What are the factors that contribute to the formation of deltas, how are deltas formed, and explaine

If you look closely at the world map, you will see that most of the world's major rivers have a delta at the mouth of the sea. For example, at the mouth of the Nile River in Egypt (the second largest river in the world), there is a huge delta with an area of 24,000 square kilometers; The delta at the mouth of the Mississippi River (the fourth largest river in the world) in the United States is in the shape of a bird's foot with an area of 26,000 square kilometers; China's Yangtze River (the world's third largest river), the Yellow River (the world's fifth largest river) and the Pearl River also have large deltas.

A delta is an alluvial plain in the estuarine area, which is formed by the sediment deposited by the river when it flows into the sea. About 16 billion cubic meters of sediment in the world are carried into the sea by rivers every year. When these sediments mixed in the river water flow from upstream to downstream, because the riverbed gradually expands, the drop difference decreases, when the river flows into the sea, the water flow is dispersed, and the flow rate decreases suddenly, coupled with the tide water influx from time to time has the effect of blocking the river water, especially the sea water dissolves a lot of ionizing sodium chloride (salt), it produces a large number of ions, can make those sediment suspended in the water also precipitate.

As a result, the sediment accumulates here and grows, and finally comes to the surface. At this point, the river had to flow on both sides around the sand piles. Because the frontal surface of the sand pile is directly impacted by the river, it is constantly eroded by the flowing water, and often forms a tip shape, while the water surface in the north is relatively wide, making the sand pile a triangle, and people name them "delta".

The delta area is generally low and flat, with a dense network of rivers, so it is mostly a good agricultural area. For example, the deltas of the Pearl River, the Yangtze River and other estuaries in China are all high-yield agricultural areas. Although the land in the Yellow River Delta is fertile, the agricultural conditions are slightly worse due to the influence of salinity.

Of course, there are also rivers whose estuaries do not form deltas. For example, there is no delta at the mouth of the Qiantang River in China. This is because the sediment content in the Qiantang River is scarce, and the estuary is a very wide trumpet-shaped estuary, and the huge scouring force of the rising tide makes the sediment unable to be deposited, even if some sediment is lucky to stay.

Piled up at the mouth of the Qiantang River, it is also difficult to increase the height, and can only form an underwater sand barrier under the water surface, which cannot form a delta that is exposed to the water surface.

In estuarine areas with strong wave action, the river flows into the sea in a single strand, or only crosses on a small scale, in this case, only the sedimentation at the outlet of the main stream exceeds the erosion of the waves, so that the delta is centered on the main stream and elongates outward in a pointed shape, which is called a pointed delta. The Yangtze River Delta is of this type.

In addition, contrary to the delta formation process, the amount of sediment from the river is small, and the estuary is strongly scoured by the current, and there is no delta formation, often in the shape of a trumpet, this kind of estuary is called a triangle port or a triangle bay, such as Hangzhou Bay (Qiantang River Estuary) is a typical triangle port. The Qiantang River carries less sediment, the estuary coast falls, and the tidal range is large, which not only makes the sediment carried by the river unable to accumulate in the estuary, but also causes a strong erosion, deepens and widens the estuary, and gradually forms a triangular port. The triangular port has a larger tidal range, thus forming the famous Qiantang River tide.

The tidal surge causes sediment to accumulate in the upper part of the estuary area into raised barrage sand.

A delta is a common surface morphology, which is a triangular land part formed by the siltation of sediment at the mouth of a river into the sea. From a chemical point of view, the formation of the delta is due to the phenomenon of colloidal aggregation and sedimentation.

At the mouth of the river, the sediment and other impurities carried by the river will encounter the seawater, because the salinity of the seawater is higher than that of the river, so the ionization ions contained in the seawater will cause the sediment and other impurities in the river water to accumulate and settle, and gradually form a delta.

Specifically, when the river water flows through the estuary, due to the gradual expansion of the riverbed, the flow velocity of the river decreases, the kinetic energy decreases, and the seawater contains a large number of ionizing ions, which can be adsorbed on impurities such as sediment, causing them to aggregate into larger particles, and eventually make them precipitate to form a delta.

In addition, since the delta is usually formed by the accumulation and sedimentation of impurities such as sediment, its composition is mainly composed of fine particles such as clay and silt. These particulate matter has good stability in water and is not easily carried away by flowing water, so it can form a stable sedimentary layer on the delta.

In general, the formation of the delta is caused by the phenomenon of colloidal agglomeration and sedimentation in chemistry, and its composition is mainly composed of fine particulate matter such as clay and silt.

Yangtze river delta.

The formation process of blind hunger reflects the properties of colloids:

When the river water is mixed with the sea water, due to the different properties of the charges carried by the colloidal particles they contain, due to the electrostatic action, the opposite charges attract each other, causing the particles of the colloids to become larger, and the Buyan finally precipitates, this process is called polysedimentation.

The formation of the delta is the result of the accumulation of a large amount of sediment at the mouth of the river under the condition that the action of the river exceeds the action of the water body.

The river carries a large amount of sediment into the ocean, and if the slope of the bottom of the river section near the estuary is small, coupled with the jacking effect of seawater, the river velocity will decrease sharply, causing the ridge of sediment carried by the river to be deposited in front of the estuary.

If the erosion and transport of waves and currents are weak and do not carry away too much sediment from the river, a large amount of sediment will be deposited and fixed in front of the estuary, and a delta will eventually be formed.

The formation of estuarine deltas is the result of the accumulation of a large amount of sediment in the estuary under the condition that the action of the river exceeds the action of the water body.

Alluvial deposits accumulate in the estuary, starting with a series of underwater shoals, heart banks or spits, where the currents bifurcate and form an underwater delta sloping seaward sloping. With the ebb and flow of the various forks and the merging and expansion of the heart beach, the front edge of the underwater delta continues to push seaward and the rear edge becomes an integral part of the water delta due to the siltation of the beach and the cover of flood deposits. Due to the continuous change of the fork road, many staggered riverside beds, sand banks and lake and marsh depressions are often formed on the delta.

2. Types The main types of deltas are pointed deltas, fan-shaped deltas and ornithiste-shaped deltas. point-shaped deltas, such as the Yangtze River Delta in China; fan-shaped deltas, such as the Nile Delta in Africa; Ornichio-shaped deltas, such as the Mississippi River Delta in the United States. There are many famous deltas in the world, mainly including the Nile Delta, the Mississippi River Delta, the Danube Delta, the Mekong Delta, the Ganges Delta and the Yangtze River Delta in China.

The river carries sediment to the estuary, where the flow slows down due to the influence of seawater jacking, and the sediment is deposited to form a delta.

Reasons for the formation of estuarine deltas.