Surface water-groundwater exchanges under conditions of daily river stage fluctuations : implications for fluid, solute, and heat dynamics in dam regulated river cooridors

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2020-05-08

Authors

Ferencz, Stephen Bruce

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Abstract

The damming of rivers has resulted in multitude of environmental impacts to river ecosystems. The focus of this dissertation is to assess how dams used for hydroelectric power generation influence surface water-groundwater exchange. The exchange of surface water with groundwater has been demonstrated to be an important mechanism regulating nutrient dynamics in rivers. Hydroelectric dams are often “hydropeaked,” which is when discharge from the dam is increased/decreased in accordance with daily patterns of electricity demand. This creates high frequency river stage fluctuations that enhance the connectivity between rivers and groundwater. The central theme explored throughout this dissertation is how hydropeaking effects the movement of water, dissolved solutes, and heat between rivers and their sediments. Much of the research presented in this dissertation uses numerical models to explore how properties of the river (stage, chemistry, temperature) and subsurface (hydraulic conductivity, groundwater levels) control exchanges of fluid, heat, and solutes, and characterizing how those fluxes alter the chemistry and temperature of sediments near the river. Two of the studies are entirely modeling-based, focusing on the lateral exchanges of fluid and solutes between rivers and their riparian aquifers caused by hydropeaking. The results provide insight into how dam release and subsurface properties influence surface water-groundwater exchange and also the distance downstream from a dam that exchanges occur over. Other key findings relate to how hydraulic conductivity and groundwater flow conditions of the riparian aquifer control fluid and solute exchange, how far river-sourced solutes propagate into the riparian zone, and factors influencing aerobic respiration in the banks of hydropeaked rivers. In another study, field observations of riverbed temperatures from a hydropeaked river are combined with flow and heat transport modeling. The results demonstrate that hydropeaking releases can strongly influence the temperature of the riverbed near the river bank, but not over the majority of the channel. A final study uses temperature observations from hundreds of rivers across the United States to characterize the mean daily temperature and daily temperature range in different ecoregions. The results from this assessment demonstrate how the thermal regimes of rivers near dams have been altered.

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