Reviews and syntheses: Dams, water quality and tropical reservoir stratification

Abstract

Abstract. The impact of large dams is a popular topic in environmental science, but the importance of altered water quality as a driver of ecological impacts is often missing from such discussions. This is partly because information on the relationship between dams and water quality is relatively sparse and fragmentary, especially for low-latitude developing countries where dam building is now concentrated. In this paper, we review and synthesize information on the effects of damming on water quality with a special focus on low latitudes. We find that two ultimate physical processes drive most water quality changes: the trapping of sediments and nutrients, and thermal stratification in reservoirs. Since stratification emerges as an important driver and there is ambiguity in the literature regarding the stratification behavior of water bodies in the tropics, we synthesize data and literature on the 54 largest low-latitude reservoirs to assess their mixing behavior using three classification schemes. Direct observations from literature as well as classifications based on climate and/or morphometry suggest that most, if not all, low-latitude reservoirs will stratify on at least a seasonal basis. This finding suggests that low-latitude dams have the potential to discharge cooler, anoxic deep water, which can degrade downstream ecosystems by altering thermal regimes or causing hypoxic stress. Many of these reservoirs are also capable of efficient trapping of sediments and bed load, transforming or destroying downstream ecosystems, such as floodplains and deltas. Water quality impacts imposed by stratification and sediment trapping can be mitigated through a variety of approaches, but implementation often meets physical or financial constraints. The impending construction of thousands of planned low-latitude dams will alter water quality throughout tropical and subtropical rivers. These changes and associated environmental impacts need to be better understood by better baseline data and more sophisticated predictors of reservoir stratification behavior. Improved environmental impact assessments and dam designs have the potential to mitigate both existing and future potential impacts.

The carbon footprint of a Malaysian tropical reservoir: measured versus modelled estimates highlight the underestimated key role of downstream processes

“…The model relies strongly on surface CH 4 patterns to predict excess CH 4 in the deep layer, which could explain why it underestimates CH 4 degassing in Batang Ai. Similar strong stratification patterns are ubiquitous in the tropics, with a recent study suggesting a large majority of tropical reservoirs are monomictic or oligomictic (Lehmusluoto et al, 1997;Winton et al, 2019) and hence more often stratified than temperate and boreal ones. This suggests that CH 4 degassing is potentially more frequently underestimated in low-latitude reservoirs.…”

“…Reservoirs provide a variety of services to humans (water supply, navigation, flood control, hydropower) and cover an estimated area exceeding 0.3 million km 2 globally (Lehner et al, 2011). This area is increasing, with an expected rapid growth of the hydroelectric sector in the next two decades (International Hydropower Association (IHA), 2015), mainly in tropical and subtropical regions (Zarfl et al, 2015). The flooding of terrestrial landscapes can transform them into significant greenhouse gas (GHG) sources to the atmosphere Rudd et al, 1993;Teodoru et al, 2012).…”

Characteristics, Main Impacts, and Stewardship of Natural and Artificial Freshwater Environments: Consequences for Biodiversity Conservation

“…Building dams and reservoirs not only impacts lowland wetlands but it is well known that dams break the natural longitudinal river connectivity [167], causing many alterations to river characteristics and function [563], such as the geomorphology [564] and the physical and chemical properties of channels, besides being barriers to the dispersal of many running-water species, i.e., macroinvertebrates and fishes, so that populations above and below dams become effectively isolated. The changes in hydrologic pattern and longitudinal connectivity also affect water quality: a comprehensive discussion of this subject, with special attention to tropical areas, can be found in [565]. Dam building is an environmental problem that today is located mainly in the tropics, considering that ongoing and proposed major dam projects are concentrated at low latitudes [566].…”

An Assessment of Self-Purification in Streams

“…Nitrate and phosphate concentrations are greatly influenced by man-or animal-made dams. The formed ponds act as precipitators, resulting that significantly less nitrate concentrations flow out from the pond [43]. Research in the Nevėžis River Basin [22] shows that beaver ponds (comparing annual concentrations of inputs and outflows) retain relatively more phosphorus than nitrogen and more soluble mineral N and P compounds: on average 28% nitrate and ammoniacal nitrogen and 43% of orthophosphate phosphorus.…”