In northeastern Africa, the Nile River flows through deserted deserts into the Mediterranean Sea, forming a fertile land at the mouth of the sea. You might say it looked like a rose, but the ancient Greeks called it delta (delta in English) because its shape resembled the letter "delta". This name later became a geographical term for a plain formed by alluvial silt at the outlet of the river. The top of the delta points upstream of the river and the outer edge faces the sea. Why do deltas form at estuaries that enter the sea? It turns out that when a river enters the sea, the water flow is gradually dispersed and the flow speed continues to decrease due to the widening of the estuary and the influence of tidal currents. A large amount of sediment carried by the river is silted up in the estuary, and the sedimentation rate is greater than the erosion rate of ocean power, so the river enters the sea. A fan-shaped accumulation gradually forms where it enters the sea. Another reason for the large amount of sediment deposition is that there are a large amount of very fine clay in rivers. These clay often exists in a colloidal state. When it encounters seawater, it will form floccules and precipitate under the action of salt. As the accumulation gradually emerges from the water, a delta plain is formed. Most major rivers in the world have a delta where they enter the sea, such as the Ganges River, Nile River, Mekong River, Mississippi River, etc. There are also large deltas where China's Yangtze River, Yellow River and Pearl River enter the sea.
In fact, there are not only land deltas that we can see at the estuary, but also very large underwater deltas underwater, which is a continuation of land deltas. Usually, when we talk about deltas, we also include underwater deltas in addition to onshore deltas. For some large river deltas, the area of underwater deltas often exceeds that of onshore deltas. For example, the ratio of land and underwater areas in the Yangtze River Delta is 0.78.
However, there are also very few rivers whose estuaries do not form deltas, such as the Amazon River, the world's second longest. Due to the low sediment content of the river, the strong hydrodynamic force at the estuary, and the transport of coastal currents, the Amazon River only forms many sand islands at the estuary, without forming obvious deltas, and instead becomes a trumpet-shaped delta port. The Qiantang River, which is world-famous for its tidal surges in China, is also very wide, trumpet-shaped because of its low sediment concentration in the water and less sediment entering the sea. The strong hydrodynamic forces of the tidal bore make it difficult for sediment to deposit. Only a small part of it is deposited in the Qiantang River estuary, making it difficult to grow tall. Only a sand bank is formed below the water surface, and a delta above the water surface cannot be formed.