Where do the Yangtze River Delta and the Pearl River Delta refer to?

When the river flows into the ocean or lake, due to the decrease of velocity, a large amount of sediment is deposited and gradually develops into an alluvial plain. 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 upper layer, dense water network, flat surface and fertile soil. For example, the Yangtze River Delta and Pearl River Delta in China. Delta can be divided into: 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. 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.

The main types of deltas are fan delta, bird foot delta and pointed delta.

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 there is a delta at the entrance of the world's major rivers. 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, due to the influence of salt, the farming belt is slightly poor.

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. The accumulation of Qiantang River estuary is difficult to rise, and only underwater sandbars can be formed under the water surface, rather than 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.

■ Yangtze River Delta

China Yangtze River and Qiantang River are alluvial deltas where they enter the sea. Include Jiangsu province, southeastern Shanghai and northeastern Zhejiang. It is a part of the middle and lower reaches of the Yangtze River. It covers an area of about 50,000 square kilometers. The apex of the delta is in Zhenjiang and Yangzhou, and the north reaches Xiaoyangkou. South bank of Hangzhou Bay. The elevation is mostly below10m, and the low hills (such as Huishan, Tianpingshan, Yushan and Langshan) are scattered, with an elevation of 200-300m. The annual sediment discharge of the Yangtze River is 400-900 million tons. In general, 28% of the sediment is deposited in the Yangtze River, and in some years it is as high as 78%. The delta continues to extend to the sea. The ancient sand mouths in Changzhou, Changshu, Taicang, Jinshan and other counties south of the Yangtze River are mostly 4 ~ 6 meters above sea level; The ancient sand mouth in Yangzhou, Taizhou, Taixing and Rugao, north of the Yangtze River, is 7-8 meters above sea level. The ancient sand mouths in Jiangnan and Jiangbei developed gradually after the highest sea level stabilized in the late glacial period. By about 2000, the north bank sand mouth extended to Liaojiaozui, and the south bank sand mouth extended to the southeast with the mainstream of the Yangtze River and connected with the mouth sand mouth of Qiantang River. Sediment continues to accumulate, advancing annually 1958 ~ 1973. It belongs to the northern subtropical monsoon climate, with abundant rainfall, criss-crossing waterways and dotted lakes, and is known as a water town and a country. The land is fertile, and agriculture mainly produces rice, cotton, wheat, rape, peanuts, silk, fish and shrimp. It is one of the most densely populated areas in China. There are many important towns in the lower reaches of the Yangtze River and on both sides of the Shanghai-Nanjing line, such as Shanghai, Suzhou, Changzhou, Wuxi, Zhenjiang, Yangzhou, Taizhou and Nantong. Among them, Shanghai is the largest industrial and commercial city in China and a world-famous foreign trade port, while Suzhou and Wuxi are scenic spots and emerging industrial cities.

Top ten river deltas?

1) Ganges River ... Brahmaputra River Delta (Bangladesh, India): 80,000 square kilometers.

(2) Yangtze River Delta (China): 50,000 square kilometers.

(3) Mekong Delta (Cambodia and Thailand): 44,000 square kilometers.

(4) Nirile River Delta (Nigeria): 36,000 square kilometers.

(5) Irrawaddy River Delta (Myanmar): 30,000 square kilometers

(6) Lena River Delta (Russia): 30,000 square kilometers.

(7) Mississippi River Delta (USA): 26,000 square kilometers.

(8) Orinoco River Delta (Venezuela): 26,000 square kilometers.

(9) Nile Delta (Egypt): 24,000 square kilometers.

(10) Red River Delta (Vietnam): 15000 square kilometers.

Delta, also known as estuary plain, is formed by the accumulation of a large amount of sediment carried by the river from the upstream at the estuary. Seen from the plane, it looks like a triangle, with the top pointing upstream and the bottom as its outer edge, so it is called a delta. The Yangtze River Delta in China is formed by the accumulation of a large amount of sediment under the Yangtze River belt. The apex of the delta is near Zhenjiang, and its bottom gradually expands eastward and seaward. About 20 million to 30 million years ago, the Yangtze River estuary was still a triangular harbor. The Yangtze River estuary below Zhenjiang is like a big horn opening eastward, with a vast water surface and strong tides. Every year, 450 million tons of sediment carried by the Yangtze River will be dumped into the sea. Due to the decrease of estuary velocity and the change of physical and chemical environment, most of the sediment will gradually deposit in the estuary area and eventually form a pointed delta.