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