The shadow of the Balbina dam: A synthesis of over 35 years of downstream impacts on floodplain forests in Central Amazonia

Abstract

The Balbina hydropower dam in the Central Amazon basin, established in the Uatumã River in the 1980s, is emblematic for its socio‐environmental disaster. Its environmental impacts go far beyond the reservoir and dam, however, affecting the floodplain forests (igapó) in the downstream area (dam shadow), which have been assessed using a transdisciplinary research approach, synthesized in this review. Floodplain tree species are adapted to a regular and predictable flood pulse, with high‐ and low‐water periods occurring during the year. This was severely affected by the operation of the Balbina dam, which caused the suppression of both the aquatic phase at higher floodplain elevations and the terrestrial phase at lower floodplain elevations (termed the ‘sandwich effect’). During the period of construction and reservoir fill, large‐scale mortality already occurred in the floodplains of the dam shadow as a result of reduced stream flow, in synergy with severe drought conditions induced by El Niño events, causing hydraulic failure and making floodplains vulnerable to wildfires. During the operational period of the dam, permanent flooding conditions at low topographical elevations resulted in massive tree mortality. So far, 12% of the igapó forests have died along a downstream river stretch of more than 125 km. As a result of flood suppression at the highest elevations, an encroachment of secondary tree species from upland (terra firme) forests occurred. More than 35 years after the implementation of the Balbina dam, the downstream impacts caused massive losses of macrohabitats, ecosystem services, and diversity of flood‐adapted tree species, probably cascading down to the entire food web, which must be considered in conservation management. These findings are discussed critically, emphasizing the urgent need for the Brazilian environmental regulatory agencies to incorporate downstream impacts in the environmental assessments of several dam projects planned for the Amazon region.

Impacts of dams on freshwater turtles: a global review to identify conservation solutions

“…As a consequence of dam reservoir the flood plain vegetation disappears, as it becomes permanently submerged (Guo et al, 2021; Schöngart et al, 2021). Additionally, wetland (Dudgeon, 2000; Santoro et al, 2020) and terrestrial (Resende et al, 2019; Schöngart et al, 2021; Zhang et al, 2021) environments are also adversely impacted (Schöngart et al, 2021).…”

“…As a consequence of dam reservoir the flood plain vegetation disappears, as it becomes permanently submerged (Guo et al, 2021; Schöngart et al, 2021). Additionally, wetland (Dudgeon, 2000; Santoro et al, 2020) and terrestrial (Resende et al, 2019; Schöngart et al, 2021; Zhang et al, 2021) environments are also adversely impacted (Schöngart et al, 2021). As a consequence, the habitat and food resources used by freshwater turtles changes, which may directly harm nutrition (Pezzuti et al, 2016; Tucker et al, 2012), growth (Bondi and Marks, 2013) and survival (Bennett et al, 2009; Bennett and Litzgus, 2014).…”

“…The loss of feeding habitat, together with the changes in the flood pulse, can strongly impact the availability of food resources (Bennett et al, 2009; Schöngart et al, 2021). This was shown in Australia where the diet of three freshwater turtle species was evaluated, comparing places with and without damming impact (Tucker et al, 2012).…”

Modeling the Ecological Responses of Tree Species to the Flood Pulse of the Amazon Negro River Floodplains

“…Indeed, some observational evidence already supports this idea (Junk et al, 2015). Particularly notable are mass mortality events of the highly flood-tolerant tree species M. acaciifolium and E. tenuifolia downstream of the Balbina hydropower dam on the Uatumã River (central Brazilian Amazon), which were correlated to extreme hydrologic events due to dam constructions and operation (Assahira et al, 2017;Resende et al, 2020;Schöngart et al, 2021).…”

“…More intimate knowledge of species ecological response to flooding in the Negro River basin is likely to have a variety of regional applications. For example, a growing number of field studies document cumulative impacts on Amazonian floodplain forests resulting from hydroelectric dams and the alteration of downstream river hydrology that their operation causes (i.e., Wittmann et al, 2015;Assahira et al, 2017;Forsberg et al, 2017;Latrubesse et al, 2017;Lobo et al, 2019;Rocha et al, 2019;Resende et al, 2020;Schöngart et al, 2021). However, parameterization of predictive or simulation models about these impacts are not possible without better information on speciesflooding relationships.…”