H3) A single low-permeability logjam creates a comparable increase
in transient storage to multiple high-permeability logjams
We compared transient storage between a single low-permeability logjam
and multiple high-permeability logjams to better understand the relative
effects of these logjam characteristics. Mean arrival times for the
flume and model show a clear increase in time the tracer takes to move
through the surface and subsurface between a single low-permeability jam
and multiple high-permeability jams (Figure 7). In the flume, the tracer
moves significantly slower when multiple jams are present
(p<0.0001). Modeling results also showed approximately a 120%
increase in subsurface mean arrival time with the presence of multiple
high permeability jams compared to a single, low permeability jam
(Figure 7).
Skew response was different based on discharge in the flume (Figure 7).
Under low-discharge conditions, more skew is observed in a single
low-permeability jam compared to multiple high-permeability jams
(p=0.002). Under high discharge conditions, more skew is observed in
multiple high-permeability jams compared to a single low-permeability
jam (p=0.0007). In other words, permeability is playing a greater role
in enhancing transient storage in the flume at a low discharge compared
to high discharge. Skew was greatest in the multiple high-permeability
logjam numerical modeling simulations relative to a single
low-permeability logjam, regardless of the discharge (Figure 7). We
observed approximately a 24% increase in hyporheic exchange at low flow
and 12% increase at high flow in the multiple high-permeability logjam
simulation compared to the single low-permeability logjam simulation
(Supplemental Table 5). We simulated a scenario with both increased
logjam longitudinal distribution density and decreased permeability (not
shown in the flume), which resulted in the greatest magnitude of
hyporheic exchange and surface backwater increase of all logjam
scenarios.