A cone of debris deposited by running water at the mouth of a canyon in an arid area is known as an alluvial fan
Explanation:
An alluvial fan is a fan- or cone-shaped heap of sediment mixed and built up by currents. If a fan is built up by debris flows it is properly named a debris cone or colluvial fan. The sediment left behind that forms the alluvial fan is called alluvium. Alluvial fans are generated when water and sediments pass within a narrow gap between hills, mountains or canyon walls and then slows down and flattened out when it relinquishes an open plain.