• Environmental Impact Assessments (EIAs) and Strategic Environmental Assessments (SEAs) are essential tools for identifying the potential impacts of water infrastructure projects. These should consider cumulative effects of multiple projects, as well as alternative approaches, mitigation, compensation, and/or restoration, and the option to cancel the project if the impacts are shown to be too severe or data gaps prevent a robust threat assessment.
  • System scale planning is critical to designing the best overall energy mix, including informing alternative location options to minimise the ecosystem impacts of any potential hydropower dams, while still allowing energy needs to be met.
  • Financial institutions can require the installation of an Independent Scientific and Technical Advisory Panel (ISTAP) and the implementation of their recommendations as a condition for financial support for water infrastructure and navigation projects to avoid activities that negatively impact river dolphins.
  • The implementation of environmental flows and water reserve legislation can reduce the impact of water infrastructure projects, while still benefiting people and nature.
  • River dolphin rescue expertise and processes can save individual animals and help to raise awareness and involvement of local communities, but should be viewed as a last resort, rather than a mitigation strategy.
  • Fish ladders have shown to be ineffective at enabling fish migration in river dolphin rivers.
  • The impacts of underwater noise and risk of ship strikes can be mitigated by routing ship traffic away from critical dolphin habitats, and implementing speed regulations and technical adaptations for the vessels involved.


How do water infrastructure and navigation pose threats to river dolphins?

Infrastructure, such as hydropower dams, dikes and dredging of channels to facilitate shipping and transport, alters river flows and connectivity, disrupting sediment transport and deposition patterns, fragmenting habitat, and potentially changing species compositions in waterways.

River dolphins depend on free-flowing water to navigate between preferred habitats, which may change between high and low water seasons, or in relation to the migrations and movements of their fish prey1, 2. These movements often cover extensive areas with dolphins heading both up and down the river’s main stem as well as laterally into tributaries, channels or flooded forests1, 2, and river dolphins have adapted specifically to these seasonal movements and environments. Although preferred habitats differ from species to species, all river dolphins rely on the availability of channels deep enough to navigate from one part of the river to another, and they use echolocation to guide their navigation, as well as to find food and communicate35. Water infrastructure and navigation can impact these essential functions in multiple ways:

  • The construction of dams and other infrastructure (e.g. irrigation barrages, channelisation and improvements for navigation, dikes, embankments, and even roads) can create physical barriers to dolphin movements along a river’s main stem, or laterally into tributaries and channels. These barriers can cut dolphins off from essential feeding, breeding or resting habitats, reducing their overall fitness. The problem is severe: it is estimated that only three free-flowing tributaries will remain in the Amazon if all of the 277 planned dams are built in the next few decades6. Dam construction and planning is also prevalent, and poses threats, along the Ganges in India and the Mekong in Laos.
  • These physical barriers can also fragment river dolphin populations, preventing exchange between populations, and rendering isolated populations more vulnerable to threats and more likely to suffer from loss of genetic diversity711. For example, the Kaptai dam on the Karnaphuli River in Bangladesh trapped an unknown number of dolphins in its reservoir, which had all apparently disappeared by the late 1990s12. Where infrastructure projects do not create direct physical barriers, they indirectly prevent dolphin movements and connectivity by affecting water flow and availability, resulting in areas that are too shallow for dolphins to traverse.
  • Physical barriers and decreased flow can leave dolphins ‘stranded’ when they have moved laterally into flooded forests or channels during high water levels, and are then unable to return to the deeper main stem when water levels drop1315.
  • Infrastructure projects can also negatively impact dolphins’ prey, particularly when fish are blocked from undertaking  long-range migrations upstream for breeding, or when important fish habitat, such as natural vegetation and gravel banks, are replaced with man-made embankments.  The construction of the Three Gorges Dam on the Yangtze, for example, was associated with a 90% decrease in the fish fry (juveniles) of four commercially important carp species, which are also prey for Yangtze finless porpoises12
  • Construction activities and vessel traffic in rivers generate underwater noise that can negatively impact river dolphins by (1) masking echolocation; (2) inducing stress or changes in behaviour that can have cumulative, long-term negative impacts; (3) causing hearing impairment; or (4) impacting fish prey1618. A study of Yangtze finless porpoises found a clear correlation between levels of vessel traffic and increased levels of stress-related hormones19. A study of Ganges river dolphins found that chronic underwater noise caused by shipping resulted in lower feeding rates and higher energy expenditure, potentially decreasing population fitness over time16.
  • Vessel traffic can also directly cause mortality and injury through collisions. In the Yangtze River, an interview study in 2008 concluded that a four-fold increase in the annual mortality rate of finless porpoises in the main stem over 20 years had been predominantly caused by vessel strikes20.

Mitigation strategies - water infrastructure

It is difficult to effectively mitigate the impact of water infrastructure projects or vessel traffic in rivers. Rivers are by nature constricted areas where there is little room for manoeuvre. Many projects alter river habitats so significantly and cumulatively, that there is no way to avoid all their negative impacts. Nevertheless, projects around the world offer lessons in how to significantly reduce these impacts.

Mitigating impacts of water infrastructure:

  • (Strategic) Environmental Impact Assessments are required to fully assess the consequences of any water infrastructure project within, upstream or directly downstream of river dolphin habitat before it is approved. Both types include social and cultural aspects as well, apart from the environmental ones. In the case of one specific project, an Environmental Impact Assessment (EIA) is necessary, while a series of projects requires a broader Strategic Environmental Impact Assessment (SEA). Both types of assessments should take into account the potentially cumulative impacts of multiple projects and should investigate alternatives to the proposed project(s) that would have less impact (e.g. shifting shipping lanes so that they do not overlap with critical dolphin habitat) as well as mitigation, compensation, restoration, and the option to cancel the project if the effects are too severe or data gaps prevent a robust threat assessment analysis. A sound (Strategic) EIA must include21, 22, 23:
    • An evaluation of all negative impacts caused by the (series of) project(s) on river dolphin habitat. In the case of hydropower dams, eight impact categories need to be understood and addressed including impacts on: (1) river dolphin and fish migration; (2) water flow; (3) sediment flow; (4) water quality; (5) nutrient flow; (6) erosion; (7) river fragmentation and (8) fish breeding/ spawning. If there is not enough information for a thorough assessment of (part of) these impacts, the project should not be approved; 
    • Proposals for actions to mitigate or compensate  for the impacts identified, following internationally-agreed guidelines21, 22, 23. If it is not possible to mitigate and/or compensate the impacts, the project should be substantially revised;
    • A thorough stakeholder consultation process, including the full range of stakeholders affected by the project; and
    • Guaranteed political uptake of the recommendations and conclusions of the EIA/SEA.

An inspiring example of a well conducted SEA is IFC’s Strategic Environmental Assessment of the Myanmar Hydropower Sector24.

  • Independent Scientific and Technical Advisory Panels (ISTAPs) can provide advice to ensure that  activities do not negatively impact river dolphins. Project funders can make the installation of an ISTAP and the implementation of its recommendations a contractual requirement. The ISTAP should have access to monitoring data throughout the project so that it is involved in measuring potentially negative impacts and designing responses. In this way, financial institutions can play a significant role in conserving river dolphins.
  • System scale planning/hydro by design25, 26 are frameworks to ensure that a robust, integrated planning process is implemented on a river basin scale to select the most practical, efficient, environmentally-friendly and cost-effective option for a project or series of projects. If hydropower generation is part of the best overall renewable energy mix, carefully locating new dams can significantly reduce their negative impacts on connectivity and river health while still ensuring that energy needs are met. This approach can be extended beyond country boundaries; for example, barrages in India have impacted river dolphin habitat and migration routes in Nepal. Planning at a higher, sometimes cross-boundary, level provides opportunities to consider alternatives to energy or navigation infrastructure projects that would fragment dolphin habitat – such as opting for wind or solar power rather than new hydropower dams. This kind of system scale planning can also reduce costs. For example, choosing a wise location and design of inland ports, shipping lanes and vessels can reduce the amount of costly dredging that is required – and its negative environmental effects.
  • Free-Flowing Rivers (FFR)27 are critical strongholds for river dolphins and biodiversity in general. New river infrastructure should avoid fragmenting the world’s remaining free flowing rivers. Case studies show how the FFR approach gives insight into specific country/regional situations. Analyses in both the Amazon28 and Myanmar29 conclude that there should be no-go zones: areas where new infrastructure must be avoided as it would impact the river systems too much and reduce the health of river dolphin populations, including critical migratory routes – and that alternative options should be pursued to meet energy or water resources needs. In some cases, the removal of an existing (obsolete) dam could create huge positive benefits for the river system, river dolphin populations and/or fish and food security. The Myanmar example shows how a FFR analysis can be the basis to ensure that a National Energy Plan does not compromise healthy rivers and their ecosystem services24, 29.
  • Maintaining or implementing environmental flows (e-flows) can reduce the negative impacts of existing water infrastructure projects by mimicking the natural water flow regime in terms of quantity, quality and timing – fluctuations that trigger key biological processes, such as fish spawning. Water managers should assess the ecological needs of the system and establish a flow regime (quantity, quality and timing) that can maintain the essential processes required for a healthy river system as much as possible, while also meeting human needs. In Mexico, water reserves legislation ensures the socioeconomic need for functioning river ecosystems, securing a minimum water amount/flow for the environment in designated rivers before allocating water to other users (drinking water, irrigation, industry)30, 31. An environmental flows programme for the Three Gorges dam was initiated in 2011, primarily to promote the spawning of a commercially important carp species whose population had collapsed after the dam was built; since then the carp population has partly recovered12.
  • Fish ladders are not considered a best practice as they appear to be ineffective for the majority of tropical freshwater fish species, which vary in their swimming strength and may be too numerous to pass through constricted ladder areas3235.
  • River dolphin rescue programmes are an important last resort for when river dolphins become trapped in channels, canals or isolated pools and need to be translocated to the main river. River dolphin rescues conducted by government institutions, trained community members and/or research and conservation networks, have proven effective in translocating trapped dolphins. Successful rescue operations have been conducted in the Amazon, as well as in India, Cambodia and Pakistan1315, 36, 37.

Mitigation strategies - shipping

Mitigating impacts of vessel traffic:

  • Acoustic sanctuaries or ‘no go’ zones can route ships away from important habitats and have been created in marine, coastal and inland waters to reduce the risk of vessel strikes and noise disturbance to dolphins and manatees in areas that are important for reproducing, feeding, or resting38, 39. However, river systems are more constrained, so the options for closing off particular parts of the river to vessels are more limited than in marine environments16.
  • Speed regulations can be an effective way of reducing underwater noise and the risk of vessel strikes4043 and have proven to be effective in reducing mortality rates of Florida manatees in settings similar to those where river dolphins are found32. Both the vessels and the animals have more time to observe and avoid each other, and vessel-strikes at a lower speed are less likely to be lethal.
  • Technical adaptations to protect dolphins from propeller damage and make vessels quieter can help reduce the risk of injury to cetaceans if strikes occur, and can also reduce the amount of underwater noise they generate. Examples can be found here and here.


  1. Paudel S, Pal P, Cove MV, et al. The Endangered Ganges River dolphin Platanista gangetica gangetica in Nepal: abundance, habitat and conservation threats. Endangered Species Research 2015;29:59-68.
  2. Martin AR, da Silva VMF. River dolphins and flooded forest: seasonal habitat use and sexual segregation of botos (Inia geoffrensis) in an extreme cetacean environment. Journal of the Zoological Society of London 2004;263:295–305.
  3. Jensen FH, Rocco A, Mansur RM, Smith BD, Janik VM, Madsen PT. Clicking in Shallow Rivers: Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow, Acoustically Complex Habitat. PLoS ONE 2013;8:e59284.
  4. Sugimatsu H, Ura T, Kojima J, Bahl R, Behera S. Underwater behavioural study of Ganges river dolphins by using echolocation clicks recorded by 6-hydrophone array system. OCEANS 2009, MTS/IEEE Biloxi – Marine Technology for Our Future: Global and Local Challenges 2009.
  5. Pilleri G, Zbinden K, Gihr M, Kraus C. Sonar clicks, directionality of the emission field and echolocating behaviour of the Indus River Dolphin (Platanista indi Blyth 1859). Investigations on Cetacea 1976;7:13-44.
  6. Castello L., Macedo M. M., 2016, Large scale degradation of Amazonian freshwater ecosystems, Global Change Biology, 22, 990 – 1007, doi: 10.111/gcb.13173
  7. Taylor BL, Abel G, Miller P, et al. Ex situ options for cetacean conservation: December 2018 workshop, Nuremberg, Germany. Gland, Switzerland: IUCN; 2020.
  8. Krützen M, Beasley I, Ackermann CY, et al. Demographic collapse and low genetic diversity of the Irrawaddy dolphin population inhabiting the Mekong River. PLOS ONE 2018;13:e0189200.
  9. Pavanato HJ, Melo-Santos G, Lima DS, et al. Risks of dam construction for South American river dolphins: a case study of the Tapajós River. Endangered Species Research 2016;31:47-60.
  10. Braulik GT, Arshad M, Noureen U, Northridge S. Habitat fragmentation and species extirpation in freshwater ecosystems: causes of range decline of the Indus River dolphin (Platanista gangetica minor). PLoS ONE 2014.
  11. Chen M, Fontaine MC, Ben Chehida Y, et al. Genetic footprint of population fragmentation and contemporary collapse in a freshwater cetacean. Scientific Reports 2017;7:14449.
  12. Smith, B.D., Ahmed, B., Edrise, M. and Braulik, G. 2001. Status of the Ganges River Dolphin or Shushuk Platanista gangetica in Kaptai Lake and the southern rivers of Bangladesh. Oryx 35: 61–72.
  13. Braulik GT, Noureen U, Arshad M, Reeves RR. Review of status, threats, and conservation management options for the endangered Indus River blind dolphin. Biological Conservation 2015;192:30-41
  14. Khan U. Cetacean Conservation: A case study of the Indus dolphin rescue project: a collaboration for rescuing stranded river dolphin from Sindh to Punjab. Lahore: WWF-Pakistan; 2005.
  15. Ganguli U, Ghosal S. Dolphin activity in South Asia: Rescues in West Bengal and Pakistan, Report on Captive Facility in Tamil Nadu. Zoos’ Print 2000;15:2.
  16. Dey M, Krishnaswamy J, Morisaka T, Kelkar N. 2019. Interacting effects of vessel noise and shallow river depth elevate metabolic stress in Ganges river dolphins. Scientific reports, 9: 15426.
  17. Erbe, C., S. A. Marley, R. P. Schoeman, J. N. Smith, L. E. Trigg, and C. B. Embling. 2019. The Effects of Ship Noise on Marine Mammals—A Review. Frontiers in Marine Science 6(606)(Review) doi: 10.3389/fmars.2019.00606
  18. Popper, A. N., and M. C. Hastings. 2009. The effects of anthropogenic sources of sound on fishes. Journal of Fish Biology 75(3):455-489. doi: 10.1111/j.1095-8649.2009.02319.x
  19. Nabi, G., Y. Hao, R. W. McLaughlin, and D. Wang. 2018. The Possible Effects of High Vessel Traffic on the Physiological Parameters of the Critically Endangered Yangtze Finless Porpoise (Neophocaena asiaeorientalis ssp. asiaeorientalis). Frontiers in Physiology 9(1665)(Original Research) doi: 10.3389/fphys.2018.01665
  20. Turvey, S. T., C. L. Risley, J. E. Moore, L. A. Barrett, H. Yujiang, Z. Xiujiang, Z. Kaiya, and W. Ding. 2013. Can local ecological knowledge be used to assess status and extinction drivers in a threatened freshwater cetacean? Biological Conservation 157(0):352-360.
  21. Senécal, P., Goldsmith, B., Conover, S., Sadler, B. and Brown, K., 1999. Principles of Environmental Impact Assessment Best Practice. IAIA
  22. IUCN World Heritage Program, 2013. World Heritage Advice Note: Environmental Assessment
  23. UNITED NATIONS ECONOMIC COMMISSION FOR EUROPE, 2012. Simplified Resource Manual: to Support Application of the protocol on Strategic Environmental Assessment
  24. International Finance Corporation. (2018). Strategic Environmental Assessment of the Myanmar Hydropower Sector. World Bank Group.
  25. Opperman, J., G. Grill and J. Hartmann, The Power of Rivers: Finding balance between energy and conservation in hydropower development. 2015. The Nature Conservancy, Washington D.C. 
  26. Harrison, D., Opperman, J.and Grill, R, 2013. The next frontier of hydropower sustainability: planning at the system scale, Inter-American Development Bank; IDB report. 
  27. Grill, G., Lehner, B., Thieme, M. et al. Mapping the world’s free-flowing rivers. Nature 569, 215–221 (2019).
  28. Caldas, B., Thieme, M.L.;Shahbol, N; Eduarda Coelho, M; Grill, G; Van Damme, P; Aranha, R; Collazos, E.E.H; Canas, C; Fagundes, C; Frale, N; Jezequel, C; Montoya,  M; Mosquera Guerra, F; Oliveira- da-Costa, M; Paschoalini Frias, M; Petry, P; Oberdorff, T; Trujillo, F; Tedesco, P; Lambert, M.(2021; in review). Freshwater connectivity corridors of the Amazon Basin 
  29. Bonzi, C., Grill, G., Phyoe Kyaw Htet, Shahbol N. and Van der Valk, F, 2020. Mapping Myanmar’s Free-flowing rivers – Assessment of current and future impacts of dam-infrastructure development on river connectivity. WWF and McGill University
  30. Inter-American development Bank Technical note, 2015. National Water Reserves Program in Mexico. Experiences with Environmental Flows and the Allocation of Water for the Environment.
  31. Salinas-Rodríguez, S.A., et al., 2021. What do Environmental Flows Mean for Long-term Freshwater Ecosystems’ Protection? Assessment of the Mexican Water Reserves for the Environment Program. Sustainability, 2021 13(3), 1240. 
  32. Kemp 2016. Meta‐analyses, Metrics and Motivation: Mixed Messages in the Fish Passage Debate. River research and applications.
  33. Noonan, M.J., Grant, J.W. and Jackson, C.D. (2012): “A quantitative assessment of fish passage efficiency.” Fish and Fisheries, 13(4), pp.450-464.
  34. David W Roscoe & Scott G Hinch (2010). Effectiveness monitoring of fish passage facilities: historical trends, geographic patterns and future directions. Fish and Fisheries 11, pp 12-33.
  35. Ana T. Silva et al. (2017). The future of fish passage science, engineering, and practice. Fish and Fisheries. 
  38. Notarbartolo di Sciara G, Hanafy MH, Fouda MM, Affi A, Costa M. Spinner dolphin (Stenella longirostris) resting habitat in Samadai Reef (Egypt, Red Sea) protected through tourism management. Journal of the Marine Biological Association of the United Kingdom 2008;89:211-6.
  39. Guerra M, Dawson SM. Boat-based tourism and bottlenose dolphins in Doubtful Sound, New Zealand: The role of management in decreasing dolphin-boat interactions. Tourism Management 2016;57:3-9.
  40. Currie, J., S. Stack, S. Easterly, G. Kaufman, and E. Martinez. 2017. Modeling whale-vessel encounters: the role of speed in mitigating collisions with humpback whales (Megaptera novaeangliae). Journal of Cetacean Research and Management 17:57-63.
  41. Joy, R., D. Tollit, J. Wood, A. MacGillivray, Z. Li, K. Trounce, and O. Robinson. 2019. Potential Benefits of Vessel Slowdowns on Endangered Southern Resident Killer Whales. Frontiers in Marine Science 6(344)(Original Research) doi: 10.3389/fmars.2019.00344
  42. Leaper, R. 2019. The Role of Slower Vessel Speeds in Reducing Greenhouse Gas Emissions, Underwater Noise and Collision Risk to Whales. Frontiers in Marine Science 6(505)(Original Research) doi: 10.3389/fmars.2019.00505
  43. Calleson, C. S., and R. Kipp Frohlich. 2007. Slower boat speeds reduce risks to manatees. Endangered Species Research 3(3):295-304.