4.2 Management Implications
Our simulation of multiple low-permeability logjams enhanced hyporheic exchange by the greatest magnitude, suggesting that this configuration maximizes hyporheic exchange potential. Thus, when thinking about designing engineered logjams to promote surface water-groundwater exchange or restoration targeting hyporheic exchange, we infer that those scenarios with opportunities to recruit more wood or facilitate the formation of multiple jams, as well as retain coarse particulate organic matter that reduces logjam permeability, will best maximize transient storage. Although the specific magnitudes of surface transient storage and hyporheic exchange at the field scale cannot be determined from flume-scale experiments, the general characterization (for example, the substantial increase in exchange rates and surface water storage with more logjams and more fine material in a logjam) should be consistent across flume to field scales.
Zones of transient storage are important components of stream systems, and, similar to other stream habitats, have suffered degradation as a consequence of human activity. Efforts to incorporate channel elements that promote hyporheic exchange have started to emerge as a step toward restoring stream ecosystems and associated hydrologic functions (Crispell and Endreny, 2009; Hester et al., 2018). Additions of complex logjam formations in altered river reaches may increase hyporheic interactions by slowing stream water velocity, forming backwaters, increasing flow complexity, and diverting water to the subsurface. Some morphologic features may also retain coarse particulate organic material that decreases the permeability of a logjam while creating the redox gradients necessary for certain biogeochemical functions. Nevertheless, successful applications of stream management and restoration utilizing logjams is dependent upon understanding the role of different large wood distributions (ranging from more dispersed distributions to many successional jam structures to tightly packed jams) on transient storage. Although the hyporheic benefits of ongoing installation of morphologic features may be considerable, to our knowledge they are not currently included as project design goals (Hester and Gooseff, 2010) and few studies quantify the impact of restoration features on hyporheic function (Kasahara and Hill, 2006; Crispell and Endreny, 2009; Wade et al., 2020). Adding hyporheic benefits of existing installations as an explicit design goal where appropriate, modifying the design parameters of such features to maximize benefits as possible, and considering additional features explicitly for hyporheic benefit are all needed to incorporate and quantify transient storage benefits in restoration projects.
Conclusions
Results from tracer experiments in our flume and numerical modeling simulations provide insight into how logjam characteristics influence transient storage. The presence of multiple channel-spanning logjams and decreased logjam permeability facilitates more opportunities for solute retention and processing in zones of transient storage. Solute breakthrough in the flume is more sensitive to surface flow paths than subsurface flow paths, suggesting much of the observed transient storage occurs in backwater zones behind jams. Configurations with greater transient storage also have the greatest rates of hyporheic exchange and longest residence times in the subsurface. From the observed differences in surface transient storage between stream segments with greater and lesser amounts of large wood and more or less permeable jams, we infer that river management designed to foster surface transient storage can effectively focus on retaining wood (either by continuing recruitment and transport or fixing engineered logjams in place) and retaining coarse particulate organic matter. We conclude that transient storage is maximal when there are many jams (large longitudinal distribution density) with low permeability (tight packing of wood pieces). An increase in transient storage can, in turn, improve stream health and makes a case for river managers to strategically implement wood in river restoration designs.
Moving forward, additional work is needed to explore whether a change in longitudinal distribution density or permeability has a nonlinear effect on transient storage. Other studies have shown that logjams create nonlinear effects on stream metabolism (Day and Hall, 2017); spatial heterogeneity of physical channel characteristics (Livers and Wohl, 2016; Livers et al., 2018), including channel and floodplain planform (Buffington and Montgomery, 1999; Wohl, 2011); retention of particulate organic matter (Beckman and Wohl, 2014); and animal biomass and biodiversity (Herdrich et al., 2018; Venarsky et al., 2018). We expect that transient storage might follow a similar pattern of nonlinearity, but the binary nature of this experimental set-up did not allow for such exploration.