What type of sediment deposit forms at the river's mouth where it meets a lake or ocean?

The formation of a delta occurs when a river reaches a body of standing water, such as a lake or ocean. At this point, the river loses its velocity as it enters the larger water body, leading to a decrease in its capacity to carry sediment. Consequently, the sediment that was previously transported by the river begins to settle out, accumulating over time. This accumulation creates a fan-shaped or triangular landform known as a delta.

Deltas are characterized by a complex network of distributaries and typically have fertile soil, making them important regions for agriculture. Additionally, they are significant ecosystems that support various forms of wildlife. The sediment in a delta consists of sand, silt, and clay, which contribute to its unique characteristics and functions.

Estuaries, while related, refer to the areas where fresh water from rivers meets and mixes with salt water from oceans. Floodplains are flat areas beside rivers that are prone to flooding and primarily composed of alluvial soils. An alluvial plain is a broader term that can refer to any area formed by sediment deposited by flowing water, not specifically at the mouth of a river. Thus, the specific nature and formation processes associated with a delta make it the correct choice.