Tree Roots Excavated By Flood Waters (un-edited)

The tree in this un-edited photo was the result of a rain storm 4 months ago, that could be considered a 100 year event (A one-hundred-year flood is a flood event that has a 1% probability of occurring in any given year.) The flash flooding was tremendous in this arroyo as the tree roots that are unearthed in this photo attest. The colors on the tree are most unusual and are the direct result of two days of lazy rainy days, of which only happens in our high desert country once every five years if we are lucky.

Arroyo is a Spanish term for stream, but in the Southwest the term is commonly applied to streambeds that are dry most of the time. Some geographers have tried to restrict arroyo to streambed shapes that are eroded narrow and deep as opposed to washes that are wide and shallow, but some streambeds alternate between the two shapes either from year to year or along their courses downstream. Streambeds with eroded vertical banks evoke negative reactions from most viewers, and arroyos are commonly seen as a symptom that something is wrong and that someone or something is to blame. The natural function of arroyos is complicated and depends on several independent and linked variables of landscape, climate, vegetation, and land use.

The primary reason streambeds exist is that water passes from higher in the drainage basin to the mouth of the drainage basin. Water moving downhill has energy to transport loose soil particles (sediment) and does work to do so. The size and shape of the streambed is a direct reflection of the amount of water (both quantity and duration of flow), the energy gradient, and the characteristics of the sediment along the stream channel as well as resistant features along the path, such as bedrock and vegetation. Arroyos tend to respond quickly to precipitation and have flashy flow—streamflow that rises to floodstage and wanes quickly.

Each one of these variables (such as runoff-water from rainfall) is complicated in its own natural behavior, and alterations in any one of the variables affect others in more than one way. For example, the amount of water a drainage basin processes during the year is related to the amount of precipitation and its fate across the landscape. Precipitation in New Mexico is extremely variable. It is measured in amount and duration (e.g. rainfall intensity). Intense thunderstorm rainfall may do more landscape work than melting snow, but may not aid the growth of vegetation. Vegetation depends on the amount, frequency, and sequence of precipitation as well as other landscape variables (bedrock, soils, and orientation of slopes). If vegetation is dense enough to slow overland flow down slope to the streambed, much flow may be trapped by the vegetation and seep into the ground, nurturing the vegetation. Soil is held in place. If vegetation is less dense, some flow reaches the channel and affects flow downstream. With less vegetation, runoff may increase and flow may increase downstream. If climate shifts to less precipitation, vegetation may die, affecting overland flow and flow within the channel. If soil is eroded from the hill slopes and overwhelms the ability of the channel to transport it, the channel becomes choked with sediment, and the slope in the channel may decrease, affecting the energy to transport sediment. Unsaturated loose sediment may absorb more water, reducing flow until a threshold is met, after which both water and sediment continue to move down gradient. Excess runoff leads to erosion and transport of sediment within the channels.
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