Groundwater-surface water exchange processes in alluvial rivers

Recent research on braided rivers in New Zealand has revealed how groundwater is recharged from braided rivers, and alluvial rivers in general. A defining feature of alluvial rivers is the development of a thin (2-5m thick) high permeability braidplain aquifer/ bed reservoir. This feature forms through the process of sediment mobilisation during flood events. Field observations in braided rivers reveal that water is freely exchanged between river channels and the bed reservoir, which accommodates parafluvial flow. The river channels and associated reservoir together constitute a river system (Wilson et al. 2024). Exchange between the river system and regional aquifer is controlled by the water level in the bed reservoir rather than river stage. This has important implications for how groundwater-surface water exchange occurs in alluvial systems.

Firstly, flow exchange between channels and the bed reservoir is preferentially lateral via the banks rather than vertically via the bed. In isotropic sediments, Darcy’s Law predicts that flow exchange will occur vertically through the river bed, and that flux will vary with river stage. However, alluvial sediments are strongly anisotropic, with lateral hydraulic conductivities magnitudes greater than the vertical. Groundwater temperature observations show that specific discharge adjacent to the bed reservoir is much greater than that beneath the bed reservoir. The flux beneath the river system is stable, with variations in flux only occurring along the river margins.  

Secondly, exchange between the river system and adjacent aquifer is primarily controlled by transmissivity rather than hydraulic gradient. It is commonly considered that groundwater recharge is controlled by the head difference between the river system and adjacent regional aquifer. Under this scenario, we would expect river losses to increase when groundwater levels in the regional aquifer become very low due to an increased hydraulic gradient. However, in the Wairau system we observe the opposite, and that river flows are sustained during periods of low groundwater level. Because the river bed reservoir is very thin, changes in hydraulic gradient are small compared to changes in reservoir saturation. When groundwater levels are low, the transmissivity along the bed reservoir margin is lower, resulting in lower exchange rates. 

Wilson, S. R., Hoyle, J., Measures, R., di Ciacca, A., Morgan, L. K., Banks, E. W., Robb, L., & Wöhling, T. (2024). Conceptualising surface water–groundwater exchange in braided river systems. Hydrology and Earth System Sciences, 28(12), 2721–2743. https://doi.org/10.5194/hess-28-2721-2024