Limnological effects of a large Amazonian run-of-river dam on the main river and drowned tributary valleys

Abstract

Run-of-river dams are often considered to have lower environmental impacts than storage dams due to their smaller reservoirs and low potential for flow alteration. However, this has been questioned for projects recently built on large rivers around the world. Two of the world’s largest run-of-river dams—Santo Antônio and Jirau—were recently constructed on the Madeira River, a major tributary to the Amazon River in Brazil. Here we evaluate the effects of the creation of the Santo Antônio dam on the water chemistry and thermal structure of the Madeira River mainstem and back-flooded valleys of tributaries within the reservoir inundated area. In contrast to the mainstem river, some back-flooded tributaries periodically developed thermal stratification, which is associated with higher water residence times. Additionally, biochemical oxygen demand, partial pressure of CO2, and organic carbon all increased in the tributary valleys inundated by the reservoir, possibly due to increased input of allochthonous organic matter and its subsequent mineralization upon back-flooding—a common feature of newly flooded impoundments. The mainstem did not show detectable dam-related changes in water chemistry and thermal structure. Although the majority of the reservoir area maintained riverine conditions, the lateral valleys formed upon back-flooding—corresponding to ~30% of the Santo Antônio reservoir area—developed lake-like conditions akin to a typical reservoir of a storage dam.

Hydropeaking Operations of Two Run-of-River Mega-Dams Alter Downstream Hydrology of the Largest Amazon Tributary

“…A remote sensing analysis has revealed that the area inundated by the Jirau and Santo Antônio dams is 60% larger than initially predicted in pre-dam environmental impact assessments (Cochrane et al, 2017), which may be in part related to changes in project design. Although the residence time of the Madeira dams is short (Figure 1), drowned tributary valleys created by the dams show significant limnological alterations, including thermal stratification and increased availability of organic matter (De Faria et al, 2015;Almeida et al, 2019a).…”

Low Diffusive Methane Emissions From the Main Channel of a Large Amazonian Run-of-the-River Reservoir Attributed to High Methane Oxidation

“…Wind and drifting speed do not show grouping statistics because of the small number of measurements, and consequently, not meeting assumptions requirements for the statistical test. potentially mixed water column in the reservoir's main channel, indicated by a homogeneous temperature profile in a location in the main channel of the reservoir near Site 1 (Almeida et al, 2019a), could sustain oxygen levels in the water at the bottom and help exchange O 2 from the water into the sediment surface, potentially increasing sediment CH 4 oxidation at the sediment water interface.…”

“…Although this study was focused on the reservoir's main channel, it is important to highlight that the large drowned areas, especially in tributary valleys, can develop lake-like conditions and water column stratification (Almeida et al, 2019a). The higher water residence time in these areas allows sedimentation of fine organic sediments and may create oxygen stratification that favors CH 4 accumulation in the hypolimnion.…”

“…Furthermore, low land Amazonian reservoirs such as Santo Antônio have lower power density than reservoirs in high elevation and steeper areas (Almeida et al, 2019b), and because of the high temperatures throughout the year in tropical systems, high rates of CH 4 production and emission can be expected (Barros et al, 2011;Marotta et al, 2014). Recent studies have indicated minimal changes in water chemistry, thermal structure, and fine sediment transport in the Santo Antonio ROR reservoir in the Maderia River (Almeida et al, 2019a;Rivera et al, 2019), andde Araújo et al (2019) found that CO 2 emissions in the Belo Monte ROR reservoir's main channel in the Xingu River remained similar to non-affected river areas. These findings suggest that the main channel of ROR reservoirs can keep the original river characteristics.…”

Evolution of Flood Regulation Capacity for a Large Shallow Retention Lake: Characterization, Mechanism, and Impacts

“…In view of the comprehensive ecological effects of lake-river interaction, lake water retention has received more concerns in floodplain regions [6,7]. Water residence time mainly focuses on reservoirs or run-of-river dams [8]. Hydrological regimes of lake and river are extensively altered by damming and water diversion [9,10], as an enlarged channel storage lake plays an important role in alleviating flood and maintaining wetland ecology [11,12].…”