Water quality in tropical rivers is changing rapidly. The ongoing boom of dam construction for hydropower is one of the drivers for this change. In particular, the stratification in tropical reservoirs induces oxygen deficits in their deep waters and warmer surface water temperatures, which often translate into altered thermal and oxygen regimes of downstream river systems, with cascading consequences for the entire aquatic ecosystem. Operation rules of reservoirs, involving water intakes at different levels, could mitigate the consequences for downstream water quality. However, optimized water management of deep reservoirs relies on predictive models for water quality, but such predictive capability is often lacking for tropical dams. Here we focus on the Zambezi River Basin (southern Africa) to address this gap. Using the one-dimensional General Lake Model, we reproduced the internal dynamics of the transboundary Lake Kariba, the world’s largest artificial lake by volume, created by damming the Zambezi River at the border between Zambia and Zimbabwe. Through this modeling approach, we assessed and quantified the thermal and oxygen alteration in the Zambezi River downstream of the reservoir. Results suggest that these alterations depend directly on Kariba’s stratification dynamics, its water level and the transboundary policies for water withdrawal from the reservoir. Scenario calculations indicate a large potential for mitigating downstream water quality alterations by implementing a hypothetical selective withdrawal technology. However, we show that a different and cooperative management of the existing infrastructure of Kariba Dam has the potential to mitigate most of the actual water quality alterations.

Studies of future 21^st century climate warming in lakes along altitudinal gradients have been partially obscured by local atmospheric phenomena unresolved in climate models. Here we forced the physical lake model Simstrat with locally downscaled climate models under three future scenarios to investigate the impact on 29 Swiss lakes, varying in size along an altitudinal gradient. Results from the worst-case scenario project substantial change at the end of the century in duration of ice-cover at mid to high altitude (−2 to −107 days), stratification duration (winter −17 to −84 days, summer −2 to 73 days), while lower and especially mid altitude (present day mean annual air temperature from 9 °C to 3 °C) dimictic lakes risk shift to monomictic regimes (seven out of the eight lakes). Analysis further indicates that for many lakes shifts in mixing regime can be avoided by adhering to the most stringent scenario.

As the demand for hydroelectricity progresses worldwide, small hydropower operators are increasingly examining the feasibility of using existing infrastructure (e.g., settling basins) in run-of-the-river schemes for intermittent power production. Such flexible production causes short-term discharge fluctuations (hydropeaking) in downstream reaches with potential adverse effects for the sensitive fauna and flora in alpine streams. In an experimental field study on a previously unregulated section of the upper Rhone River (Switzerland), we measured density and composition of macroinvertebrate drift in two habitats (riffle, pool) following a 15-minute hydropeaking wave. The experimental hydropeaking was replicated five times over 14 days with decreasing recovery times between peaks (8, 3, 2 days, and 24 h), and drift measurements were compared with kick samples for the benthic community. Results from the kick sampling showed that benthic macroinvertebrate abundance and composition did not significantly change between the experimental peaks. There were habitat specific reactions in macroinvertebrate drift to hydropeaking, with the pool experiencing more pronounced drift abundances than the riffle. Overall, drift abundance was not significantly correlated with recovery time, but results indicate taxa-specific differences. This research advocates for the importance of completing more in-situ field experiments in order to better understand the ecological impact of flexible power production in small hydropower plants.

Lake Kivu is a 485 m deep tropical rift lake in East-Africa and well-known for its very high concentrations of dissolved carbon dioxide and methane in the stratified deep waters. In view of future large-scale methane extraction for power production, there is a need for predicting the evolution of gas concentrations and lake stability using numerical modelling. However, knowledge about the geochemical origin and transport processes affecting dissolved gases in the lake is still partially missing. Due to their inert nature, the analysis of dissolved noble gases can help to shed light on such questions. To learn more about transport processes in Lake Kivu, we extended a well-established sampling method for dissolved noble gases to work in the lake's high gas pressure waters. The results of our analysis show a distinct non-atmospheric isotopic signal in the deep waters (below 250 m) with ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios ~250% and ~20% higher than air saturated water (ASW). Moreover, the gas concentration profiles reveal a striking lack of atmospheric noble gases in the deep waters with respect to ASW. While Ne is depleted by ~45%, the more soluble ³⁶Ar and Kr even decrease by ~70%. In contrast, ⁴He concentrations increase with depth by a factor of up to ~600. We attribute this excess He and the increases in ³He/⁴He and ⁴⁰Ar/³⁶Ar to the inflow of magmatic gases into Lake Kivu, along with a significant contribution of radiogenic ⁴He. To explain the depletion of atmospheric noble gases, we present and discuss three different scenarios, namely continuous outgassing, the inflow of depleted groundwater and a large past outgassing event. Due to the best agreement with our observations, we conclude that the inflow of depleted groundwater is likely responsible for the observed atmospheric noble gas depletions.

The assessment of ecological impacts of pumped-storage (PS) hydropower plants on the two connected water bodies is usually based on present climatic conditions. However, significant changes in climate must be expected during their long concession periods. We, therefore, investigate the combined effects of climate change and PS operations on water temperature and quality, as well as extent and duration of stratification and ice cover, using a site in Switzerland. For this purpose, a coupled two-dimensional hydrodynamic and water quality model for the two connected water bodies is run with 150 years long synthetic stochastic meteorological forcing for both current and future climate conditions under two PS and two reference scenarios. The results show relevant synergistic and antagonistic effects of PS operations and climate change. For example, hypolimnion temperatures in September are projected to increase by < 0.6 °C in a near-natural reference scenario and by ~ 2.5 °C in an extended PS scenario. Ice cover, which occurs every year under near-natural conditions in the current climate, would almost completely vanish with extended PS operation in the future climate. Conversely, the expected negative impacts of climate change on hypolimnetic dissolved oxygen concentrations are partially counteracted by extended PS operations. We, therefore, recommend considering future climate conditions for the environmental impact assessment in the planning of new or the recommissioning of existing PS hydropower plants.

The extraction and disposal of heat from lakes and rivers is a large yet scarcely exploited source of renewable energy, which can partly replace fossil fuel heating and electrical cooling systems. Its use is expected to increase in the near future, which brings attention to the impacts of discharging thermally altered water into aquatic systems. Our review indicates that thermal discharge affects physical and ecological processes, with impacts recorded at all levels of biological organization. Many in situ studies found local effects of thermal discharge (such as attraction or avoidance of mobile organisms), while impacts at the scale of the whole water body were rarely detected. In complex systems, diffuse impacts of thermal discharge are difficult to disentangle from natural variability or other anthropogenic influences. Discharge of warm water in summer is likely to be most critical, especially in the context of climate change. Under this scenario, water temperatures may reach maxima that negatively affect some species. Given the diversity and complexity of the impacts of thermal pollution on aquatic systems, careful planning and judicious management is required when using lakes and rivers for extraction and disposal of heat. We discuss the drivers that influence the severity of potential impacts of such thermal use, and the options available to avoid or mitigate these impacts (such as adapting the operating conditions).