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What China dams mean for Mekong

The worries and woes about the impacts of China's dams along the Mekong River have been simmering since their construction more than two decades ago.

But there is one big difference today. Recorded data, such as that reported in the Mekong Dam Monitor online platform, describes — and confirms — that the way these dams are being operated is upsetting the river system’s natural balance and undercutting its life-sustaining rhythms such as the Mekong flood pulse.

The monitor’s data are used to issue early warnings to downstream communities about changes in river levels of half a metre or more due to releases from China’s dams, events that can lead to flooding.

“When the threshold reaches one metre or more, the Thai disaster risk authorities issue an alert to their provincial counterparts, who then issue an alert to local counterparts,” said Brian Eyler, co-lead of the Mekong Dam Monitor.

Below are excerpts from a conversation between Reporting Asean’s Johanna Son with Mr Eyler, director of the Southeast Asia programme of the US-based Stimson Centre that runs the dam monitor.

How does the Mekong Dam Monitor tell the story of how dams are changing the region?

We’re able to examine why the river is the way it is. So is the river low now because of the way dams are operating? Or is it low now because of drought, or the combination of the two, and how much is dams and then we would assume the rest is related to drought or flood? So the monitor is for anyone really to gain insight into the impacts of the dams.

How do you know that the change in the Mekong flood pulse is due to the dams and not other factors?

We’ve ruled out other factors. And we have models that compare actual information about the river — river levels, river gauge readings — that incorporate everything that nature and humans are doing to the river. We’ve ruled out essentially that there are no other aspects of human intervention than dams, particularly (at) that Golden Triangle gauge, where we can gain the most insight for China’s upstream dam impacts.

In the early 2000s, a common narrative we heard is downstream countries should be grateful because if they need water, it can be released from the Chinese dams. But it’s not that simple, is it?

Right, that’s a false narrative. And it’s a narrative that China, particularly, has not changed, despite a huge amount of evidence that something like dry-season water releases are not good for the natural resource base and the people of the Mekong. The dry-season water releases raise the level of the river during the dry season and essentially put more water in it. So again, the Chinese discourse is that, well, you get more water.

The problem with that is it’s a reflection of water being taken out during the wet season. So there’s a symmetrical relationship between the amount of water that is held in reservoirs during the wet season and then released during the dry season.

And then that cycle repeats — more water held on the next wet season and released during the dry season. Dry-season releases produce hydropower. That’s an important benefit that people in the Mekong or importantly, people outside of the Mekong benefit from, because the power goes to Guangzhou in China or to Bangkok, which aren’t in the Mekong.

What these restrictions and releases do is they flatten the curve, they erase the pulse, and the wet season peak or the high pulse, which is what drives the expansion of the Tonle Sap. When that is lower than normal, the benefits of that flood pulse do not accrue.

Can you tell us more about the changes to the Mekong’s balance that one can see?

For China’s dams impact — and we focus on China’s dam impacts not because we have an anti-China agenda — we focus on China’s dam impacts because the dams are huge. They’re two of the largest dams in the world there, and they wield tremendous power over the outcomes of the downstream, and the outcomes are more pronounced the closer one is to those dams. For Southeast Asia, that would be the Golden Triangle, where Thailand, Laos, and Myanmar meet.

During the wet season, those dams can reduce natural flow by 60 to 70% over the course of six months — and six months’ stock of water, 60 to 70% reduction, is huge. Then during the dry season, they can triple or quadruple the amount of water that should be in the river at any one month.

So, information from the monitor provides other countries and communities some degree of agency?

Every time the Thai government issues the alerts to (the) provincial level, they also send a diplomatic letter to China saying “you should issue an alert”. So that’s agency.

We see plenty of ways, actual actionable ways, that China’s dams can be operated in a more sensible manner for the downstream that would still allow China to produce as much hydropower as they would like. And it’s a matter of timing.

What are some of these ways?

The Tonle Sap expansion typically begins to kick off in June and in July, and really takes off in August. So it’s the early months of the wet season where the kind of ignition of the expansion happens. That’s also the time when China’s big dams start to take the water out. They actually take a lot out first, and then they slow down as the wet season goes on. So there’s a kind of a conflict there, where water is needed downstream to ignite the flood pulse and expansion of the lake at a time when China is taking the most water out.

So if there’s a situation where it’s known and forecasted that the later months of the wet season are going to be very wet — well, it’s possible for those big dams to wait to first allow flow to move through the dams and give the Tonle Sap the kind of ignition that it needs. And then when the rains are coming in August, September, and in October, that’s when the reservoirs can fill.

Now what the data shows isthat by the time September and October come, the large dams are all mostly already filled. And that means lots of water running through them, and it’s coming down to the downstream. But the Mekong would be healthier if it had water earlier in the wet season.

When you look at this same dam story against the backdrop of climate change, what do you see?

Well, the wet season is compressing. The ends, the sides, the tails of the wet season are getting shorter. The wet season itself is getting shorter. So that’s the time when the dams would be filling up for their later hydropower production.

From 2015 to 2021, the wet seasons had a lot less rain than normal. And whether that was due to climate change or just a climate effect — normally, you can’t have these periods of drought during the wet season — has yet to be determined. But those dams throughout the Mekong some of them filled up. In 2020, the Chinese dams filled up close to normal, and 2020 was the worst year on record for drought during the wet season.

So when those dams filled up normally, (they) took a proportionately higher amount of water out of the river system. So that’s an effect of climate change and dams making things worse for people downstream.


Johanna Son is editor-founder of the Reporting Asean series.

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