Restoring the liver of the river: Instream wood as a nature-based solution to nutrient pollution in agricultural watercourses

The hyporheic zone, the regions in the river sediment where surface water and groundwater interact, acts as the liver of the river that often can attenuate nutrients and other pollutants from mixing groundwater and river water. Nutrients can be retained and transformed by sediments to environmentally benign products (e.g. N₂) by microbially mediated reactions. However, in most watercourses the capacity for this filtering function has been reduced by centuries of mismanagement such as channel straightening and the removal of instream wood and riparian buffers. The addition of instream wood, directly in restoration or indirectly by rewooding riparian zones, could restore the functioning of hyporheic zones and provide nature-based solutions to persistent water quality challenges.

Nutrient transformation in the river corridor is limited by two primary mechanisms which could be abated by wood introductions. Firstly, reaction kinetics, including the availability and quality of organic matter. Here, the decomposition of wood could provide a local and sustained source of labile organic matter. Secondly, transport, namely the total flux of water into the hyporheic zone and its residence time therein. Instream wood causes an obstacle for river flow which can induce hyporheic exchange of suitable properties to allow favourable nutrient transformations.

Here we present the results of two linked experiments which have been designed to investigate the effect of wood introductions at different scales: the microbial, restoration feature and reach scales. An incubation experiment focuses on the microbial scale, which allowed us to demonstrate that additions of wood to river sediments increases concentrations of dissolved organic carbon, leading to increased rates of nutrient transformation and increased microbial metabolic activity and production of greenhouse gases. The restoration feature and reach scale impacts are investigated using a before-after-control-intervention field experiment – including a series of smart tracer injections to estimate (metabolically active) transient storage. Preliminary results from this study suggest that wood introductions in river restoration increased reach-scale residence time and ecosystem metabolism, which are good indicators for reach scale nutrient turnover. 

Our results could provide evidence that supports the use of wood in river restoration and of nature-based solutions to water quality challenges.