A(n) is a cone of debris that forms where an ephemeral stream emerges from the confines of the canyon. Its runoff spreads over the gentler slopes at the base of the mountains and quickly loses velocity, dumping most of its sediment load within a short distance.

alluvial fan

Explanation:

Alluvial fans are a mix of sediments made up of mostly sand and gravel that are created when canyons, hills or mountains interact with flowing water.  They help with the sustenance of life and irrigation farming as the main source of water.

alluvial fan

Explanation:

A(n) alluvial fan is a cone of debris that forms where an ephemeral stream emerges from the confines of the canyon. Its runoff spreads over the gentler slopes at the base of the mountains and quickly loses velocity, dumping most of its sediment load within a short distance.

it sounds like either a or c