When the normally murky brown Mekong River turned a brilliant blue late last year, villagers in northeast Thailand were surprised.
Then, this week, unusually large patches of green algae appeared, clogging up nets and making it almost impossible to fish.
Both the Mekong’s strange color and the algae have heightened worries about the health of the river that more than 60 million people in Southeast Asia depend on for their livelihoods.
Illustration: Mountain People
“This is unnatural,” fisherman Tongchai Kodrak said of the algae and the blue waters, which both signal a lower level of life-bringing sediment in the water.
He said a bit of algae appears every dry season, but this year it is more than anyone can remember.
This year is shaping up to be crucial for the Mekong River, which is under threat from climate change and faces uncertain changes wrought by two new hydropower dams that have come online in Laos in the past three months.
Many fear the new Laotian dams — the first on the Lower Mekong that generates most of the vast system’s sediment — would impede the nutrient flow.
Scientists say the Mekong’s new blue color — first seen in November last year in northern Thailand and now evident down the river into Cambodia — is caused by shallow, slower moving waters.
Normally, sediment flows all through the 2,390km Lower Mekong, nourishing fishing grounds and farmlands as it flows in from China then winds through Myanmar, Laos, Thailand, Cambodia and Vietnam.
It is the sediment that keeps the river a muddy brown, but when the water flow slows, the sediment can settle.
The fishers and farmers of the Thai village of Bungkhla said they have seen the Mekong gradually changing before their eyes in the past decade, with less predictable rainy and dry seasons, and fewer, smaller fish.
However, in the past year the change has been more dramatic as a drought during the last rainy season — from May to October — extended into this dry season.
“I’ve never seen the river this dry,” said Tongchai, 52, who has plied the river since he was a teenager.
The drought — which saw the Mekong’s levels at lows not seen for 50 years — is fueled by El Nino, a naturally occurring weather phenomenon related to warming seas that climate scientists predict is to become more frequent as average global temperatures rise.
The Mekong’s lower regions are also endangered by rising seas threatening to inundate its delta in southern Vietnam.
Ocean levels are already increasing saline levels in parts of the river, rendering the water useless for irrigation and threatening native fish.
Climate change would mean more unpredictable and extreme weather, said the Mekong River Commission, an intergovernmental organization.
That is bad news for farmers and fishers, who for centuries have relied on the river’s yearly ebb and flow.
Environmental activists said that while hydropower does not add greenhouse gases to the atmosphere in the way coal-fired plants do, the region’s planned hydro projects nevertheless represent a huge threat to the river.
Advocates of hydropower argue that the fast-growing region needs electricity, and that dams with reservoirs are useful in controlling floods and storing water during droughts.
How much of the changing Mekong is caused by climate change and how much by damming is at the center of a controversy surrounding hydropower development as Laos, backed by Thailand’s state electricity company, and now Vietnam’s state power company, moves ahead with plans to build nine new hydropower dams on the Mekong.
The first dam, the 1,285 megawatt Xayaburi Dam, began commercial operations in October last year.
The second, the 260 megawatt Don Sahong near the Laos-Cambodia border, began supplying electricity last week.
While 11 dams upstream in China have affected the water’s flow in the past decade, the new dams in Laos are worrisome because the plains of that nation are where the Mekong begins to pick up most of the sediment that feeds the system, said Tuanthong Jatagate, professor of agriculture at Thailand’s Ubon Ratchathani University.
Tuanthong said that there is no doubt that the Xayaburi Dam’s operations have contributed to the changes.
“There are many factors overall, but the main one is that the Xayaburi holds water by blocking the flow for electricity generation,” Tuanthong said.
Satellite photographs of the Xayaburi Dam on Jan. 3 by Planet Labs Inc showed the blue-water phenomenon was visible on both sides of the dam, though the image did not make clear how far upstream it extended.
In addition, environmentalists said that dam-building blocks the migration of dozens of fish species that feed millions of people, especially in Cambodia.
The Xayaburi Dam’s main developer, Thailand’s CK Power, has declined requests for interviews and did not respond to written questions sent this week.
CK Power, a subsidiary of Thai construction company CH Karnchang, has spent 6 billion baht (US$198.2 million) on “fish ladders” and sediment gates that the company has said would ensure that the river’s fragile ecosystem would not be harmed, but activists said those systems are untested.
The controversy should gain new clarity later this year — once the Mekong River Commission begins testing water downstream of the Xayaburi Dam for sediment and examining the fish channels.
In the meantime, villagers are keeping a watchful eye on the water and the weather.
In Bungkhla, fishers such as Tongchai do not go out on the water so much any more.
They mostly catch algae, they said.
“There have been fewer fish in the past few months and some types have disappeared,” Tongchai said.
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