Search the web Search this site

Desert Alluvial Fans

Characteristics of a Desert Alluvial Fan

Alluvial fans are beautiful places in the desert. They are products of low moisture, minimal vegetation, and rain events. Basically an alluvial fan is the washing of broken rocks and sediment from the mountains to the valleys. You may notice them as you are driving around in the desert. You'll see steeply sloped hills and mountains. Then, you'll also see (and be driving on) very mild slopes that start at the base of the mountain and taper off gradually as they slope down to the open valley. Within these gradual slopes will be depressions, ravines, and seasonal washes. These depressions change over time because the soil is very rocky, loose, and contains minimal vegetation.

Alluvial Fan
Desert Alluvial Fan Just Below Large Mountain Range

Typically, the smaller sediment washes to the bottom of the fan, and is sometimes called silt. This fine material makes a very clay-like soil that can be very difficult to work with in your landscape. When it's dry it's almost as hard as concrete. When it's wet it's like glue, sticking to your shoes and everything it touches. If you step in it while it's wet, you could get stuck.

Alluvial Fan
Alluvial Fan in Death Valley National Park

If your landscape does have this clay type soil, there are some advantages and disadvantages. The main disadvantage is that is it extremely difficult to dig and move it around. If it is dry, you'll need a pick-axe to chip a hole for your plant. It's easier to dig if it's wet, but it's very sticky and messy to work with.

Alluvial Fan
Mildly Sloping Alluvial Fan

The advantage of clay type soils is that they hold water longer. This means you need to water much less during the summer months in the desert. You'll also want to choose plants that are adapted to those kind of silty soils. Because the clay soil consists of very fine particles, it compresses very easily. That's what makes it so hard to break up. The roots of your plant need to be strong enough to penetrate the compacted material. Otherwise, the roots will stay in the original hole you dug and will eventually die because of the lack drainage and chemistry change from irrigation. So if you plant desert adapted shrubs and trees in desert clay soil that can handle the higher salt content, you can almost get away with no irrigation water at all, once the plant is established.

As you rise up along the alluvial fan, the soils become more rocky. There's a gradual change in average rock size as you go up the hill. Once you reach the top of the alluvial fan the boulders are huge and the drainage is great. So as you go up the hill of an alluvial fan water drains faster. This makes a difference in how much you'll need to water some species of plants in the desert. However, as in the case with silty soils, you can find plant species that are adapted to fast draining rocky lands, thereby minimising the need to irrigate.

Las Vegas Valley, basically sits in a bowl surrounded by alluvial fans, which are rimmed by a ring of mountains. That's why Las Vegas has been attractive to animals and humans for thousands of years. The snow pack and rain from the surrounding mountains drain into Las Vegas valley through the alluvial fans and by underground percolation that comes up as spring water. Residential property development around Las Vegas has swollen to engulf most of the alluvial fans up to the mountain's bottom edge. For those living in Las Vegas, your landscape success is dependent on how you incorporate the dynamics of the fan that your property was built on.

Plants Adapted to Mojave Desert Alluvial Fans

Many plants have adapted to the turbulent structure of alluvial fans. We highlight the Mojave desert alluvial fans because they are most pronounced across vast areas of the desert. If you want to see a few, take the drive on Highway 15 between Barstow and Las Vegas. As you rise up along the fan there will be gradual changes in plant communities.

  • Alluvial Basin Plant Community The environment here is dry, sunny, flat valley bottoms, low land flood plains, ephemeral stream channels, and playa margins. These plant communities generally occur below the moister sagebrush or creosote zones.

    Bottom of the Alluvial Fan Plant Community
    Bottom of the Alluvial Fan Plant Community

    This elevation is typically 0 to 2000 feet above sea level. The soils typically are high saline (salty). Typical plant species that can be found here are Greasewood (Sarcobatus vermiculatus), Salt Bushes (Atriplex species), and Salt Grass (Distichlis spicata).

  • Creosote and White Bursage Plant Community This plant community covers most of the flat, open, and mildly sloping alluvial fans of the Mojave Desert. It is dominated by Creosote Bush (Larrea tridentata) and White Bursage (Ambrosia dumosa). This elevation is typically 2000 to 4000 feet above sea level.

    Creosote and white bursage on alluvial fan
    Creosote Bush and White Bursage Community on Alluvial Fan

    This is also frequented by Nevada joint-fir (Ephedra nevadensis), Fremont's dalea (Psorothamnus fremontii), brittlebush (Encelia spp.), spiny menodora (Menodora spinescens), white burrobrush (Hymenoclea salsola), desert-thorn (Lycium spp.), spiny hopsage (Grayia spinosa), littleleaf ratany (Krameria erecta), Mormon tea (Ephedra viridus (green stems), several species of fragrant sage (Salvia dorryi and Salvia mojavensis), and various species of rabbitbrush (Chrysothamnus spp.).

  • Mojave Yucca and Cactus Plant Community The creosote community continues upward, but then you start to see medium tall Mojave Yuccas (Yucca schidigera) towering above the shrubs. This elevation is typically 3000 to 5000 feet above sea level.

    Mojave Yucca Habitat
    Mojave Yucca Habitat on Alluvial Fan

    Also appearing along side washes and rock spills are several species of Mojave native cactus. You'll see the Cotton Top cactus (Echinocactus polycephalus), the Strawberry Hedgehog cactus (Echinocereus engelmannii), the Beaver Tail cactus (Opuntia basilaris), and a host of other prickly pear cactus.

  • Joshua Tree and Black Bush Plant Community

    Joshua Trees and Blackbrush on alluvial fan
    Joshua Tree and Blackbrush Plant Community

    At this elevation, the creosote begin to thin out and Joshua trees (Yucca brevifolia) and Blackbrush (Coleogyne ramosissima) begin to appear. This elevation is typically 3000 to 6000 feet above sea level.

    Big Joshua Trees
    Big Joshua Trees on Alluvial Fan

    The Blackbrush appears as a carpet of low dead shrubs most of the year. However, in the spring they turn green with many small leaves. Then they flower yellow. A Black-brush community can be over 800 years old.

    Agave utahensis flower stalks can also be seen growing on distant rocky cliffs.

    Joshua Trees and Blackbrush on alluvial fan
    Agave Utahensis Can be Found in a Joshua Tree and Blackbrush Plant Community

  • Juniper Tree Plant Community The Utah Juniper (Juniperus osteosperma) habitat starts to appear at the upper range of the Joshua Tree community at the top of the alluvial fan, then continue up the mountains until they meet the pine trees in snow country.

    Juniper Sage Habitat
    Juniper Sage Plant Community near the Top of an Alluvial Fan

    This elevation is typically 5000 to 8000 feet above sea level. Growing along with the junipers are Great Basin sage (Artimisia tridentata), Curl leaf Mountain Mahogany (Cercocarpus ledifolius), and Rabbit brush (Chrysothamnus species)

  • Pine Tree Plant Community The pine trees are in the mountains above the alluvial fan. They are joined by a whole host of plants that are different than what exists in the creosote community. Most of these plants enjoy mild summer temperatures and rains. They must also be able to handle very long cold winters.

Landscaping Ideas Menu

Try Cactus