Rivers of Andean origin contribute approximately half of the Amazon mainstem’s annual flow. These rivers also are responsible for a large amount of sediment and nutrients deposited in the lowland Amazon. They also control processes which affect the geomorphology, sediment deposition, and formation of floodplains for thousands of kilometers downstream of the headwaters.
The Amazonian rivers have the highest richness and diversity of fish species in the world with 2,258 known species and total species estimates ranging from 3,500 to 5,000 different species of fish. Many of these fish have very long migration routes. Connectivity between the Andes headwaters and lowland Amazon is critical for many of these different fish species.
Unfortunately, the installation of large hydroelectric dams threatens these patterns and processes in the Andes Amazon region. A recent study by Amazon Dams Networks researcher Dr. Elizabeth Anderson and colleagues published in Science Advances in January 2018 showed that the number of dams planned to be installed in the Andean Amazon has been severely underestimated. Current dams in this region are already severing the connections between the Andean highlands and the Amazonian lowlands, with some rivers experiencing losses of as much as 20% of the connectivity. If planned dams continue to be built, these losses by increase by 35%-50%, depending on the watershed. Significant fragmentation of the mainstem is possible in five out of eight of the major Andean Amazon rivers.
This hydropower development is also concentreated in the Andean highlands, which houses many fish species that are adapted to survive in waters with fast currents. Dams in these regions create an uncrossable barrier for these fish species. This fragmentation will only be made worse by climate change, as species ranges contract to smaller habitable climatic zones. Losses in connectivity in the river can affect species diversity at multiple scales, reducing both diversity at both the local scale and across the entire watershed. In addition to creating migratory barriers, dams alter flow patterns of the river. Regular seasonal floods give fish cues for both migration and reproduction. Altering these flood pulses can have detrimental affects on the fish biology, leading them to miss or delay these important cycles.
Also concerning is the potential these dams have for trapping sediments that influence river morphology and floodplain creation. Dams are expected to trap as much as 100% of all sediments that originate in the Andes, and given that 93% of sediments in the Amazon are of Andean origin, this is cause for great concern. Not only will trapping of these sediments affect the morphology of the river, there will also be impacts on important fishery species and floodplain agriculture, therefore directly impacting the many people who depend on these resources for nourishment and livelihood.
Given the high impact of hydropower development in the Andean Amazon, it is important that any future dams be built with best practices in mind. If Andean Amazon nations ratify the United Nations Watercourses Convention, this will provide a legal basis for transnational cooperation for sustainable water resource management. Since these dams affect rivers and resources across geographical boundaries, it is important for countries too cooperate and coordinate to ensure protection and preservation of these important ecological and cultural resources. Additionally, researchers must also frame questions surrounding these Andean Amazon dams at multiple scales to better understand how fragmentation affects waterways not only near the dam, but across the entire watershed. There is also growing concern surrounding cumulative impacts of these dams. These impacts can only be understood at larger scales that capture multiple dams across a single watershed.