A new dam on the Mekong could cause a biodiversity crisis

The Tonle Sap Lake is called the Great Lake by Cambodians for good reason. The scenic lake spans a vast area as the largest freshwater body in Southeast Asia. Its quaint waterborne villages are popular with tourists.

Yet Cambodia’s Great Lake does more than just provide for great photo ops. It also provides around 2 million people with their livelihoods, ranging from fishing to trading and transportation. Fisheries around the lake contribute around 75% to the nation’s production of protein.

But trouble could be brewing for Tonle Sap. The lake’s annual flood cycle largely depends on water flowing from the increasingly beleaguered Mekong River, whose water recently plummeted to its lowest level in a century. During the monsoon season the rising of water levels in the Mekong, triggered by heavy rains, causes the usual flow of the river to reverse, pushing a large amount of water into the Great Lake.

This causes the lake to overflow and inundate surrounding areas. Tonle Sap and its surrounding floodplains are rich in nutrients and home to numerous fish species and other aquatic lifeforms. Some fish spawn in these floodplain areas, whereas many migrate upstream to their spawning grounds in the Mekong River in northern Cambodia at the beginning of the monsoon season.

Large amounts of their eggs then drift downstream with the help of fresh rainwater in rivers and make their way into floodplains like Tonle Sap. These nutrient-rich floodplains, therefore, serve as important nursery grounds for young fish. The Great Lake’s fish are facing severe threats from overfishing and a changing climate. And soon they could also be facing additional threats in the form of a planned hydroelectric dam that could cut off their migratory routes.

Cambodia has major energy challenges. Half of the country’s energy consumption is imported from neighboring countries like Thailand. High energy prices and chronic blackouts are common.

“Last year there were blackouts but not as bad as this,” said Em Sokyenn, a print shop owner in Phnom Penh. “It affects business: work we are supposed to finish in a day takes two or three.”

The scarcity of power has prompted Cambodia to consider building dams in order to produce energy for domestic consumption. A planned hydropower project called the Sambor Dam could soon be in the works upstream on the Mekong River. If the country’s government decides to go ahead with the massive project, it would produce 1,800 megawatts of power.

However, environmentalists are concerned that the hydropower project would have a massive impact on fisheries. The Sambor Dam, which is backed by China, could be built at a location between where fish migrate upstream to their spawning grounds and where eggs and larvae drift downstream to nursery habitats. “A huge volume of fish biomass pass through this river corridor every year, and that migration sustains the extraordinarily productive fishery of the lower Mekong, making the Sambor site the least suitable place for a physical barrier in the Mekong Basin,” noted a recently published report.

The environmental assessment was conducted by an international team of experts directed by the California-based Natural Heritage Institution (NHI) under a formal agreement with Cambodia’s government. “NHI’s Sambor Alternatives Assessment developed a set of environmental performance standards (e.g. max fish survival and sediment passage) selected to maintain the natural functions of the Mekong river system, and then considered how a major hydropower facility could be sited, designed and operated to achieve those standards while maximizing power production and maintaining economic viability. NHI studied 10 alternative sites, designs and operations and a hybrid solar PV/hydro option,” NHI explained to Sustainability Times.

If constructed without due environmental considerations, the proposed hydropower project, and similar projects like it, could destroy natural habitats in the lower Mekong basin and floodplains, including the Tonle Sap lake, for various fish species. It would also disrupt their life cycle stages. “Sediment capture at Sambor CSP [China Southern Power (Grid Company)] would deplete the flow of the sediments and nutrients into the Tonle Sap Great Lake, where the productivity of this fishery is directly correlated to the volume of water containing the sediments and nutrients passing through the lake,” the report explained.

If Cambodia does decide to pursue this proposed project, the nation would have to pay a high price. Biodiversity would suffer and so would the environment. “Even the most advanced mitigation measures still pose high risks,” Gregory Thomas, NHI’s executive director, was quoted as saying. “There is no evidence that any large dam on a tropical river has ever been successful in the use of the latest fish mitigation technology.”

NHI recommends an alternative to hydropower dams with no harmful environmental impacts. “[We] partnered with the Solar Energy Institute of Singapore (SERIS) to carry out a comparative study of a floating solar PV facility to augment power at the recently completed Lower Se San 2 (LSS2) hydropower project as no-dam option to Sambor Dam,” NHI told Sustainability Times.

“We concluded that solar augmentation of LSS2 is the only alternative to the original Sambor Dam — even more than the most mitigated hydropower option — that provides a net positive economic benefit and no impacts to the Mekong River fishery,” it added.