A controversial plan to dam one of the longest and most economically significant rivers in the world will have a devastating ecological impact on the waterway and the wildlife it supports, according to a report leaked to The Guardian.

The report on the proposed Sambor dam in Cambodia warns that if construction goes ahead, it will “literally kill” the Mekong River. It goes on to state that the location chosen is one of the worst possible places for a dam, as it will prevent the largest freshwater fish migration in the world and threaten the survival of the rare Irrawaddy dolphin.

Commissioned in 2014 by the Cambodian government, the purpose of the report was to look at the impact that the massive hydropower plant would have on the river, before it was kept secret and hidden from the public. This raised concerns that those in power might simply ignore the damaging findings and go ahead with construction.

The dam project is backed by a Chinese energy company, who want to construct a 56-meter-high (183-foot-high), 18-kilometer-wide (11-mile-wide) dam at the Sambor site. This is expected to create a reservoir in the region of 82 kilometers (51 miles) long with a surface area of 620 square kilometers (240 square miles), flooding vast tracts of land and displacing many people in one of the most biodiverse regions on the planet, second only to the Amazon.

However, blocking the migration of the fish is the real concern here. The Mekong River is one of the most productive inland fisheries in the world, supplying fish to an estimated 60 million people who live along the river's 4,350-kilometer (2,703-mile) length. This fishery is thought to be worth a staggering $17 billion annually between Thailand, Laos, Vietnam, and Cambodia.

The future and sustainability of this system relies heavily on the annual migration of fish up this section of the river, which the dam falls bang in the middle of.

“A dam at this site could literally kill the river, unless sited, designed and operated sustainably,” the authors of the leaked report write. “The Sambor reach is the worst possible place to build a major dam.”

As if risking the destruction of a fishery that supports tens of millions of people wasn’t enough, the dam will also likely have a significant impact on the rare Irrawaddy dolphin that lives in the Mekong.

Once present along much of its length, its numbers have been dramatically reduced. Only last month, conservationists announced that for the first time in 20 years, the Mekong Irrawaddy dolphin numbers have risen, with just 92 of the cetaceans thought to be clinging on.

Whether or not the government will heed these dire warnings is yet to be seen.