The root cause of delta formation

Let's briefly describe the delta as an alluvial plain gradually developed due to the decrease of velocity and the deposition of a large amount of sediment when rivers flow into oceans or lakes. Delta, also known as estuary plain, looks like a triangle in plan, with the top pointing upstream and the bottom as its outer edge, so it is called delta. The delta has a large area, deep soil layer, dense water network, flat surface and fertile soil. For example, the Yangtze River Delta, the Pearl River Delta and the Yellow River Delta in China. Delta can be divided into pointed delta, fan delta and bird foot delta according to its shape. Delta region is not only a good agricultural region, but also a region rich in oil, natural gas and other resources.

When the river is injected into the ocean or lake, the water flows outward, and the kinetic energy is obviously weakened, and the subsequent sediment accumulates, forming a flat land extending to the ocean or lake, which is often delta-shaped, so it is called delta.

From the dynamic characteristics of the estuary area, in the range where the tidal current boundary moves up and down, the river is supported by the tidal current, and the flow velocity is relatively low, so it is most likely to form the beach and Jiang Xinzhou, which makes the river bifurcate. At the estuary, due to the diffusion of water, the velocity slows down, and sediment often accumulates into shoals and crosses the estuary, so it is named sandbar, which is the main obstacle to navigation in the estuary.

The formation of the estuary delta is the result of a large amount of sediment accumulation in the estuary when the river is greatly affected by the water body. Alluvial deposits accumulate in estuaries, and a series of underwater shoals, beaches or sand mouths first appear, and the water flows diverge, forming an underwater delta inclined to the sea. With the rise and fall of each fork and the merger and expansion of the beach, the front edge of the underwater delta is constantly pushed to the sea, and the back edge is covered by flood deposits because of the high siltation of the beach, becoming a part of the aquatic delta. Due to the constant change of fork roads, many staggered riverbed sand dikes and lakes and marshes are often formed in the delta. Secondly, the main types of deltas are pointed delta, fan delta and bird-foot delta. (1) pointed delta, such as the Yangtze River Delta in China; (2) Fan deltas, such as the Nile Delta in Africa; ③ Bird's Foot Delta, such as Mississippi Delta in the United States. There are many famous deltas in the world, including Nile Delta, Mississippi Delta, Danube Delta, Mekong Delta, Ganges Delta and Yangtze Delta in China.

In areas where shallow waves of seawater can scour and level the sand mouth extending out of the estuary, arc fan delta is often formed. The Yellow River Delta in China is a fan delta formed under the condition of weak tide and weak sand. Its characteristics are: the amount of sediment discharged from rivers into the sea is large, the rivers in the delta change frequently, and sometimes several streams appear. Sediment quickly silted up in the estuary, forming a large estuary sand mouth, which extended to a certain extent. Because of the decrease of slope and poor water flow, it changed its course and quickly formed a new sand mouth in the new estuary. However, after the old estuary was cut off, influenced by waves and currents, the sand mouth was gradually eroded and retreated, forming a fan-shaped outline. The coastline didn't move forward quickly until a new river flowed through it. Therefore, with the constant change of the estuary, the delta coast is pushed forward alternately, and there are many sand mouths distributed on the coast, which makes the delta coastline slightly jagged.

At the mouth of the river with weak wave action, the river branches into several streams and flows into the sea at the same time. The sediment deposition of each tributary exceeds the scouring of the waves, and the sediment accumulates and extends along each tributary channel, forming a strip-shaped sand mouth that extends into the sea, making the delta look like a bird's foot. Because this bifurcation is relatively stable, natural dikes are often developed on both sides, which can restrain the water flow and make the bifurcated water continue to extend to the sea. Once the natural dike is flooded, it will produce new cross flow. The Mississippi River Delta in the United States is a typical bird's foot delta. Bird's foot delta is also common in estuaries that pour into lakes. For example, there are many bird's foot deltas of different sizes along Poyang Lake and Dianchi Lake in China. If you carefully observe the map of the world, you will find that most major rivers in the world have a delta where they enter the sea. For example, where the Nile River in Egypt (the second largest river in the world) enters the sea, there is a huge delta with an area of 24,000 square kilometers; The delta where the Mississippi River in the United States (the fourth largest river in the world) enters the sea is bird-footed, covering an area of 26,000 square kilometers; China's Yangtze River (the third largest river in the world), Yellow River (the fifth largest river in the world) and Pearl River all have large deltas.

Delta is an alluvial plain in the estuary area, which is deposited by the sediment carried by the river when it enters the sea. About 654.38+0.6 billion cubic meters of sediment are brought into the sea by rivers every year in the world. When the mixed sediment in the river flows from upstream to downstream, the riverbed gradually expands and the drop decreases. When the river flows into the sea, the current is dispersed and the velocity suddenly decreases. In addition, the influx of tides from time to time has the effect of blocking rivers, especially the seawater contains a lot of ionized sodium chloride (salt), which produces a large number of ions and can also precipitate suspended sediment. As a result, more and more sediment accumulated here and finally surfaced. At this time, the river had to bypass the sand and flow from both sides. Because the front of the sand pile is directly impacted by the river and constantly eroded by the flowing water, it often forms a sharp shape, while the water surface in the north is relatively wide, making the sand pile triangular, so people name it "Delta".

The delta area is generally low-lying and densely covered with river networks, so it is mostly a good agricultural area. For example, the Pearl River Delta and the Yangtze River Delta in China are both high agricultural production areas. Although the land in the Yellow River Delta is fertile, the breeding conditions are slightly poor due to the influence of salt.

Of course, there are also some rivers whose estuaries have not formed deltas. For example, there is no delta at the mouth of Qiantang River in China. This is because the sediment content of Qiantang River is scarce, and the estuary is a wide trumpet-shaped estuary. The huge scouring force of tidal bore makes it impossible to deposit sediment, even if some sediment is lucky enough to stay. It is difficult to raise the height of the pile at the mouth of Qiantang River, and it can only form underwater sandbars under the water surface, but not deltas exposed to the water surface.

In estuaries with strong wave action, rivers enter the sea in the form of single flow, or only cross in a small area. In this case, only the sediment at the outlet of the main stream exceeds the scouring of waves, which makes the delta extend outward in a sharp angle with the main stream as the center. This delta is called a sharp delta. The Yangtze River Delta belongs to this type.

In addition, contrary to the process of delta formation, the river sediment is small, and the estuary is strongly washed by tidal current, and there is no delta formation, which is often trumpet-shaped. This estuary is called Triangle Harbor or Triangle Bay, such as Hangzhou Bay (Qiantang Estuary) is a typical triangle harbor. The Qiantang River's sediment load is small, the estuary coast drops, and the tidal range is large, which not only prevents the sediment carried by the river from accumulating in the estuary, but also causes strong erosion, which makes the estuary deepen and widen, and gradually forms a triangular port. The tidal range of the triangle port is larger, thus forming the famous tidal bore of Qiantang River. Tidal bore makes sediment accumulate in the upper part of estuary area into raised bar sand.