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.
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.
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.
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.
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.
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).
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.).
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.
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.
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.
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)