H2) Decreasing the permeability of a single logjam enhances
transient storage
Results support our hypothesis that a decrease in logjam permeability
enhances the magnitude of transient storage (Figure 5). Visual
observations of backwater extent in the flume suggest a less permeable
jam has a more profound impact on increasing retention in surface water
relative to a more permeable jam (Figure 6). As more wood and coarse
particulate organic matter are added to a single logjam, greater
backwater and low-velocity flow fields form. Model simulations further
show that an increase in rising surface-water level behind a less
permeable logjam resulted in a greater magnitude of water infiltrating
into the water-sediment interface in front of the logjam and exfiltrate
out behind the logjam (Figure 4). Thus, hyporheic exchange is increased
around the logjam.
Surface mean arrival time increased as the permeability of the jam
decreased in the flume (p=0.0001). In other words, a more tightly packed
jam results in slower movement of the tracer down the channel regardless
of discharge. We observed the same trend in subsurface mean arrival time
in the model (Figure 5). Pairwise comparisons of skew and permeability
in the flume results suggest that a less permeable logjam increases
surface water retention compared to a more permeable logjam in the flume
(p=0.0087). Skew in the model increases by 300% as the permeability
decreases at low discharge and increases by over 160% as the
permeability decreases at high discharge.