How does sediment deposition occur when a stream overflows onto a floodplain?

Sediment deposition during a flood event on a floodplain typically occurs due to the decreasing flow velocity as the water spreads out from the channel. When a stream or river overflows its banks onto the adjacent floodplain, the water slows down, and this reduction in velocity causes particles suspended in the water to settle.

Coarse materials, such as gravel and sand, are heavier and therefore settle first as the flow exits the channel and spreads out. This process results in a gradient of sediment deposition: larger particles accumulate closest to the river bank where the water velocity decreases most rapidly, while finer materials, such as silt and clay, can be carried farther out onto the floodplain before settling.

This pattern of deposition is crucial for the development of fertile floodplains, as it replenishes the soil with nutrient-rich sediments. The other options provided do not accurately reflect how sediment transport and deposition work during flooding. Fine materials may be deposited as well, but the sequence of deposition starts with coarser sediments near the channel.