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.