Tuesday, September 6, 2011

Sonoran Desert Soil Distributions

Dry mountain canyon wash looking out on the desert below.  Canyons such as this one are the gateways for nearly all soils in the Basin and Range.
When most people think of desert soils they think of rolling sand dunes.  The problem is that sand dunes are not soil at all, rather they are blowing, shifting piles of sediment.  And sand dunes are relatively rare in the Sonoran Desert anyway.  Instead of blowing sediments, the life of Sonoran Desert soils begins as rock in the scattered small mountain ranges common to the region.  A process called weathering breaks down this rock into smaller and smaller particles.  These particles then wash out of the mountain due to heavy rainfall causing erosion, debris flows, or flash floods.  Odd as it might sound, nearly all desert soils are built by these types of water movements through the landscape.  Contrary to intuition, the very rock and dirt fabric of the desert is carved out and formed by water.

Knowing the above process is essential to understanding soil distributions in the Sonoran Desert.  And knowing where soils are located is key to understanding the Sonoran Desert landscape.  Both hydrology and plant communities are strongly defined by the desert soil.  In a previous post on soil mapping we discussed generally how to locate soils in the landscape.  In this post we will discuss specifically how to locate soils in the Sonoran Desert.  Believe it or not, desert soils are complex in structure but relatively easy to map.  The general rules that we discussed before apply in a rather neat fashion.

The alluvial fan is circled in white in the below image.  The canyon through which all the sediments came from for the alluvial fan is lined with blue.
Sediments washed off a mountain are deposited in landforms called alluvial fans.  Deriving their name "alluvial" due to being water deposits, and "fan" from the fact that they are often fan shaped.  These fans are created through flash floods or debris flows which cascade sediments through mountain canyon washes to basins below.  Typically, once these flow reach the wash outlet, sediments are deposited radiating out from the outlet in a fan like shape.  Over long periods of time these fans can become quite large, often merging with other fans being deposited from other canyon washes nearby.  Then, from this first alluvial fan, sediments can be eroded off and deposited below forming a second alluvial fan.  This process of erosion and deposition can repeat itself several times and several alluvial fans can be deposited in succession away from the mountain range.  Each of these successive fans will produce a unique soil.  Erosion and deposition will often destroy the fan-like shape initially present when first deposited.
This diagram shows generally how alluvial fans, or soils, are arranged in a bajada.  Each soil  developed from an alluvial fan.  Each fan was deposited in succession outward from the mountain.  The oldest fan, or soil, being where it is labeled soil 1.  The youngest fan, or soil, is labeled soil 4.  Soil 1 and 2 are the oldest and in the Sonoran Desert are generally very similar with red soil, an argillic, and caliche horizons.  Soils 3 and 4 are also similar to each other and generally lack argillic and caliche horizons, also lacking red coloration.
The resulting succession of different alluvial fans radiating out from a mountain side is known as a bajada.  Each section of the bajada is a different alluvial fan with a different soil type.  These individual fans, or sections of the bajada, can be identified by basic soil mapping principals.  Each fan soil will have a particular soil color, horizons, a general rock type cover, slope, plant community, and landscape position.  Soils, or fans, closer to the mountain, higher up the bajada are much older.  Soils, or fans, further away from the mountain and lower on the bajada are much younger, in-fact they may have just recently been deposited.  The older a soil is the redder it will be (previous post on soil color), therefore upper bajada soils are red in coloration while younger lower bajada soils lack red coloration.  The red coloration indicates the presence of caliche and argillic soil horizons in most cases (previous post on desert soil horizons).  Upper bajadas will also have steeper slopes while lower bajadas will nearly be flat.  Upper bajadas will also generally have rockier soils and fewer rocks will be present lower in the bajada.

An upper bajada soil surface.  Cacti of several species, Palo Verde, Triangle-Leaf Bursage,  and some Creosote make up this upper bajada plant community.  These plants, except the Creosote, have generally shallow roots which are adapted to caliche and argillic soils common to upper bajadas such as this one.
Soils in the desert seem to have a particularly strong effect on plant life.  We will discuss this further in a future post but desert plants are a strong indicator of what lays below the surface.  Some plants prefer argillic and caliche horizons while others can stand them.  Generally shallow rooted plants such as Chollas, Prickly Pears, Saguaros, Triangle-leaf Bursage, and Foothills Palo Verde prefer caliche and argillic horizons in some circumstances.  Therefore shallow rooted plants prefer upper bajadas.  Creosote bush having deeper roots prefer the absence of caliche and argillics, therefore preferring lower bajadas.
A lower bajada soil surface.  This plant community is primarily Creosote.  There are a very few cacti also present though.  The deep roots of creosote prefer the absence of impeding caliche and argillic horizons upslope on the bajada.

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