Alluvial Systems

Alluvial fans form where high-gradient mountain streams flow into valleys or onto plains and deposit their sediment load. Such is the case along the foot of the Panamint Mountains bordering Death Valley in California. As with overbank floodplain deposits, the coarsest sediment is generally deposited closest to the mountains and finer sediment can be carried many miles. Geologists often refer to the sediment deposited near the mountains as proximal and the sediment deposited far from the mountains as distal. Alluvial fans typically form three-dimensional cones that resemble folding fans when viewed from above, hence their name. The main stream branches out into many channels that distribute sediment across the alluvial fan. Alluvial fans formed by streams that drain large watershed tend to be larger than those that drain small watersheds. They can grow in size until they begin to merge with fans created by neighboring streams, at which point they coalesce into a broad sloping surface known as a bajada.