The feature described in your question is an alluvial fan. An alluvial fan is a widely sloping deposit of sediment that forms where a river exits a mountain range and spreads out onto a flatter area. This sediment is typically deposited due to a decrease in the river’s velocity as it meets the lowland, allowing it to lose its capacity to transport larger sediment particles.
A delta, on the other hand, forms at the mouth of a river where it meets a body of water, such as a lake or ocean. A floodplain refers to the flat area adjacent to a river that is subject to regular flooding. An oxbow lake is a crescent-shaped lake formed from a meander in a river.
Based on your description, the correct answer is alluvial fan.
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