What is formed at the mouth of a river where it meets a still body of water?

When a river meets a still body of water, such as an ocean, sea, or lake, it can deposit sediment that has been carried downstream, forming a delta. A delta is characterized by its triangular or fan-shaped landform created by the accumulation of sediment, which splits the river's flow into multiple channels as it spreads out before entering the larger body of water. This sediment deposition occurs because the velocity of the river water decreases significantly when it enters the still water, allowing particles to settle out.

In contrast, an estuary is a broader term that describes the area where freshwater from the river meets and mixes with saltwater from the sea. While estuaries can be found at river mouths, they are not specifically formed by sediment deposition in the same way that deltas are.

Alluvial fans and plains describe different geological features. An alluvial fan is formed typically at the base of a mountain when a river's flow is slowed as it exits a steep gradient, resulting in sediment deposition. A plain refers to relatively flat areas of land and is not specifically associated with the dynamics of river and body of water interactions.

Therefore, the formation of sedimentary structures at the river mouth that leads to delta formation uniquely defines that geographical feature in question.