The Yangtze River is the mother river of the Chinese nation, it originated from the northeast of the Qinghai-Tibet Plateau known as the "roof of the world" on the southwest side of the Tanggula Mountain Geladan Winter Peak, the main stream meanders through China's Qinghai, Tibet, Sichuan, Yunnan, Chongqing, Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Shanghai 11 provinces (autonomous regions, cities), through plateaus, canyons, mountains, basins, hills, plains and other terrain areas, and finally into the East China Sea.
According to the length and average annual runoff of the river, the Yangtze River ranks third in the world with a total length of 6387 kilometers and an average annual runoff of 960 billion cubic meters, ranking behind the Nile, Amazon and Amazon rivers and congo rivers respectively.
One of the Three Gorges of the Yangtze River: Wuxia
However, in terms of basin area, the basin area of the Yangtze River is 1.8 million square kilometers, which is not in the top ten in the world (Amazon River, Congo River, La Plata River-Paraná River, Ob River, Mississippi River, Nile River, Yenisei River, Lena River, Niger River, Heilongjiang River 1.856 million square kilometers).
However, unlike the Amazon River, the Nile River and other long rivers, these are international rivers, and the Yangtze River is the longest river in the world completely in the territory of one country in China, and crosses the three-step ladder of the terrain, the middle and upper reaches of the river section difference is large, the water flow is turbulent, is one of the largest rivers in the world.
The Yangtze River source braided water system
I don't know if you have noticed such a problem, the Yangtze River is huge, especially in the summer rainfall, the river water level is high, the water flow is turbulent, there are many fish in the Yangtze River are directly washed into the sea, where do these fish and freshwater fish end up? Can they adapt to the saltwater environment of the ocean?
The sea is so salty, will the freshwater fish drown? This matter must be understood first!
Seawater is very salty, and it is basically well known that you cannot drink it directly, and if you have drunk seawater, you should know that seawater has a bitter taste and the more you drink it, the more you thirst. This is because seawater is rich in minerals, with an average of 0.0375 tons of minerals per cubic meter of seawater, of which about 90% is sodium chloride, which is a variety of salts.
The salt content of the four oceans of the world is not much different, with an average salt content of 35 ‰, which means that each ton of seawater contains 35 kg of salt. However, due to the influence of rainfall, rivers, evaporation, ocean currents, seawater mixing and other factors, the salt content of offshore waters varies relatively largely, especially in places where rivers are injected. The Amazon River, the "king of rivers," for example, flows at 220,000 cubic meters per second, and a large amount of freshwater is injected into the estuary so that the salt content within 160 kilometers of the estuary is particularly low.
Overlooking the mouth of the Yangtze River
Careful friends will find that the concentration of sodium chloride injection that people usually use is 0.9%, while the normal saline (spss) used for humans or other mammals is 0.85 to 0.9%, and this sodium chloride solution can maintain the normal morphology of cells, that is to say, the salt content of seawater is close to 4 times that of human blood.
Usually, liquids with low concentrations penetrate into liquids with high concentrations due to the presence of osmotic pressure. Take drinking seawater as an example, because the salt content of seawater is greater than the salt content of human body fluids, which causes a certain amount of body fluids to penetrate into seawater with high concentrations, and finally not only seawater cannot be absorbed, but also leads to a reduction of body fluids, if the amount of seawater drinking exceeds the limit of the human body, it will lead to serious water loss and life-threatening.
Because the salt content of freshwater fish living in freshwater is very different from the salt content of seawater, whether freshwater fish in the Yangtze River are washed into the sea or saltwater fish in the ocean enter freshwater waters, they will be affected by osmotic pressure. Because the concentration of salt in fish cells is not the same, the salt concentration in saltwater fish cells is greater than that of freshwater fish, if the freshwater fish is directly put into the sea, the water molecules will enter the seawater because of the difference in density, causing the fish to dehydrate, on the contrary, after the saltwater fish enters the freshwater waters, the water molecules will penetrate into the cells of the fish, resulting in a larger volume, and finally absorb too much water and swell to death.
At first glance, it is true that freshwater fish cannot survive in the seawater, but why are there not a large number of floating dead freshwater fish in the Yangtze River estuary?
There is a problem that needs to be corrected, the main stream of the Yangtze River has the interception of Gezhou Dam and Three Gorges Dam, coupled with the relatively flat terrain in the middle and lower reaches, the river channel is wider, the water flow rate is greatly reduced, and it is impossible for live fish to be directly washed into the sea by the river water.
Global hydropower base, shipping and logistics channel: three gorges dam
On the contrary, fish in the gushing river water prefer to swim against the current, because the oxygen content in the water is the greatest, and they swim more cheerfully. And the ability of fish to swim upstream is beyond people's imagination, for example, the salmon in North America has the "ability" to cross the ocean and fly over low waterfalls.
However, in addition to the inflow rivers and lakes on the earth, the outflow rivers and lakes are connected with the ocean, and in the long-term evolutionary process, a broad salinity fish has been formed, which is mainly divided into two types: osmotic regulation type and osmotic adaptation type. And it is not difficult to understand by literally, fish adjust or adapt osmotic pressure through their own functions to meet the survival of different salinities.
Salmon migration
However, even if it is a wide saline fish, its own regulatory effect or adaptability can only be carried out within a certain salt content range, so such fish mainly survive in the offshore sea with low salt content, or in the sea where ocean currents pass through and the evaporation is small.
According to preliminary statistics, there are a total of 424 species of fish in the Yangtze River Basin (including 183 species of endemic fish), including the common "four big fish" grass carp, bluefish, silver carp, bighead carp, as well as "Yangtze River three fresh" pufferfish, anchovies, Yangtze River knife fish, as well as catfish, catfish, mandarin fish, cochineal, Chinese sturgeon and so on. Due to the construction of Gezhou Dam, the Three Gorges Dam, overfishing, busy shipping in the "Golden Waterway" and water pollution, the survival of some fish and the environment of spawning grounds have been destroyed, and the number of naturally occurring wild populations has decreased sharply and is on the verge of extinction, in 2006, the white dolphin was identified as functionally extinct, and on December 23, 2019, the Yangtze River white sturgeon was also declared extinct...
Yangtze River knifefish
"Thousand pounds of wax and ten thousand pounds of elephants", once China's largest freshwater fish Yangtze River white sturgeon was declared extinct, the largest fish in the Yangtze River is now the largest fish Chinese sturgeon, is a kind of haihe migratory fish, also belongs to the broad saline fish, they travel between freshwater and brackish waters, in the upper reaches of the Yangtze River water flow is rapid, the water temperature is low, the water quality is better and the bottom of the river is more gravel, Sichuan Yibin and other river sections spawning, growing in fish, shrimp and various mollusks rich in the Yellow Sea, the East China Sea and the northern part of the South China Sea and other shallow sea areas. But the construction of Gezhou Dam cut off its lifeline of migratory spawning.
How do you ensure that the lifeline of fish migration is unblocked?
China is the most developed country in the world with large-scale water conservancy facilities, with nearly 100,000 large and small dams built on more than 9.6 million square kilometers of land, and stored nearly 900 billion cubic meters of storage capacity, which is only more than 60 billion cubic meters less than the average annual net flow of the Yangtze River, equivalent to 2.5 times the runoff of the Pearl River.
Three Gorges Dam Locks
However, due to the large water level difference between sand dams and dams built in rivers and streams, fish cannot swim upstream, which often blocks the life path of some migratory fish migrating and spawning, posing a direct threat to their survival and reproduction.
In order to facilitate the normal migration of migratory fish to spawn in the upper reaches of the river, some biologists and ecologists suggest that when building the dam, a relatively gentle continuous stepped sink should be built on the side of the dam, and the fish can struggle up through this stepped sink until they climb over the dam to spawn upstream, and when they are tired, they can rest in the tank for a while, and will not be directly washed down by the water.
Fish ladder
The construction of the "fish ladder" can not only solve the problem that the fish cannot migrate and spawn to a certain extent, but also cause no loss to the entire water conservancy hub project, which can be called the best of both worlds.