The report, compiled by Vattenfall Power Consultant with support from the Asian Development Bank, which will finance the Nam Ngum 3 Hydropower Project, was presented at a meeting in the Lao capital Vientiane Tuesday.
The Lao government plans to build a series of dams in the Nam Ngum basin in Vientiane Province to generate electricity for export - mostly to neighbouring Thailand.
The dams include Nam Ngum 2, which is under construction; Nam Ngum 3, which is being studied, while Nam Ngum 5, Nam Lik and Nam Bak are still in the pipeline.
The study, which considered three scenarios for hydropower and irrigation development, found the projects would block fish migration routes, destroy riverine habitat, and affect water quality.
A serious impact was seen on aquatic ecology.
Water quality in the existing Nam Ngum 1 reservoir would be significantly affected and key migration routes would be blocked by construction of Nam Ngum 2 and Nam Lik, according a summary of the report.
"The existing migration route to and from the Mekong River is the only remaining, unregulated connecting channel in the Nam Ngum Basin, and used by important species such as Pangasids and Cyprinids. The Nam Lik river supports a population of over 30,000 people, many of whom have a strong dependence on fisheries," it said.
"Subsistence farmers, the poor, the landless, ethnically and otherwise marginalised groups with few alternatives are likely to be hit hardest by any impact on habitats and wildcapture fisheries. The new reservoirs (Nam Ngum 2) are mainly expected to have moderate to low potential for reservoir fisheries," said the report, a copy of which was seen by The Nation.
The hydropower sector is developing fast in Laos. The government expects that the Nam Ngum projects could generate between 1,800 and 2,100 Megawatts by 2020.
Laos hopes the projects will generate income for poverty reduction.
However, the hydropower cascade could undermine the Laos' poverty reduction plans, particularly in the absence of any revenue management or legally enforceable contracts to share the benefits from dams with affected people.
"The impact of hydropower development on the rural poor will depend largely on the existence of concrete mechanisms to guarantee affected villagers benefit directly from the revenue earned by hydropower projects through formal benefitsharing mechanisms," the report said.
Environmentalists say the Lao authorities' plans and way of operating lack transparency and have not allowed full participation by local people who might be affected by the projects.
"Decisions have been taken to proceed with hydropower projects even before individual and cumulative environmental and social impacts have been fully assessed," said Shannon Lawrence, the Lao program director for the International Rivers Network (IRN).
"Uncoordinated development and poor basin management pose major risks to local communities, as well as to investors," she said.
No environmental impact assessment or resettlement plan had been disclosed for the Nam Ngum 2 dam, which has been under construction since 2006 - in violation of Laos' National Policy on Environmental and Social Sustainability of the Hydropower Sector, an IRN statement said.
Thai investors include Ch Karnchang, Ratchaburi, Bangkok Expressway Plc and GMS (Thailand), which also have stakes in Nam Ngum 2 and Nam Ngum 3. These projects have a combined installed capacity of 1,075 megawatts.
By Supalak Ganjanakhundee