Examining the impact of instream wood additions on hyporheic exchange in a lowland agricultural catchment

Increasingly, Instream wood  is (re)introduced into river systems to reverse decades of catchment mismanagement and to deliver nature-based solutions to contemporary water resource challenges such as flooding and pollutant attenuation. Most Research concerned with instream wood has focused on its ability to modify morphological and ecological conditions within a reach, but less work has considered the implications for hyporheic connectivity, a primary control of many ecosystem functions. Here, we investigate the impact of wood additions in river restoration on hyporheic exchange, at both the feature-scale and reach-scale,  with the application of a before-after-control-intervention experimental design.

Research was conducted over a 200m long reach of Wood Brook (Staffordshire, UK), a lowland river, which drains a 3.1km² catchment dominated by mixed-arable farmland and deciduous woodland. The experimental reach included 3 treatment sites where channel-spanning wood features were installed, 2 sites with natural wood features, and 3 control sites that were appropriate for treatment but received no intervention. High-resolution-temperature-sensors (HRTS) were installed at these sites to capture the temperature in the surface water and at 3 hyporheic depths, up to 25cm, at 3-minute intervals. Furthermore, a series of smart tracer injections allowed us to estimate (metabolically active) transient storage before and after intervention, in both the treatment sub-reach which had received wood additions and the control sub-reach which had not.

Results indicate, once background conditions are excluded from the dataset, that the mean difference between hyporheic and surface water temperatures across the treatment sites reduced by 31% over the course of the study whilst the control sites remained unchanged. Further examination determined that the daily mean temperatures observed at treatment sites were significantly different to those witnessed at the control sites. This suggests that the introduction of instream wood fostered an increase in the magnitude of hyporheic exchange. This is supported by the analysis of before-after intervention data, where a smaller deviation was observed between surface water and hyporheic temperatures across the treatment sites when compared with the control group. Preliminary analysis of smart tracer injections suggests that wood additions increase reach-scale residence times of surface water and reach-scale metabolism.

The current research supports observations previously derived from flume and model-based studies, suggesting that the addition of instream wood alters the magnitude of localised hyporheic exchange. Enhanced hyporheic exchange can offer numerous benefits to a reach including: increased habitat diversity, improved primary production, and greater attenuation and transformation of pollutants. Therefore, research within this area offers valuable insights for water resource managers who are increasingly under pressure to improve the health of our riverine environments as stipulated by international policies such as the European Unions’ Water Framework Directive. While our research has contributed to advancing current knowledge surrounding how instream wood alters hyporheic connectivity, there remains numerous questions which need addressing prior to its widespread application to global watersheds.