alluvial fan

Alluvium is the sediment deposited by the flow of a river, so an alluvial fan is the mass of sediment, sand and gravel, silt and clay, deposited in a fanlike shape by the flow of a mountain stream or river when it has left a confined channel and opened onto a broad plain. Alluvial fans occur in humid as well as arid and semiarid landscapes, although the most prominent ones are in deserts. A series of fans, formed by parallel streams flowing out of a range of mountains, sometimes forms a continuous and overlapping apron of sediment which is called a bajada or alluvial apron. Norman Hinds notes an instance in his Evolution of the California Landscape: “Because of the height of the ranges around Death Valley and their consequently steep slopes, a host of valleys has been eroded into them and the streams have carried quantities of debris into the basin forming alluvial fans and aprons.”

Robert Hass