Why when a river flows out of a steep an alluvial fan forms?

An alluvial fan forms when a river flows out of a steep gradient and enters a flatter area, causing a reduction in its velocity. This decrease in speed leads to the deposition of sediment that the river was carrying, as it can no longer support the same load. Over time, these sediments accumulate in a fan-shaped pattern, creating the characteristic structure of an alluvial fan. This process is driven by the river's transition from erosion to deposition as it interacts with the surrounding landscape.