How Environmental Flow Practices Keep Rivers and Communities Healthy

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River infrastructure projects must consider natural flows in project planning to save aquatic ecosystems and downstream communities.


Humans have relied on rivers for thousands of years and have been modifying river flows for nearly as long. Enhanced technologies and infrastructure capabilities have only led to greater alteration of the natural flow of rivers for the purpose of irrigation, flood control, water supply, and electricity generation. Globally today, only 37% of rivers longer than 1,000 kilometers are free-flowing.

Infrastructure projects, such as dams and hydropower plants, cause direct upstream impacts, including flooding for reservoir filling, necessity of relocation of communities residing in the submergence areas, and more. Scientific studies have indicated that these projects can also bring more negative impacts on downstream ecology than climate change, especially when project planning is poor.  

These negative effects highlight the importance of considering the environmental flows (EFlows) concept in projects with the potential to induce significant alteration of a river flow. EFlow is defined as the quantity, timing, and quality of freshwater flows and levels necessary to sustain aquatic ecosystems that in turn support human cultures and well-being, sustainable livelihoods, and economies.

How do EFlows work?

EFlows concept considers the traits of natural flow regimes in a river infrastructure project, to be able to maintain aquatic ecosystems. A change in the quantity of water in a river can lead to water scarcity downstream, erosion of riverbanks, and alteration of the physical characteristics of ecosystems. The timing of released water (for example, for sediment flushing from the reservoir, or from peak hydropower plants) is also important, as every river has a natural variance in release frequency, that it is used by several migratory aquatic species to synchronize their spawning period and ensure that eggs or larvae have the best chances to survive. Projects that release water too regularly or with too much sudden variance are both dangerous to people and ecosystems. Water quality considers not just pollution levels but also the temperature, salinity, oxygenation, and nutrients concentration. People and aquatic organisms are well attuned to specific qualities of water so slight changes can have significant impacts. Projects like dams also often block sediment and biota flows, such as sand and fish migrations, which can lead to erosion of beaches and extinction of species, if these effects are not properly mitigated.

These river changes ultimately impact people. Many rivers, such as the Ganges, have cultural or religious significance. They can contribute to national economies and can play essential roles in various industries, such as transportation, agriculture, fishing, and tourism. Rivers also support local livelihoods and healthy living by providing water for irrigation, drinking, and laundry, to name a few. Through the healthy environment, diet, and scenic spots that they provide, rivers can help improve the well-being of people.

Figure 1. The Impact of EFlows on People

Source: Alta Alonzi

EFlows assessments can help balance the trade-offs between infrastructure projects and the benefits of ecosystem services from natural flows throughout river basins. A good, holistic EFlows assessment allows policy makers and project designers to consider all factors of river flows (including the quantity, timing, and quality of water plus the sediments, fish, and other life flowing through river basins) as well as the impacts of these factors on the livelihood, cultural practices, and the overall well-being of local people. This information can help shape the project plan and make management adaptive to avoid or mitigate any changes in natural river flows that would negatively impact downstream communities.

What is the status of EFlows around the world?

Many countries have no national policies or guidance regarding EFlows. This means project developers and operators have few incentives to consider the downstream impacts of their plans. In these cases, EFlows are unlikely to even be assessed so governments and communities are unaware of the true costs and impacts of a project.

There are countries that include the words “environmental flows” or similar language in national water or environmental strategies. However, the definition given usually refers to the minimum flows, which requires only a set percentage of river flow (usually 10%) to remain in the river for environmental purposes. Unfortunately, this is a rule of thumb that does not assess or consider the context of the entire river basin and the people living nearby, and it is not considered a good international practice. Minimum flow guidelines do not help in balancing trade-offs and do not consider the other aspects of EFlows, such as timing, quality, sediments, and biota.

While no country has perfectly implemented EFlows, best practices in design and policy guidance can be found in Australia, South Africa, and United States.

How should EFlows be implemented?

To achieve the best results, EFlows should be planned at the national level first, then considered on the river-basin level, and incorporated at project level.

National level

Rivers management should be coordinated between the relevant ministries at the national level to apply the following best practices:

  • EFlows should be clearly defined and included in national-level policies beyond mere minimum flow requirements.
  • EFlows assessment should be required at the river-basin and project levels. It should be required as well before project renovations and relicensing.
  • National capacity for reviewing and implementing EFlows recommendations must be developed.
  • A strategic assessment of national priorities and comprehensive needs analysis, involving the economic, ecological, and social demands on rivers should also be conducted to inform decision-making on acceptable trade-offs between the needs of all stakeholders.
  • The strategic assessment can also include evaluations of technological and siting options, locations of set-aside areas, and predetermined environmental and social costs in financial models for future assessments.

River-basin level

To improve outcomes and save time and resources, EFlows should be assessed at the river-basin level before projects are planned. This involves 

  • Collecting data on the environmental, social, and hydrological needs/status quo;
  • Assessing climate resilience;
  • Engaging with stakeholders; and
  • Evaluating options for siting and technologies.

Project level

EFlows should be required for all infrastructure projects that impact river flows. Project-level EFlows assessments must rely heavily on national and basin-level data and determinations, and must include:

  • Collection or extrapolation of data not readily available from the basin-level assessment;
  • Analysis of alternative siting or technology options;
  • Analysis of no-project option; and
  • Any additional project-specific EFlows calculations.

EFlows assessments and recommendations should also inform project decision-making throughout the planning process, especially to inform:

  • Feasibility studies,
  • Purchasing power agreements (for hydropower projects only),
  • Environmental impact assessments,
  • Project designs,
  • Licensing agreements,
  • Adaptive management decisions, and
  • Relicensing agreements.
How is the Asian Development Bank working to implement EFlows?

Lao People’s Democratic Republic

Completed in 2010, the Nam Theun 2 Hydroelectric Project involved a transfer of water that resulted in more and faster water flows, which could flood habitats, homes, and gardens. The Downstream Program was included in the project to work with stakeholders and mitigate the negative impacts on communities and biodiversity. A 2017 evaluation of the program showed improvements in consumption, diets, housing, asset ownership, and sanitation levels in downstream communities. However, fish catch and per capita incomes had declined overall in the same time period. The Lao PDR government has taken over the program and plans for its continuation are underway.


The Gulpur Hydropower Project included a holistic EFlows assessment of the entire river basin in the early stages of project planning. The basin-level assessment included quality data and robust modeling, which was later used to inform the EFlows assessment of the larger Balakot Hydroelectric Development Project located in the same river basin.


In the Alaoa Multi-Purpose Dam Project, the EFlows assessment was conducted early in the project planning process, allowing ADB staff to require adjustments to the project design. Additionally, as part of this project, ADB is working with the Samoan government and national utility company to develop national  EFlows assessment guidelines, to be used on the entire river basin for the future and, if possible, to retrofit previous projects.


A. Arthington et al. 2018. The Brisbane Declaration and Global Action Agenda on Environmental Flows (2018). Frontiers in Environmental Science 6.

A. Forslund et al. 2009. Securing Water for Ecosystems and Human Well-Being: The Importance of Environmental Flows. Swedish Water House Report. 24.

Asian Development Bank (ADB). Pakistan: Balakot Hydropower Development Project (formerly Hydropower Development Investment Project).

ADB. Lao People's Democratic Republic: GMS Nam Theun 2 Hydroelectric Project.

ADB. Pakistan: Gulpur Hydropower Project.

ADB. Samoa: Alaoa Multi-Purpose Dam Project.

Australian Government Commonwealth Environmental Water Office. Flow-MER Is the Commonwealth Environmental Water Offices On-ground Monitoring, Evaluation, and Research Program.

B. Arheimer, C. Donnelly, and G. Lindström. 2017. Regulation of Snow-Fed Rivers Affects Flow Regimes More Than Climate Change. Natured Communications. 8.

C. Brown. 2018. Environmental Flows (EFlows): Concepts and Methods. Presentation at the Training Course on Pollution, Health, and Safety Management in ADB Projects - Water Quality Module on 22 November at the Asian Development Bank. Youtube.

C. Brown, D. Campher, and J. King. 2020. Status and Trends in EFlows in Southern Africa. Natural Resources Forum.

C. Brown et al. 2019. Achieving an Environmentally Sustainable Outcome for the Gulpur Hydropower Project in the Poonch River Mahaseer National Park, Pakistan. Sustainable Water Resources Management. 5. pp. 611–628.

Conservation Gateway. Environmental Flows Policy and Implementation.

M. Acreman and M. Dunbar. 2004. Defining Environmental River Flow Requirements? A Review. Hydrology and Earth System Sciences Discussions. European Geosciences Union, 8 (5). pp. 861–876.

World Bank Group. 2018. Environmental Flows for Hydropower Projects: Guidance for the Private Sector in Emerging Markets. Washington, DC: International Finance Corporation.

Francesco Ricciardi
Senior Environment Specialist, Office of Safeguards, Asian Development Bank

Prior to joining ADB, Francesco Ricciardi worked for about 10 years as a researcher focusing on the impact of environmental contamination on natural ecosystems and wildlife. After leaving academia, he worked as an environment and ecology consultant in several projects around Asia, including renewable energy plants, large coastal infrastructures, and natural resources development and protection. He is a passionate underwater and wildlife photographer. Some of his photos have been published in international magazines and papers.

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Alta Alonzi
Former Intern, Sustainable Development and Climate Change Department, Asian Development Bank

Alta Alonzi has a master's degree in Public Policy at the Lee Kuan Yew School of Public Policy, National University of Singapore. She has experience working with international development organizations around Asia from the grassroots to international levels. She completed her undergraduate education in Global Liberal Studies at New York University, where her senior thesis explored environmental policies in the United States and the People’s Republic of China.

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