First, let's state the conclusion:

Because no matter how deep the sea is, it will have a not-so-deep continental shelf, the coast usually has a gentle slope. However, freshwater lakes do not have a similar "shelf," so the shores of deep lakes are extremely steep.

  What is the continental shelf?

  Schematic diagram of the continental shelf

The continental shelf in the figure is the extension of the continent into the ocean. The slope of the sea floor on the continental shelf is very gentle, and the water depth is relatively shallow, usually not exceeding 200 meters.

On the side of a passive continental margin, where there is no active tectonic movement along the coast, the width of the continental shelf can be hundreds of kilometers. For example, in the East China Sea, the water depth is still less than 200 meters 400 kilometers away from the coast, which is within the continental shelf. The width of the continental shelf in the South China Sea is also 200 kilometers, while the entire Bohai Sea and Yellow Sea are part of the continental shelf.

Outside the continental shelf (with land on the inside) is the steeper continental slope, also known as the continental slope in the diagram, where the water depth increases over a relatively short distance. For example, on the continental slope of the South China Sea, about 200 kilometers from the coastline, the water depth increases from 200 meters to over 2000 meters within a distance of 30 kilometers.

On the side of an active continental margin, where there is active tectonic movement along the coast, the continental shelf can be very narrow, but there will still be a continental shelf. The western coastline of California in the United States is a typical example of an active continental margin. Geological activity is prominent here, and the famous California State Route 1 runs along the sea on the edge of steep cliffs. Even so, the narrowest part of the continental shelf here is still about 2 kilometers wide. Beyond 2 kilometers, the water depth is no more than 100 meters, after which the slope increases and enters the continental slope area.

It can be easily calculated that the average slope of an active continental margin is only 5%. Walking forward one meter in the water, the depth increases by 5 cm. At an average person's height, when the water depth reaches 1.5 meters, the feet can still touch the bottom. At this point, the distance from the shore is already 30 meters. In contrast, the average slope of a passive continental margin is only 1/1000 of this. In most coastal areas of our country, the water remains shallow even at a considerable distance from the shore. The coast of northern Jiangsu is so shallow that, except for Lianyungang, it is impossible to build a port.

No matter how deep the center of the sea is, people always enter the sea from the shore. Whether you are swimming or boating, when you are a considerable distance from the coast, the depth of the sea is still quite limited, so there is no reason to feel afraid.

  Why is there a continental shelf?

The reason for the formation of the continental shelf is the result of sedimentation after the silt carried by rivers to the sea drifts along the coast. Sound a bit confusing? Let me explain it slowly.

All rivers that flow into the sea carry sediment. Where does the sediment go after it enters the sea? Does it sink to the bottom or float on the surface?

  Xiaoyangshan Port seawater

The water of Yangshan Port, located in Shengsi County, Zhejiang Province, can be described in one word: turbid.

The service object of Yangshan Port is Shanghai, but its location is in the Zhengsui Archipelago. It is already a distance from the mouth of the Yangtze River, but the water is still turbid.

  Change the satellite map to see more clearly.

  Satellite photos of the southeastern coastal region of China

The sediment carried by the Yangtze River not only turns the entire Hangzhou Bay yellow but also travels southward all the way to Ningde, Fujian. The entire coastal area of Zhejiang is covered.

This is not a phenomenon unique to China; similar situations occur in rivers around the world. Below are the river mouths of the Mississippi River in the United States and the Gironde River in France, which also form plumes of turbid water that drift with the ocean currents.

  Mississippi River Delta

  Gironde Estuary

Why does the sediment carried by rivers float on the surface of the sea? Because seawater is salty, even river water carrying a large amount of sediment is generally lighter than seawater. Therefore, when river water flows into the sea, it will float on the surface of the seawater for a considerable distance.

Regarding coastal currents, I won't go into detail here. Just remember: the net direction of seawater flow in coastal areas is usually parallel to the coastline. This is quite easy to understand. From the principle of continuity of matter, it is clear that seawater cannot flow into the land, nor can seawater spontaneously arise from the land and flow into the sea (except at river mouths).

Under the influence of coastal currents, river water carrying sediment will drift along the shore to coastlines where no rivers flow into the sea. During this process, the sediment in the river water will gradually settle and precipitate to the seabed. Over time, this forms the continental shelf.

Due to the shallow depth of the continental shelf (not exceeding 200 meters), which is less than the maximum depth that waves can affect, waves can resuspend sediments from the bottom of the continental shelf, transport them over some distance, and then deposit them, thus making the bottom of the continental shelf relatively flat.

  Why don't freshwater lakes have "jia"?

Understanding why the sea has a "rack" also helps to understand why freshwater lakes do not have a "rack." Freshwater lakes are fresh, and any river water carrying sediment that flows into a freshwater lake is heavier than the lake's water. The sediment carried by the river will not float on the surface but will sink directly to the bottom.

The mouth of the Goloustnaya River on the north shore of Lake BaikalGoloustnaya River

The image above shows a river flowing into Lake Baikal. Lake Baikal is the world's deepest lake, with a maximum depth of 1,637 meters. It can be seen that the river carries a considerable amount of sediment, forming a delta. However, it is clear that the sediment from the river does not float on the surface but sinks directly to the bottom upon entering the lake.

This means that apart from the river mouths where rivers flow into Lake Baikal, there is no littoral current carrying drift sediment along the other shores of the lake, hence no "shelf." The lake bottom slopes directly from the shore to the deep parts, making it very steep.

  Baikal Lake shore

  Baikal Lake shore

On one hand, the water of Lake Baikal is indeed very clear. On the other hand, it can be seen that the lake shore is entirely composed of gravel produced locally, with no silt transported from the water. Due to the refraction of light in the water, it is difficult to tell how steep the lake bottom is from the shore. After entering the water, one would find that the lake is almost bottomless. Swimming four or five meters from the shore, you can no longer touch the bottom. Swimming tens of meters into the lake, what you see through your diving mask is a deep, dark blue world. In contrast, whether at Beidaihe, Sanya, or Shenzhen Dapeng Peninsula beaches, anyone can walk into the water for twenty to thirty meters without fear, and the seawater would only just reach their chest.

  About the east coast of Yizhou

First, regarding the issue of the eastern coast of Yizhou. The eastern coast of Yizhou is located at a tectonic plate boundary and is known for its steepness, with cliffs extending directly to the sea. However, it is precisely because of this that the rivers here have a very high sediment content, delivering large amounts of silt to the sea. Therefore, the continental shelf here remains very narrow.

The above image shows the seabed topography of the east coast of Yizhou provided by nautical charts online. The depth contours are at 200 meters, 500 meters, 1000 meters, 2000 meters, and 3000 meters. The horizontal scale can be referenced by the latitude on the right. One minute of latitude is equivalent to 1.85 kilometers.

It can be seen that even on the east coast of Yizhou, the continental shelf, which is mostly less than 200 meters deep, is still at least 2 kilometers wide, with an average width of 4 kilometers. This means that near the coast, the seabed slope is 5%-10%. The steepest part of the seabed is from the 200-meter depth contour to the 1000-meter depth contour. In many places, the distance between these two lines is only 3-4 kilometers. This means that the seabed slope here reaches 20%-27%.

  About Hangzhou Bay

The upstream of Hangzhou Bay is of course the Qiantang River, not the Yangtze River, but the Qiantang River carries very little sediment. The sediment in Hangzhou Bay is almost entirely brought by the Yangtze River, which bypasses the Nanhui Promontory to enter Hangzhou Bay.

About Fjord

Coasts are generally elevated. However, there are exceptions, and one of these is a fjord. What is the essence of a fjord? It is a glacial U-shaped valley that, after the sea level rises and the glacier melts, is filled with seawater. The appearance of glaciers at such low altitudes must occur in high-latitude regions. Therefore, fjords are found along the coasts of high-latitude areas such as Norway, Iceland, Alaska, Greenland, Svalbard, Novaya Zemlya, southern Chile, northern Alaska and Canada, and the South Island of New Zealand.

What does a glacier look like when it hasn't melted? We can see them today in high latitude and high altitude areas. This is the Baltoro Glacier in the Karakoram Mountains:

The glacier itself is hundreds of meters thick, and the U-shaped valleys it has carved can be thousands of meters deep.

The image shows a melted glacier in Bomi, Tibet, China. The U-shaped characteristic of the valley formed by the glacier can be clearly seen: the bottom is relatively flat while the sides are very steep.

Sognefjorden is a famous fjord in Norway, over 200 kilometers long. The underwater contour lines in the image are at 100 meters, 200 meters, 400 meters, 600 meters, 800 meters, and 1000 meters. It can be seen that the underwater topography of the fjord still retains the characteristic of a U-shaped valley: steep slopes and a flat bottom. The width of the fjord is approximately 5 kilometers. It is evident that within 1 kilometer from the shore, the water depth reaches 1000 meters, with a slope of 45 degrees, and the maximum slope approaches 60 degrees, which can basically be described as vertical.

The reason is that the fjords are very young. The last ice age ended only about 10,000 years ago, which is not long enough for enough sediment to accumulate and flatten the slopes.

  Is the continental shelf a plain from the Ice Age?

Some people say that the continental shelf is not formed by sedimentation of silt in the sea, but rather by plains from the Ice Age. During the last Ice Age, the sea level was about 120 meters lower than it is now. Some believe that during the interglacial period, the sea level rose to its current position, causing these plains to be submerged and become the continental shelf.

This argument seems reasonable, but it fails to explain one issue: coastal areas are not all plains, and the continental shelf is not only found off plain coasts, but is continuously distributed off almost all continental coasts and large island coasts.

Taking the coast of our country as an example, the Liao River Plain, North China Plain, Yangtze River Delta Plain, Hangzhou-Shaoxing-Ningbo Plain, Pearl River Delta Plain, Zhangzhou-Xiamen-Quanzhou Plain, western coast of Taiwan, Chao-Shan Plain, Leizhou Peninsula, and most of the coastal areas of Hainan are indeed plains. However, our country also has many coastlines that are mountainous or even cliffs, such as the famous Laoshan Mountain in Qingdao:

  For example, the Sai Kung Volcanic Rock Area of the Hong Kong Geopark:

If the continental shelf is formed by the coastal plain being submerged by seawater when the sea level rises, then only areas such as the Lower Liao River Plain, North China Plain, Yangtze River Delta, and Pearl River Delta would form continental shelves. Places like Laoshan or Hong Kong's Sai Kung Volcanic Rock Region, even if the sea level rises by another 100 meters, would still be submerged slopes and would not form flat, gently sloping continental shelves.

However, the fact is that, except for the east coast of Taiwan, all of our country's coastal areas have a relatively wide continental shelf. The peak of Victoria Hill in Hong Kong is 554 meters above sea level, and it is only 2 kilometers from the peak to the coast. However, it takes 60 kilometers from the coast to reach a water depth of 50 meters. The peak of Mount Laoshan in Qingdao is 1132.7 meters above sea level, and it is only 6 kilometers from the peak to the coast. It takes 200 kilometers from the coast of Qingdao to reach a water depth of 50 meters.

In other words, whether it is the coastal plain or the mountainous coast, once you enter the sea, the slope of the terrain suddenly becomes gentle, with the steep cliffs above sea level instantly turning into a gentle continental shelf. This is clearly difficult to explain with the theory that "the continental shelf is a submerged plain from the Ice Age." The most reasonable explanation remains the one I previously proposed: the continental shelf is formed by the deposition of sediments carried by rivers and drifted along the coast.