What is an alluvial fan?

An alluvial fan is defined as a cone-shaped sediment deposit that forms when a river flows out of a mountainous region onto a flatter landscape. As the river's velocity decreases upon exiting the steeper slopes, it loses its capacity to carry sediment, resulting in the deposition of materials. This accumulation of sediment creates a fan-shaped structure that radiates outwards. This geological feature is significant because it can affect local ecosystems, water habits, and land use.

In contrast, other options describe different geological or hydrological phenomena. The structure in option B, while suggesting a broader concept of water management, does not capture the specific sedimentary nature that characterizes an alluvial fan. Option C describes a flat lake, which does not involve a cone-shaped deposit but rather a body of standing water that can form due to sediment accumulation in depressions over time. The floodplain mentioned in option D could be associated with a river but also does not have the distinct conical shape or depositional process involved in an alluvial fan.