Press Release | Tackling Myths on the Mekong: Run-of-River Hydropower

Saturday, March 31, 2018
Boats at the Pak Man Dam in Thailand
Boats at the Pak Man Dam in Thailand
International Rivers

March 31, 2018

Today, International Rivers launches a new factsheet, "Swindling the Mekong: Run-of-River Hydro." The fact sheet examines the term "run-of-river" as it is applied to understanding the impacts of hydropower dams in the Lower Mekong Basin.

A total of eleven dams are proposed for construction on the lower Mekong mainstream, and over a hundred on Mekong tributaries. Multiple studies, including the Mekong River Commission’s recently released Council Study, warn of severe impacts and trade-offs inherent in the current and proposed hydropower expansion, due to substantial and transboundary losses to fisheries, sediment transport and other ecosystem services.

Despite the warnings, the rhetoric from governments and dam developers continues to downplay the impacts of hydropower in the Lower Mekong Basin, maintaining that dams can be built "sustainably."

The existing and proposed dams in the Lower Mekong mainstream are described as "run-of-river" (ROR) dam projects. There is currently no single or clear definition of what constitutes a ROR dam, but it is generally applied to hydropower projects that have only a small reservoir or no reservoir for storing water.

The term ROR is sometimes used by dam developers to provide assurance that the impacts of a given hydropower project will not be significant. For example, proponents argue that because proposed Mekong dams have limited storage capacity, the hydrological impacts will be minimal, as they allow a "natural" flow of water through the dam.

However, this assertion doesn’t address the issue of an individual project’s operational regime, including “peaking” operations, in which large quantities of water are released in a short timeframe. In addition, focusing on the overall amount of water passing through a dam does not take into account impacts on flow patterns, including increased water levels during dry season and decreased levels during wet season. The ways in which dams alter seasonal flow patterns has severe adverse implications for downstream ecosystems and agricultural systems that are built around the seasonal flood pulse.

ROR dams, despite being touted as more sustainable than traditional storage projects, can have equally harmful impacts, particularly on ecosystems and communities downstream. Some of these impacts are inherent; others depend on how a dam is operated. Unfortunately, the impacts of ROR dams tend to be overlooked and understudied, an oversight in part due to the assumption that ROR are less destructive than traditional dams.

The fact sheet "Swindling the Mekong: Run-of-River Hydro" addresses some of the misconceptions that surround ROR dams, and examines specific case studies in the Mekong Basin. Key points outlined in the factsheet include:

  • ROR projects 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 projects that are referred to as ROR, including in the Mekong Basin, do store water, and some can in fact store water for weeks or even months.
  • The eleven 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 2-3 weeks during an average dry season, and 1-2 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 at its highest, or 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 a “death by a thousand cuts.”

Media contacts: 

Maureen Harris, Southeast Asia Program Director 
International Rivers,

Pianporn Deetes, Thailand Campaigns Director 
International Rivers,

More information: 

Read ‘Swindling the Mekong: Run-of-River Hydro’ Factsheet in EnglishThaiKhmer or Vietnamese.