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.