Tackling Hydropower Myths on the Mekong: “Run-of-River” Does Not Mean Sustainable

A boat on the Mekong in Laos.
A boat on the Mekong in Laos.
International Rivers


A few weeks ago, the 3rd Mekong River Commission Summit took place in Siem Reap, Cambodia. The Summit is held every four years, bringing together the leaders of the Lower Mekong member countries: Cambodia, Lao PDR, Thailand and Vietnam, together with representatives of China and Myanmar as MRC dialogue partners. The MRC describes the Summit as an opportunity for political leaders to address “prevailing challenges and opportunities” facing the Mekong Basin. 

To say that the challenges are considerable would be an understatement. With two of 11 planned hydropower dams under construction on the Mekong mainstream, and many more coming online on important tributaries, the Mekong faces an unprecedented ecological threat. Multiple studies, including the MRC’s own recently-released Council Study, warn of extremely severe impacts and trade-offs in the current plans for hydropower expansion. Fisheries, sediment flows, and other ecosystem services will experience substantial transboundary losses – and these impacts will only be exacerbated by climate change. 

Read our new fact sheet, "Swindling the Mekong: Run-of-River Hydro."

Despite the warnings, dam proponents continue to downplay the impacts of dams in the Lower Mekong Basin, asserting that these “run-of-river” (ROR) projects can be built “sustainably” with limited impacts or that impacts can be minimized. This argument relies on largely blind faith in the effectiveness of impact mitigation technologies, such as fish passages, ‘fish-friendly’ turbines and sediment flushing facilities, which are yet to be adequately tested or proven effective in the context of the Mekong. The argument also links to a broader discourse of so-called “sustainable hydropower” that is increasingly prominent in lower Mekong countries, but is not borne out by documented experience from existing projects in the Lower Mekong Basin.   

The term “run-of-river” is itself often used in misleading ways. Although there’s no clear definition of ROR, the term typically refers to dams that have limited storage capacity. Developers have tended to overlook and understudy the impacts of ROR dams, in part because – in the absence of any clear or meaningful definition of the term – they simply assume that ROR projects are less harmful than traditional dams.

In fact, ROR projects can have equally serious impacts, particularly on ecosystems and communities downstream, even without large reservoirs. Some of these impacts are inherent; others depend on how a dam is operated. ROR dams alter seasonal flow patterns, increasing water levels during the dry season and decreasing them during the wet season, with profound implications for downstream ecosystems and agricultural systems that are built around the seasonal flood pulse.

International Rivers’ newest fact sheet looks at the term ROR as it is applied to different types of hydropower projects in the Mekong. We find that:

  • While there’s no single definition of ROR, it’s usually applied to hydropower projects that have either a small reservoir or no reservoir. ROR projects do differ from traditional storage dams, which store large quantities of water in the wet season to allow year-round releases to generate power. However, many ROR projects in the Mekong Basin do store water, and some can in fact store water for weeks or even months.
  • The 11 dams planned and under construction on the lower Mekong River mainstream are classified as ROR projects. However, the storage capacities, reservoir sizes and operational strategies of these projects differ greatly. Some of the Mekong mainstream dams have the capacity to retain flows for two to three weeks during an average dry season, and one to two weeks during an average wet season. Some of the projects will have very large reservoirs, submerging large areas of land and displacing thousands of people.
  • ROR dams with storage, or “pondage,” can be operated to produce “peaking power.” These projects can time releases and generate power for those hours or days when energy demand is “peaking.” By releasing an entire day’s worth of flows within the span of a few hours, they create daily fluctuations between flood and drought that can wash away or disrupt fish breeding grounds and aquatic biota that are critical to the food chain. Projects operated in this manner create the opposite of a river’s natural flow. Peaking plants can also pose a significant safety risk to people living downstream, who can be exposed to unexpected dam releases.
  • ROR projects often have significant impacts on fish and other aquatic species. They block the upstream and downstream migration of fish and other biota, and prevent sediment and nutrients from flowing to floodplains downstream. They often inundate important biodiversity hotspots, which tend to occur near the rapids that attract dam developers. 
  • ROR projects are often built in a cascade, as the construction of the first dam on a river regulates its flow, making the development of subsequent projects downstream more economical. Cascades can pose impenetrable barriers to migratory fish and exacerbate impacts on riverine ecology. Cascades also have a more pronounced effect on blocking sediment from traveling downstream, impacting both dam operation and, most critically, the ecosystems and fertility of floodplains downstream. The cumulative impacts of dam cascades on river health cannot be measured by examining each project individually, as such projects can condemn a river to “death by a thousand cuts.”

Read our new fact sheet "Swindling the Mekong: Run-of-River Hydro" in Thai, Khmer or Vietnamese. 


Terms such as “run-of-river” and “sustainable hydropower” tend to minimize and conceal the very real impacts of the current and proposed Mekong mainstream dams. It is critical that Lower Mekong governments acknowledge the severity of the threats facing the Mekong basin due to hydropower construction rather than downplaying or understating them. 

In assessing the cumulative impacts of hydropower developing on the basin, the MRC Council Study is a step towards ensuring that decisions made adequately address these threats. Among its key findings and recommendations, the MRC Council Study “strongly recommends” that governments consider renewable power technologies as increasingly viable alternatives to proposed hydropower development. 

If the Mekong River and the people of the Mekong Basin are to have a truly sustainable future, the lower Mekong governments must seriously heed this advice. We are therefore urging lower Mekong leaders to support a moratorium on further hydropower construction until a regional study of renewable energy technologies and alternatives is completed.

Thursday, April 19, 2018