Impact of river management on groundwater recharge from braided rivers

A new conceptualisation describing surface water-groundwater exchange for braided rivers and their associated alluvial aquifers has been developed (Wilson et al., 2023 Preprint). This conceptualisation recognises that braided rivers create their own high-permeability shallow aquifer system through the process of mobilising bed sediments during flood events. A braided river can therefore be considered a “river system” consisting of surface channels and an intrinsically linked subsurface gravel reservoir, the “braidplain aquifer”. This conceptualisation implies that for settings where the river system is hydraulically disconnected to the regional aquifer, groundwater recharge is largely governed by braidplain aquifer width. Additionally, for settings where the river system is hydraulically connected to the regional aquifer, river bed levels will have a large control on recharge rates since they determine the hydraulic gradient. Depending on the hydraulic status, groundwater recharge can be compromised by narrowing the active braidplain, and bed degradation caused by extracting gravel from the braidplain aquifer at a rate that exceeds natural replenishment. To test the impact of river management on groundwater recharge, long-term records of river mean bed level from surveyed cross sections were compared to groundwater levels for the Wairau and Ngaruroro rivers in New Zealand. Scenario testing for different river system widths and elevations was also conducted in MODFLOW based on shorter term monitoring records.

In New Zealand, groundwater monitoring commenced after the river flood engineering schemes of the 1960’s, so the impact of river narrowing is not captured by groundwater records. However, hydraulically connected recharge reaches of the Wairau and Ngaruroro river systems have both been subject to more recent bed degradation caused by gravel extraction. The long-term groundwater level decline in the regional aquifers clearly mimics the drop in mean bed levels in the recharge reaches of the rivers. The drop in river bed elevations can also account for the decline in groundwater levels in MODFLOW scenario modelling.

Observation data for the Wairau and Ngaruroro systems show that the dynamic component of recharge pulses from flood flows propagate rapidly through their associated highly transmissive alluvial aquifers. For both hydraulically connected and disconnected braidplain aquifer-regional aquifer settings, maintaining a steady rate of recharge is therefore most beneficial for sustaining groundwater levels throughout the year. The observation data and modelling results confirm that gravel extraction in the braidplain aquifer is having the largest impact on the hydrological function of the regional aquifer in the two hydraulically connected systems studied here. In both cases, the decline in bed levels offsets the benefit of recharge sourced from flood flow events.

Wilson, S., Hoyle, J., Measures, R., Di Ciacca, A., Morgan, L. K., Banks, E. W., Robb, L., and Wöhling, T.: Conceptualising surface water-groundwater exchange in braided river systems, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-2767, 2023.