Better understanding human impacts on river thermal regimes under climate change

Abstract

Human activities, especially dam construction, greatly modify the response of river thermal regimes to climate change. Dams impound large water bodies, decrease surface to volume ratios, and increase water residence times. All of these changes affect the interaction between surface meteorology and river systems. During warm seasons, surface energy fluxes can only warm a reservoir’s top layer (epilimnion) while the bottom layer (hypolimnion) remains cold. As a result, the cold hypolimnetic releases greatly depress downstream river temperatures. Additionally, reservoir releases during cold seasons can increase downstream river temperatures. Thus far, most large-scale stream temperature studies have ignored seasonal thermal stratification and therefore underestimated the regulation impacts on downstream fluvial thermal regimes. In the papers that constitute this dissertation, I synthesized a physically-based model framework to simulate regulated river flow and temperature, explicitly considering the impacts of reservoir thermal stratification. This model framework laid the basis of this dissertation and was applied in all subsequent analyses. In Chapter 2, I applied this model framework in the southeastern United States and investigated the impacts of reservoir regulation and climate change on mean summer river temperature and cooling potentials, a metric designed to evaluate the compound impact of river flow and temperatures. Under climate change, summer river temperatures in the regulated rivers will remain colder compared to those in the unregulated rivers but under climate change the effect does not carry as far downstream. The impact of reservoir regulation on cooling potentials remains strong for rivers heavily influenced by thermal stratification, but under climate change higher river temperatures will decrease cooling potentials for all river segments. In Chapter 3, I examined extreme fluvial thermal events, i.e., high river temperatures, so as to facilitate risk management for regional aquatic ecosystem and power sectors. We introduced a standard characterization with three attributes, i.e., duration-intensity-severity, to quantify the climate change impacts on thermal extremes in a regulated river system. Thermal extremes will be greatly exacerbated by climate change. In the baseline (unregulated) scenarios, duration, intensity, and severity are projected to increase to 85.6 day/year (+77.4 day/year), 5.2 °C (+4.4°C), and 193.4 °C·day/year (+187.9 °C·day/year), respectively, by the 2080s under RCP8.5, with values in parentheses indicating the changes relative to the historical, unregulated values. Even though reservoir mitigation impacts are projected to be stronger, only 12.2%, 19.7%, and 26.0% of duration, intensity, and severity by the 2080s under RCP8.5 can be mitigated by reservoir regulations. In Chapter 4, I projected potential fish distribution due to climate change in the highly regulated Tennessee River. By coupling the model framework for regulated river systems described in Chapter 2 with a species distribution model, I simulated fish presence probability for historic and future periods considering the effects of dams on flow, thermal regime, and reach connectivity. The number of stream segments that are environmentally suitable for an exotic and lucrative rainbow trout, a coldwater species, will greatly shrink under climate change. Only 4.4% of historically suitable streams will remain, mostly located at reservoir tailwaters. For endemic coolwater species, projected higher river temperature may facilitate their expansion, but it will be constrained due to the physical blockage of dams.