Two hydropower projects along Southeast Asia’s most important river have had a measurable though moderate impact on water flows, sediment and fisheries, according to a press release on the preliminary findings of a report by the Mekong River Commission (MRC) released last week.

The report goes on to state that over the next two to three years, the commission will be able to draw a “stronger conclusion” about the impacts of hydropower development and other human pressures on the river than it can do so at present.

The press release notes that with roughly just one year’s worth of monitoring data, it is still very early in the process to draw any conclusions, as a multinational team of investigators for the MRC-Joint Environmental Monitoring (JEM) programme are still conducting test protocols that measure five key indicators of the Mekong’s health – hydrology, sediments, water quality, aquatic ecology and fisheries.

It added that by testing for these indicators at two relatively new hydropower projects – the Don Sahong and Xayaburi dams – the MRC aims to create improved monitoring methods and standards that all hydropower operators on the Mekong mainstream will then follow in the future.

Among its most significant findings, the monitoring teams observed that the overall river flow patterns did not change and that the river’s ecological health was “good” at upstream dam sites, “moderate” within the dam impoundment sites and “moderate” downstream of both dams.

“Water quality remains within human health thresholds. Within dam impoundments, there is no evidence of stratification – which can affect both water quality and fisheries. Once water passes through the dams, there is little evidence of change in downstream water quality,” according to the press release.

The monitors also detected some daily water level fluctuations downstream of the Xayaburi project in Laos, which could affect the ecosystem. However, these fluctuations didn’t occur downstream of the Don Sahong dam.

The JEM team also observed the continuation of a pattern that began in 2018 – a decrease of sediment concentration and sediment loads which bring nourishing nutrients and stabilize riverbanks – stating that the reduction “may be due to trapping at both tributary and mainstream hydropower projects”.

The JEM team observed some changes in fish diversity upstream of the Xayaburi dam, with some stability from 2017 to 2019 and then a reduction in 2020.

Downstream of the reservoir, diversity remained high with stable fish catches of high value reported in 2017, then lowering in 2018–2019, but raising again in 2020.

Around Don Sahong, though, monitors observed puzzling findings: Over the years, a fish catch decline in northern Cambodia, but an increase immediately downstream of the dam.

That said, these findings were drawn from one monitoring cycle and the JEM team suggested further monitoring over the next two to three years in order to draw a “stronger conclusion” about the impacts of hydropower development and other human pressures.

The MRC Secretariat CEO Anoulak Kittikhoun said that while this report sheds some light on the impact of hydropower, it’s too early to attribute every impact measured to the dams because climate change and other developments are also factors.

“We must understand the scope of our challenges and identify the most effective methodology to measure them,” he added.

Overall, the JEM report is part of a broader effort by the MRC – which represents member countries Cambodia, Laos, Thailand and Vietnam while maintaining a regular dialogue with Myanmar and China – to strike a balance between the economic and societal benefits of hydropower and the industry’s impact on the environment and the millions of fishing and farming families who rely on the Mekong River directly or indirectly for their livelihoods.

For the JEM pilot project, a diverse team of scientists tested their monitoring protocols for about one year, after the pandemic caused significant delays to the start of their work.

The press release stated that the report will likely become a foundation for both future policies and monitoring activities.

The report itself states that their findings can help to shape future hydropower projects “site placement and design, prediction of changes relating to the project’s operations and the development, application and evaluation of mitigation and management measures.”

Moreover, the report can provide a “basis for constructive discussions” between river-adjacent communities and MRC members.

Among its dozen-plus recommendations to the MRC’s member countries and to hydropower operators, the report recommends that to mitigate impacts on hydrology, sediment and fisheries, they should jointly introduce “targets or limits on the rate of water level change” in the Mekong River mainstream; implement a central communication notification system; jointly operate low level gates for sediment transport and in the Don Sahong dam’s case, deepen the entrance of fish passages and improve their channels.

As for future projects, the report calls for “systematic water monitoring during all phases of hydropower construction and operation, so that appropriate action can be taken if poor water quality conditions emerge.”

The MRC is also discussing how to integrate JEM monitoring into the new Core River Monitoring Network, to ensure long-term monitoring, reporting and adaptive management in order to safeguard the Mekong River.