Geology Cafe

Alluvial Fans: Deposition Along Mountain Fronts

Alluvial fans are large cones of sedimentary material that accumulate at the mouth of a canyon. Stream erosion in upland mountainous areas transport material downslope to a valley, typically and inland basin. Large quantities of sediment typically move only during major flood events, such as during a flash flood or debris flow. When the flood exits the mouth of a canyon, the mix of water, mud and rock begins to slow down as it is distributed across a broad surface of an alluvial fan. Sediments accumulate across a broad surface criss-crossed by migrating and bifurcating stream channels that fill with sediment derived from upstream.

Sediments high on alluvial fans are rich in gravel with cobble- and boulder-sized rock fragments, whereas, the toe of an alluvial fan typically consists of sand-, silt-, and mud-sized fractions. Alluvial fans merge with other fans to form apron like structures along mountain fronts (called a bajada). Many factors controls the shape an character of alluvial fans including the character of the eroding bedrock in the mountain source area, the frequency of floods, vegetation cover, and the geometry and tectonic activity of the basin into which the sediments are being deposited.
Cross section of southern Death Valley showing relationships bedrock uplifts (Panamint Range and Funeral Mountains) and the sediment filled basin beneath Death Valley. The diagram is hypothetical and not exactly to scale. The size and distribution of alluvial fans reflect the geologic history and landscape development of a region. For instance, in southern Death Valley, the alluvial fans on the east side of the valley adjacent to the Funeral Mountains are small and distinct compared to the alluvial fans on the west side of the valley along the Panamint Range. This is because the east side of the valley is sinking along a fault system along the range front of the Funeral Mountains. In contrast, the Panamint Mountains have been gradually rising for millions of years, shedding sediments onto the broad apron of alluvial fans that extend into the basin. The alluvial fans on the east side of the valley are sinking and being buried the playa-lake sediments that are accumulating in the central low area of the Death Valley basin. Badwater is near the lowest point of the surface in the basin.
In arid intermountain basins, such as Death Valley, the toe of an alluvial fan system is typically a playa (dry or ephemeral lake bed) or a trunk stream that flows between the toes of coalescing alluvial fans and eventually drains into a terminal basin. Mountain ranges in the Desert Southwest are typically surrounded by multiple alluvial fans, each beginning at the mouth of a canyon. Downslope, the alluvial fans typically coalesce into a broad, gentle slope of alluvial material called a bajada (fan apron). Click on thumbnail images for a larger view.
This aerial photography show the Furnace Creek alluvial fan. Furnace Creek flows into Death Valley at the north end of the Funeral Mountains. The coarser gravel accumulates in the upper fan, whereas finer-grained material accumulate in the lower fan. Dark patches and lines are vegetation growing along stream channels in the lower fan region. The playa/salt pan is to the left. The dark square area is vegetation around Furnace Creek Ranch (about 1 square mile) as it appeared when the aerial photograph was taken in 1948.
This view is is looking west from Dantes View toward Telescope Peak in the Panamint Range in Death Valley National Park. A large alluvial fan drains from the mouth of Hanaupah Canyon. The younger, more active stream channels are light in color compared with the older, vegetated surfaces of the alluvial fan. This view is looking downstream from the mouth of Gowen Gulch where the stream channel reaches the alluvial fan. Over time, incised channels can fill in again, and the channels will shift to new locations on the alluvial fan. This way, over time, the great pile of sediment deposited by the stream builds a great gentle alluvial fan. This view is in Death Valley National Park.
Streams draining from the Funeral Mountains dump their sediments on alluvial fans in Death Valley. This aerial view is near Badwater, the lowest place in North America (280 feet below sea level). The alluvial fans on the east side of southern Death Valley are small compared with the opposite side of the valley. This contrast is related to the geologic history of the valley. This view shows the wall of a stream channel that has cut down through a this sequence of alluvial fan deposits. Note the interbedded layers of gravel and finer mud and sand material. Each of the layers probably represents major storm events where floods or debris flows contributed a new layer of sediment to the surface of an alluvial fan. If the present is key to the past, then these deposits probably represent major storm events that happened intermittently between long periods of drought or dry seasons. This view is along Fish Creek Wash in Split Mountain, Anza Borrego Desert State Park.
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