| Summary: | The hydrodynamics of a river confluence generate significant vertical, lateral, and stream-wise gradients in the context of velocity, thereby forming a highly complex three-dimensional flow structure, including the development of large-scale turbulence structures. The above features affect the ecologically important underlying hyporheic zone, where surface and subsurface waters interact, and hence affect biological activity and result in highly varied habitats for organisms as well as the whole river environment. The influence of challenging conditions for in situ monitoring of hyporheic exchange—such as non-sinusoidal temperature signals, uncertainty in thermal parameters, and unsteady flows—have led to the development of hyporheic exchange detection methods that are based on the phase and amplitude changes in transient thermal signals. The use of heat as a tracer can require complex steps, including the isolation of the diurnal component of the temperature signal from other signals as well as stochastic variation. The focus of this study was to investigate a field campaign carried out between the Ninemile Creek and its tributary confluence, located in Marcellus, NY. Temperature data of the shallowest saturated sediment layers were measured from April to May 2019. Flux estimations were calculated using VFLUX 2, a MatLab based code, which performed data filtering and DHR (Dynamic Harmonic Regression). The patterns and rates of vertical flux exchange were then analyzed, and sampling of the temporal thermal profiles was performed. Furthermore, multiple analytical solutions of the one-dimensional heat transport model were analyzed and discussed in order to obtain the confluence hydrodynamic effect as well as the variations in the vertical flux estimation. This was achieved by utilizing different sensor pairs and porous medium characteristics, such as thermal diffusivity and conductivity. The predicted flow field shows that confluence topography—which includes the turbulent kinetic energy downstream of the junction, shear layer formations, bed stratigraphy and water table gradients—affects the magnitude and patterns of hyporheic exchange. The results of this study could help to advance the calibration of one-dimensional heat transport models in order to better understand the key hydrological, hydraulic, and ecological issues associated with river confluence.
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