Streambed restoration to remove fine sediment alters reach‐scale transient storage in a low‐gradient fifth‐order river, Indiana, USA

Abstract

Hyporheic restoration is of increasing interest given the role of hyporheic zones in supporting ecosystem services and functions. Given the prevalence of sediment pollution to waterways, an emerging restoration technique involves the removal of sediment from the interstices of gravel‐bed streams. Here, we document streambed sediment removal following a large, accidental release of fine sediment into a gravel‐bed river. We use this as a natural experiment to assess the impact of fine sediment removal on reach‐scale measures of transient storage and to document the responses of reaches with contrasting morphology (restored vs. unrestored) to changing discharge one‐field season. We conducted a series of conservative solute tracer experiments in each reach, interpreting both summary statistics for the recovered in‐stream solute tracer time series. Additionally, we applied the transient storage model to interpret the results via model parameters, including a Monte Carlo analysis to measure parameter identifiability and sensitivity in each experiment. Despite the restoration effort resulting in an open matrix gravel bed in the restored reach, we did not find the significant differences in most time series metrics describing reach‐scale transport and transient storage. We hypothesize that this is due to enhanced vertical exchange with the gravel bed in the restored reach replacing lateral exchange with macrophyte beds in the unrestored reach, developing a conceptual model to explain our findings. Consequently, we found that the impact of reach‐scale removal of fine sediment is not measureable using reach‐scale solute tracer studies. We offer recommendations for future studies seeking to measure the impacts of stream restoration at the reach scale.

Is the Hyporheic Zone Relevant beyond the Scientific Community?

“…The efficiency of river restoration structures can likely be improved by attending to design variables that can alter HZ functioning. Example variables include controlling hydraulic gradients (e.g., the height of a step [218]), manipulating hydraulic conductivities (e.g., with sediment coarsening [248]), changing flowpath geometries (e.g., with baffle walls [54,249] or hyporheic caps and increased HZ depth [33]), or shielding a structure from groundwater upwelling or downwelling (e.g., with a liner). Future research should focus on tailoring river restoration practices to deliver specific regulating ecosystem services, while also recognizing that in-channel structures alone are not able to overcome catchment-scale degradation of these services [250].…”

Spatial and temporal variation in river corridor exchange across a 5th order mountain stream network

“…Finally, we calculated the holdback of the system (H), which describes transport in a continuum ranging from piston flow (H = 0) to no movement of the solute (H = 1) [Danckwerts, 1953]. Ward et al [2018b] interpret higher values of H to indicate greater influence of transient storage on reach-scale transport. Holdback is calculated as:…”

“…While a trend was present, we also note that travel time based on tpeak exhibits less variation than discharge (coefficient of variation 1.00 for travel time compared to 1.49 for discharge). For context, a recent study by Ward et al [2018b] attempted to control for experiments with 20-min of advective time and accepted a range from 17 to 50 minutes as comparable. Thus, while our selection of study reach lengths was imperfect to achieve identical advective timescales, we contend that we have controlled for advective time in a more rigorous way than has been done in most previous studies.…”

“…Given the relative ease and low cost of this method, it is unsurprising that many studies have used solute tracer studies under different discharge conditions to assess relationships between discharge and river corridor exchange. For example, some studies repeat solute injections in a fixed reach under range of discharge conditions during different seasons [e.g., Zarnetske et al, 2007;Ward et al, 2018b], during baseflow recession [e.g., Payn et al, 2009;Ward et al, 2012], or during storm events [e.g., Ward et al, 2013b;Dudley-Southern and Binley, 2015]. Still others use spatial replication at multiple sites within a network to construct a relationship that can be used to predict behavior for unstudied reaches during a single discharge condition [e.g., Jencso et al, 2011;Covino et al, 2011;Stewart et al, 2011].…”

Fine Sediment Removal Influences Biogeochemical Processes in a Gravel-bottomed Stream

“…The Fawn River restoration provided an opportunity to examine the effects of restoration on stream metabolism and nutrient spiraling in a real-world setting in which a limited number of physical and chemical variables-streambed sediment content and macrophyte abundance-were manipulated in a semi-controlled manner. In a companion study, Ward et al (2018) found that fine sediment removal likely increased surface water exchange with the hyporheic zone in Fawn River during high-flow periods. Increased surface-subsurface connectivity is expected to promote ecological functioning, including nutrient retention (Mendoza-Lera and Datry 2017).…”