Final answer:
The type of feature that forms when a steep, narrow drainage enters a broader, more gentle valley and a stream deposits the large sediments it is carrying is a fan delta.
Step-by-step explanation:
This occurs as the river or stream moves from a narrow, steep gradient and enters a flatter area. The water flow slows down, leading to sedimentation where the waters of a river split into many channels as they deposit sand and silt they've carried from upstream.
A fan delta is often triangular-shaped and can be found in arid or semi-arid regions. These fan-shaped deposits are created by the high-energy flow of moving water, which results in poorly sorted gravel, sand, silt, and/or clay.
The pinnacle of this feature is located at the mount of the sedimentary source, typically where the mountain stream exits onto the valley floor.