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.