bed.  Though flowing water is lost, the water is not entirely lost.  Water is stored in these sediments for long periods of time after surface flow ends.  Depending on the depth of this moisture and the depth of the sediments differing plants will occupy the area.  Typically, deep sediments with relatively frequent flows will be occupied by blue palo verde and desert willow.  Areas of fewer or shorter flows typically have yellow palo verde.  Other plants such as acacia's, ironwood, wolfberry, and mesquites can be somewhere in-between. 

Fall Leaves in a Sonoran Desert Riparian Zone

A Sonoran Desert riparian area in fall along Cottonwood Creek.

The desert is most definitely not known for spectacular fall colors.  Fall colors do however, find their place along some of the wetter desert water courses.  If perennial water sources are available, even if it is hidden below ground a short distance, the roots of large deciduous trees will find there way to it.  Sycamores, cottonwoods, and willows are all relatively common along streams and washes with perennial sources of water.  Even Arizona walnut and ash trees can be found in some of the more stable riparian zones.  These trees do not display the brilliant hues of red and orange common to eastern forests but do show off bright yellows that are in stark contrast to the dried out browns and greens of the desert.    Desert fall leaves are quite a rarity and are quite unique.  Typically, perennial water sources are considered perched water tables.  A perched water table simply is water that accumulated above the surrounding water sources, most often a result of bedrock that prevents water from penetrating deeper into the soil and out of reach of plant roots.

A recent hike I took demonstrated this concept extremely well.  The hike was along Cottonwood Creek near Lake Pleasant north west of Phoenix.  The majority of this hike is along Cottonwood Creek, which really isn't much of a creek considering water only flows in this creek a few hours every year.  The rest of the year the wash remains mostly dry, except for a few locations.  Nearly all washes in the desert are called dry washes, and for good reason: they are completely bone dry the majority of the year.  A few washes, such as Cottonwood Creek are fortunate enough to have areas that always remain wet.  Cottonwood Creek owes this moisture to its underlying geology.  First off, the creek bed lays at the base of two small bajadas between two small mountain ranges.  One bajada lays to the north of the creek bed and one to the south.  These bajadas and bedrock of the mountains are relatively steep and provide ample runoff to Cottonwood creek so it will run during periods of heavy rainfall.  Moisture is quickly lost into the deep sediments of the bajada and placed out of reach of deciduous tree roots.  In areas where bedrock are shallow though, moisture cannot penetrate deeply and remains closer to the surface within reach of plant roots.  Bedrock can also push water flowing underground towards the surface.  At these locations large deciduous trees take advantage of the shallow moisture and can in a few places form small but beautiful wooded areas.

Wildlife may not be obvious in these small wooded areas, but if you look at the ground you are sure to see evidence of animals.  Javelina and mule deer heavily utilize these small areas and their hoof prints are normally abundant.  In some area, such as along Cottonwood Creek, wild donkey's are also abundant and heavily utilize these areas.  The abundance of shade, food, water, and cooler conditions during hot dry summers gives great value to these areas for every creature.