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Dynamics of Hyporheic Flow and Heat Transport Across a Bed-to-Bank Continuum in Large Regulated River

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[i] The lower Colorado River (LCR) near Austin, Texas is heavily regulated for hydropowcr generation. Daily water releases from a dam located 23 km upstream of our study site in the LCR caused the stage to fluctuate by more than 1.5 m about a mean depth of 1.3 m. As a result, the river switches from gaining to losing over a dam storage-release cycle, driving exchange between river water and groundwater. We assessed the hydrologic impacts of this by simultaneous temperature and head monitoring across a bed-to-bank transect. Rivcr-groundwater exchange flux is largest close to the bank and decreases away from the bank. Correspondingly, both the depth of the hyporheic zone and the exchange time are largest close to the bank. Adjacent to the bank, the streambed head response is hysteretic, with the hysteresis disappearing with distance from the bank, indicating that transient bank storage affects the magnitude and direction of vertical exchange close to the bank. Pronounced changes in streambed temperature are observed down to a meter. When the river stage is high, which coincides with when the river is coldest, downward advection of heat from a previous cycles' warm-water pulse warms the streambed. When the river is at its lowest stage but warmest temperature, upwelling groundwater cools the streambed. Future research should consider and focus on a more thorough understanding of the impacts of dam regulation on the hydrologic, thermal, biogeochemical, and ecologic dynamics of rivers and their hyporheic and riparian zones.

Introduction
[2] Sixty percent of the world's major rivers are dammed for a variety of purposes including hydropower generation, water storage, and recreation [Hancock, 2002; Nilsson et al., 2005]. River regulation by dams has impacts that extend up to hundreds of kilometers downstream, affecting not only riparian environments but also groundwater systems and the hyporheic zone.

[3] The streambed hosts the hyporheic zone, an ecotone where water, nutrients, heat, and contaminants are exchanged between surface water and groundwater systems [Bencala, 2005; Boulton et al., 1998; Hancock, 2002; Vervier et al, 1992]. The hyporheic zone also acts as a protective habitat for many fish and insect eggs and invertebrates [Brunke and Gonser, 1997; Findlay, 1995; Hancock, 2002]. The importance of this zone warrants a full characterization of the impacts of regulation on downstream environments, especially as the fluxes in this zone affect the water quality of both the river and underlying aquifer [Boano et al.. 2008; Brunke and Gonser, 1997; Findlay, 1995; Hancock, 2002; Verviereta!., 1992].

[4] The main objective of this study is to advance our understanding of hyporheic zone dynamics when they are subjected to regular stage fluctuations. During the summer of 2009, temperature and pressure measurements were used to identify the depth of the hyporheic zone and to determine how this varies both spatially and temporally. This work provides new insight into hyporheic exchange in a large, regulated river with detailed spatial and temporal resolution and intensity of field observations. The study is unique in its use of hydraulic and thermal measurements to link exchange responses in the streambed and the adjacent bank. A deeper understanding of hyporheic zone hydrology is critical to expanding our current understanding of hyporheic zone ecology and biogeochemistry [Bencala, 2005; Lewandowski et aL, 2009]. However, few previous studies have directly estimated hyporheic zone depths using multiple methods, as well as calculated corresponding exchange times. The results of our work provide insight on the effects of regulated releases on hyporheic processes.

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