Environmental activists have submitted more evidence to the Supreme Administrative Court that the Xayaburi dam is cutting Mekong River levels, threatening fish ecology and reducing catch sizes downstream in Thailand.
"Today we can clearly see the impacts we've been worrying about," said Ombun Thipsuna, a member of the Thai Mekong People's Network in Eight Provinces.
Since the 1,260-megawatt dam, located south of Luang Prabang in Laos, started operating last year, the Mekong's water levels have plummeted during certain periods, her colleague Channarong Wongla said.
The rapid changes in flow had led to unusual phenomena, ranging from unprecedented algae growth and plants withering on exposed rapids and islets to the appearance of "off-season fish".
Mr Channarong was referring to pla ket, fish usually caught by villagers between March and May but now appearing in other months since dam operations began.
If Mekong water levels remain low, its tributaries will eventually suffer similar effects and fish will not be able to breed there either, he said.
Mr Channarong works for a local environmental group in Loei, one of eight Mekong-rimmed provinces where villagers earn a living from the river's fish.
He and fellow activists from Chiang Rai, Nong Khai, Beung Kan, Nakhon Phanom, Mukdahan, Ubon Ratchathani and Amnat Charoen want the court to impose an injunction on purchase of the dam's electricity until further study of its environmental impacts is conducted. The Electricity Generating Authority of Thailand (Egat) buys 95% of the dam's hydropower under a contract with the Lao government.
"If we win the case, authorities must also conduct a Prior Consultation and Agreement [PNPCA] process in Thailand," Sor Rattanamanee Polkla, a lawyer for the Community Resource Centre Foundation, said.
"Previous PNPCA consultations were insufficient."
Cambodia and Vietnam have already voiced concerns over the impact of the dam on their rice-producing provinces.