China's largest freshwater lake dries up

[xchrom]

http://www.guardian.co.uk/environment/2012/jan/31/china-freshwater-lake-dries-up

For visitors expecting to see China's largest freshwater lake, Poyang is a desolate spectacle. Under normal circumstances it covers 3,500 sq km, but last month only 200 sq km were underwater. A dried-out plain stretches as far as the eye can see, leaving a pagoda perched on top of a hillock that is usually a little island. Wrapped in the mist characteristic of the lower reaches of the Yangtze river, the barges are moored close to the quayside beside a pitiful trickle of water. There is no work for the fisheries.

According to the state news agency Xinhua, the drought – the worst for 60 years – is due to the lack of rainfall in the area round Poyang and its tributaries. Poor weather conditions this year are partly responsible. But putting the blame on them overlooks the role played by the colossal Three Gorges reservoir, 500km upstream. The cause and effect is still not officially recognised, even if the government did admit last May that the planet's biggest dam had given rise to "problems that need to be solved very urgently".

"Every year, when the Three Gorges reservoir stores water – to power the dam's turbines during the winter – the flow rate in the Yangtze drops. This in turn increases the rate at which the level of Poyang lake falls, and the period of low water comes sooner," said Ye Xuchun, a researcher at China's Southwest University. In partnership with scientists at the Lake Science and Environment laboratory at Nanking University, he has published a comparative analysis of water levels in the Three Gorges basin and at the lake's northern extremity, near the city of Hukou, where the outflow from Poyang joins the Yangtze.

[baldguy]

1. This is what happens without environmental regulations.

Imagine Lake Erie getting dried up.

[TheWraith]

8. No. Poyang at its height had a volume of 25 cubic kilometers. Lake Erie is 480 cubic kilometers.

Size difference.

[baldguy]

9. A few years ago, there was a push by Big Agri corporate farms

in the SW who wanted to ship Great Lakes water to irrigate Arizona, Nevada & California. The deserts of the SW could easily drain that much water from the Great Lakes basin - and being the smallest Lake Erie would be the first to go. The only thing that stopped it was Canada & the governors of the Great Lakes states.

[TheWraith]

10. This has nothing to do with irrigation.

Poyang Lake dried up because they dammed up the inflow.

[baldguy]

11. Are you being obtuse on purpose, or are you really that dim?

Poyang Lake dried up because they diverted the water.

[wtmusic]

12. The Chinese government admits it was the dam.

"The central government admitted last week that the Three Gorges Dam had seriously affected navigation, irrigation and water supplies in the middle and lower reaches of the Yangtze River. "

http://topics.scmp.com/news/china-news-watch/article/Human-factors-blamed-as-lake-shrinks-to-a-puddle

The Chinese have been using the lake for irrigation for millenia. When a dam is built and six years later a lake downstream is dry, there's probably a connection.

[baldguy]

13. And a dam diverts the water from the lake.

What they were proposing for the Great Lakes wasn't a few irrigation channels within the watershed. They wanted to build an artificial river the size of the Mississippi TO DIVERT THE WATER FROM THE LAKES. It would have the exact same effect.

Duh.

[wtmusic]

14. The purpose of the dam was to generate electricity

I suppose backing the water up into a reservoir is "diverting" it, but only temporarily.

[baldguy]

16. Check your geography.

Poyang Lake is downstream from the dam and isn't inline on the Yangtze River. It's fed by a channel from the river, but there's no outlet.

Even in the OP there's a quote:

"Every year, when the Three Gorges reservoir stores water – to power the dam's turbines during the winter – the flow rate in the Yangtze drops. This in turn increases the rate at which the level of Poyang lake falls, and the period of low water comes sooner," said Ye Xuchun, a researcher at China's Southwest University.

So, the diversion of the water is not temporary, because the lake never gets replenished. The reason the water is diverted & not allowed to be replenished doesn't matter, just the fact that it is.

[FBaggins]

18. If there's no outlet... why are they arguing over whether or not to dam the outlet?

The Yangtze river is the outflow for Poyang.

A higher Yangtze water level blocks that outflow from the lake. It's called "river forcing".

[baldguy]

20. Wrong. Here's a map:

The main channel of the Yantze is at the top (labeled Changjiang). The shaded area is the normal surface of the lake. Is you can see (if you have eyes), the river channel does not go through the lake.



"Every year ... the flow rate in the Yangtze drops." And that cuts the lake off from the river, which dies up the lake. River forcing is a flood condition. The Yangtze and Poyang Lake are experiencing a drought right now.

[FBaggins]

22. Here's a paper on the subject from the Journal of Hydrology.

http://snr.unl.edu/climate_change/research/TGD_YZ_PY_article.pdf

Changes in the Yangtze River discharge caused by the TGD have further altered the interrelationship
between the river and Poyang Lake, disturbing the lake basin hydrological processes and water resources.
A major consequence of such changes has been a weakening in the river forcing on the lake, allowing
more lake flow to the river from July–March.

If you can read (if you have eyes), you'll correct your error. Or you could just try a dictionary. "Delay" and "divert" are not the same thing. Your example from the beginning of the thread is entirely unrelated to the situation in China. Nobody has decided to take water that should be in the lake and instead "divert" it elsewhere. What they've done is change the timing of that flow.

Depending on a number of factors, the river could flow in to the lake or the lake could flow into the river. Whether you choose to accept that as river water running "through" the lake is irrelevant. The statement that "the timing of that flow has changed" is entirely accurate and is the source of the problem (to the extent the dam is involved at all).

River forcing is a flood condition.

Duh. And dams have the effect of ending seasonal flooding downstream and replacing it with a steadier flow. Which... it should be noted... is entirely different from diverting the river.

[baldguy]

23. Your except supports my posts.

River forcing is a flood condition - from the river to the lake during the wet season (Yangtze -> Poyang). The dam has weakened this normal river forcing flow, and causes the flow to reverse - from the lake to the river (Yangtze <- Poyang). The river never gets to replenish the lake because the flow has been reversed. This, combined with the drought in the region, are causing the lake to dry up.

[FBaggins]

24. It really doesn't, sorry.

You've started moving closer to the right direction in the last couple posts, but nothing changes the fact that you started off talking about "diverting" and gave a specific example which is in no way similar to what's going on here.

from the river to the lake during the wet season (Yangtze -> Poyang). The dam has weakened this normal river forcing flow, and causes the flow to reverse

Closer, but still not there. As I said in the beginning, it's really a case where the lake usually drains to the river, and higher river levels reduce this outflow. Inflow from the river was much less common.

Try reading more than just the excerpt. It's a good read.

[FBaggins]

15. No... it really doesn't.

"Divert" implies (as in your example) that water that once traveled a particular path... now goes somewhere else. The Great Lakes example that you give would "divert" water that would otherwise go over Niagara Falls and send it instead into AZ/NV.

In this case they aren't changing the course of the river. The outflow from the dam still runs through the same channel and runs to Poyang (and then further on).

As the article pretty clearly lays out, the problem is a combination of a draught (so the rest of the lake's watershed isn't feeding it) and the fact that the dam is storing water from seasonal floods. So while the watercourse still runs through the lake, the timing of that flow has changed.

It sounds like Poyang historically has had significant seasonal variation itself. A possible solution involves daming the outflow of the lake. This shifts the problem downstream until the system fills up (possibly years later).

[baldguy]

17. The main channel of the Yangtze doesn't run through the lake.

The water is diverted to the dam's reservoir & is not replenished. It states that clearly in the article in the OP.

[FBaggins]

19. It runs past the outflow for the lake.

The water is diverted to the dam's reservoir

Sorry... that doesn't make sense. The water doesn't stay in the reservoir. When it is released it travels in the same bed it was in before.

Storing water isn't the same thing as diverting it.

It states that clearly in the article in the OP.

They must have removed that part. Nowhere does it say that the water is diverted.

[baldguy]

21. There are several rivers that run into the Poyang.

And at high river levels a channel from the Yangtze runs into it too, while the main part of the river continues on. The dam is upstream from the lake (to the left on the map), and when they divert water into the reservoir, the level of the river drops and stops feeding the lake. The other rivers don't have enough flow to maintain the lake levels in the drought, so the lake dries up. When the water is released from the reservoir (into the Yangtze river channel), it doesn't raise the level of the river enough to replenish the lake. The released water bypasses the lake, staying in the main river channel.

[dixiegrrrrl]

2. dam up the downstream flow rate to the lake and then say

" no one could have foreseen" the combined effects of a drought plus decreased inflow.

[Dead_Parrot]

3. Oops. nt.

[Judi Lynn]

4. Painful to learn this has happened. What a sad, sad situation. n/t

[Nihil]

5. That will cause a rewrite of the record books ...

> For visitors expecting to see China's largest freshwater lake ...

So, I guess the second largest has been promoted (and so on)?


> Under normal circumstances it covers 3,500 sq km, but last month only 200 sq km were underwater.

That's a hell of a change!

[wtmusic]

6. But...but...renewable power has no consequences!

This news is definitely harshing my renewable mellow.

[hunter]

7. It's an interesting place.

The lake has filled and emptied through time depending on the course of the Yangtze.

The existing lake was formed around AD 400 when the Yangtze shifted south. The people living in the basin were flooded out and forced to migrate to Wucheng Township.

http://en.wikipedia.org/wiki/Poyang_Lake

The history of the lake reminds me of the Imperial Valley and Salton Sea in California which has filled and emptied through time as the Colorado river shifts across its delta.

I hate dams... wild river systems are a lot more interesting.