Beyond harm’s reach? Submersion of river turtle nesting areas and implications for restoration actions after Amazon hydropower development

The Amazon River discharge contributes more than 15% of the total discharge of all rivers, playing a key role in maintaining global climate and hydrological cycles. The basin is home to a vast array of aquatic and terrestrial ecosystems, including freshwater turtles, which are under threat from the alteration of rivers, habitat loss, and climatic changes. Freshwater turtles are sensitive to changes in their environment, and the placement of dams can have significant impacts on their populations. The literature on environmental impact assessments of dams on freshwater turtles is limited, as most dams are constructed before any baseline ecological data are collected. There is an urgent need for refined approaches to river management, and a move away from decision-making governed by overly generalized guidelines. The study area is located in the Araguari River basin, upriver of the newly installed Cachoeira Caldeirão Dam, in the state of Amapá, Brazil. The regional climate is classified as Am (Equatorial monsoon), with an annual rainfall greater than 2,000 mm. The Araguari River rises in the Guianan Shield at the base of the Tumucumaque uplands and until recently discharged directly into the Atlantic Ocean. However, in 2015, the river course changed due to anthropogenic effects, and the Araguari River now discharges directly into both the Atlantic Ocean and the Amazon River. The Cachoeira Caldeirão is a large run-of-river dam with a capacity of 219 MW and a dam height of 30 meters. The dam was constructed in 2016, and the reservoir formed as a result of the dam's construction flooded a significant area of the Araguari River basin. The study aimed to identify the distance at which the reservoir from the dam impacted populations of the yellow-spotted river turtle (Podocnemis unifilis) and to quantify and map nesting areas pre- and post-reservoir formation. The study employed a before-after comparison, which is lacking in most studies of hydropower development. The researchers conducted surveys along 150 km of rivers upstream of the dam during the low water months that correspond to the nesting season of Podocnemis unifilis in the study area. Comparison of nest-areas before (2011, 2015) and after (2016) reservoir filling showed that reservoir impacts extended 13% beyond legally defined limits. The submerged nesting areas accounted for a total of 3.8 ha of nesting habitat that was inundated as a direct result of the reservoir filling in 2016.