Alluvial Fans

Alluvial Fans

Sediment transport and erosion.

Death Valley Alluvial Fan Variations

In Death Valley, we were able to observe firsthand the difference in size between East and West alluvial fans. The alluvial fans that form in Death Valley are particularly interesting because of the lack of water and unique tectonics occurring in the area. The Panamint Ridge (west) side of Death Valley has significantly larger alluvial fans than the Black Mountain (east) side. The reason for this is a combination of several factors which cause larger base sediment accumulation on the Panamint side.

The first factor is the tectonics of the area. Death Valley lies in between the Panamint Range and the Funeral Mountains (which are also on the east side, like the Black Mountains discussed previously). The fault zone is on the west side and the Funeral and Black Mountains are rotated towards the East. The surface is ever so slightly tipping eastwards and so the fans can also grow larger with more momentum.

In addition to the tectonics of the area, precipitation also plays a role in fan progression. The Panamint Range receives more precipitation in the form of rain and snowfall than the East side. Because of this, the Panamint has more flow down the mountainside and can build up more power to pick up sediment along the way. They are also able to form larger fans because of the increased momentum outwards.

In an aerial view we are able to see the size difference between the eastern and western alluvial fans in Death Valley. Telescope Peak is labeled on the Panamint Range. These fans are much more sprawling than the right side, which remains close to the side of the mountain range. If we zoom in, we can see that there is vegetation on the edge of the alluvial fans. In such a dry, high salinity area, this is very unique. The way this vegetation is able to thrive along the edges of alluvial fans is by being fed by the water that powers sediment movement for the fans. 

1. Alluvial Fans Observed

While we were in the Mojave Desert, we saw many alluvial fans. However the most prevalent ones we saw were in Death Valley, are shown below. 

Alluvial Fans vs Debris Flows

We observed a number of alluvial fans. Alluvial fans are constructed by a combination of fluvial and mass wasting processes. These flows were unique due to their sediment size distribution, and the way the water flows. When combined with water, Alluvial fans can be created by debris flow and sheet flow. Typically, alluvial fans made from sheet flows, have a more evenly distributed rock distribution, with smaller sized rocks being near the bottom, and larger sized rocks being nearer to the apex. On the other hand, if an alluvial fan is caused by the result of debris flow, the rocks in the alluvial fan will be more mixed up, with bigger rocks existing at both the apex of an alluvial fan and at the apron of the alluvial fan, and vice versa for smaller grained sand.

What is an Alluvial fan?

An Alluvial fan is a fan-shaped landform, formed in arid environments by heavy flowing water from intense storms interacting with mountains, hills, canyon walls, rivers and creeks. As the water flows down the slope of the elevated surface it drops an accumulation of sediments, called alluvium. Alluvium sediments are deposited by water and sometimes wind from a canyon to a valley floor, and are considered geographically young. Often, fans are formed within multiple canyons along a mountain front, this creates a continuous fan apron, called a piedmont or bajada. The parts of an alluvial fan consist of the apex, which is the top/narrowest point of the fan, the alluvial cone, where the sediments are deposited.

Historical Processes

On alluvial fans, fan deposition was triggered from the transition of damp to dry climates from the Pleistocene to Holocene. The reduction of soil moisture led to a reduction in vegetation cover, ultimately leading to an increase in soil erosion and sediment supply.

From the Pleistocene to Holocene period the gradual upward movement in elevation, and a decrease in effective soil moisture. Fan enrichment and stabilization of the alluvial fan surface followed from a decrease in sediment supply from the increase in bare slopes.

Hydrology and Ecology of Alluvial fans

As it may have been hinted earlier on, water and the hydrology of an alluvial fan is one of the most important factors to an alluvial fan. Without water, alluvial fans would not be able to form. When water flows through an alluvial fan, it starts harsh and quick in the narrow, sloped gap near the top, but quickly eases out as the water quickly spreads out and can only flow across a flat, open plane. Water will also, overtime, erode away sand, creating channels in the alluvial fan, which can result in both smaller alluvial fans on the alluvial fan forming, but also allowing a type of garnish to show up in the alluvial fans, forming dark spots where it’s been longer since the last time water came in contact with the soil.

Regarding the ecology of Alluvial fans, most living things will live a the bottom of an alluvial fan, where potential water that could still flow through the alluvial fan wouldn’t disrupt animals and uproot various plants that could potentially grow, but still close enough to the alluvial fan where water can exist. Some of the most prominent animals that will live on alluvial fans, especially in the desert are various species of snakes and lizards, and while not completely native to the Mojave area, desert tortoises are also known to live near these alluvial fans. As for the plants, there are usually two types of plants that will exist near Alluvial fans. The first type of plant are plants that require a good amount of water, and thus have deep root systems to reach underground wells of water (one example of this plant being Larrea tridentata), which are also known for existing on younger alluvial fans due to weaker, earlier disturbed soil allowing deeper water penetration. The second type of plant is plants that require very little water and thus have short roots (one example of this plant being Ambrosia dumosa) and usually exists with older alluvial fans since the alluvial fan has more settled, stronger soil that has not been disturbed in a long time. Nonetheless, as it can be seen in the picture below of one of the alluvial fans we saw that while no plants exist on the alluvial fan, it springs to life at the end of the alluvial fan, and the life certainly flourishes, as can be seen by the large extent of plants.