Construction at the controversial Don Sahong hydropower dam is rapidly moving ahead, according to environmental activist Chum Huot, who was briefly detained on Sunday with his 26-year-old twin brother, Chum Huor, by Laotian authorities.

“Laos has already blocked the water,” Huot said, adding that “villagers living there have already been displaced; their houses have been moved away and the dam emerged”.

Saying he saw evidence of dynamiting, Huot said the ecological impact is already clear.

“I used to see dolphins swimming, but this time, I did not see any of them,” he said, referring to the endangered Irrawaddy Dolphin, which dam opponents say will be severely threatened by the dam.

Opposition to the dam, both among activists and governments, has been high, with those who oppose the project arguing that it will greatly deplete downstream fish stocks, a staple food source in Cambodia.

Huot and his brother were detained on Sunday while attempting to photograph the dam’s construction. Describing the incident, Huot said he, his brother, activist Se Seamdy and a boatman arrived at Don Sahong at about 10am, and were approached by Lao authorities.

“They asked us ‘why do you come here?’ Then they took the camera to check and told us that we went into their territory illegally. I was so nervous,” Huot said, adding that he never got the camera and phone back.

Released after an hour of questioning, Huot said “Laotian authorities told us that if we return, there will be a problem”.