Characterization of Hyporheic Exchange Drivers and Patterns within a Low-Gradient, First-Order, River Confluence during Low and High Flow

Abstract

Confluences are nodes in riverine networks characterized by complex three-dimensional changes in flow hydrodynamics and riverbed morphology, and are valued for important ecological functions. This physical complexity is often investigated within the water column or riverbed, while few studies have focused on hyporheic fluxes, which is the mixing of surface water and groundwater across the riverbed. This study aims to understand how hyporheic flux across the riverbed is organized by confluence physical drivers. Field investigations were carried out at a low gradient, headwater confluence between Baltimore Brook and Cold Brook in Marcellus, New York, USA. The study measured channel bathymetry, hydraulic permeability, and vertical temperature profiles, as indicators of the hyporheic exchange due to temperature gradients. Confluence geometry, hydrodynamics, and morphodynamics were found to significantly affect hyporheic exchange rate and patterns. Local scale bed morphology, such as the confluence scour hole and minor topographic irregularities, influenced the distribution of bed pressure head and the related patterns of downwelling/upwelling. Furthermore, classical back-to-back bend planform and the related secondary circulation probably affected hyporheic exchange patterns around the confluence shear layer. Finally, even variations in the hydrological conditions played a role on hyporheic fluxes modifying confluence planform, and, in turn, flow circulation patterns.

Numerical simulation of overbank hyporheic transport and biogeochemical reactions in a compound channel

“…Typically, the hyporheic zone exhibits a variety of complex physical, chemical, and biological processes (Bryant et al, 2020; Jin et al, 2019; Roche et al, 2019) that are influenced by a variety of factors, including riverbed topography (Rana et al, 2017) and groundwater hydrological conditions (e.g., groundwater discharge). Increased hyporheic exchange can be driven by riverbed physical characteristics such as dunes, ripples, pool‐riffle sequences, and scour holes at confluences, improving the biogeochemical transformation of solutes inside the hyporheic zones (HZ) (Bardini et al, 2012; Cardenas et al, 2008; Gomez‐Velez et al, 2017; Martone et al, 2020; S. Yuan, Tang, et al, 2021). The head distribution and hyporheic flow patterns are affected by geometric elements of the compound channel, such as the angle of the bank slope (Wang et al, 2018), which could result in more complex biogeochemical processes (Azizian et al, 2015; Cardenas et al, 2008).…”

The Effect of Tubificid Bioturbation on Vertical Water Exchange across the Sediment–Water Interface

“…Previous studies have focused on the hydrological and biological factors affecting hyporheic water exchange, such as instream wood, bed form and microbial activity [7][8][9][10][11][12]. With hyporheic macroinvertebrates attracting more attention as ecosystem engineers, efforts have been undertaken to investigate the bioturbation-induced hyporheic processes [13][14][15].…”

Effect of Combined Subsurface Structures and Steps on Hyporheic Exchange

“…The main feature of the HZ is the vertical water transfer between surface and subsurface flows that moderates fluctuations of water temperature and strongly influences nutrient cycles (Bakke et al, 2020). Hyporheic exchange is characterized by downwelling and upwelling fluxes (Martone et al, 2020) that are determined by variations in hydraulic head, hydraulic conductivity of the porous medium, and bed thickness (Tonina & Buffington, 2009a, 2009bVaux, 1968). The combination of these factors will affect the exchange rate, the residence times in the porous medium, the penetration depth of exchange, and the length of the streamlines in the streambed.Natural river morphologies such as meanders or bedforms induce hyporheic exchange due to differences in hydraulic head on the river bed.…”