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

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 latitude contexts. We find that most water quality changes are driven by a two ultimate physical processes: the trapping of sediments and nutrients, and thermal stratification and oxygen depletion in reservoirs. Since stratification emerges as an important driver and there is ambiguity in the literature regarding the stratification behavior of low latitude water bodies, 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 efficiently trapping sediment and bed load, which alters downstream habitats and causes losses of floodplain and delta ecosystems. 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 the worlds tropical and subtropical river systems. The water quality changes and their associated environmental impacts could be better understood by better baseline data and more sophisticated predictors of reservoir stratification behavior.