H1) Increasing logjam longitudinal distribution density enhances transient storage
Results from the flume support our hypothesis that an increase in logjam longitudinal distribution density enhances the magnitude of transient storage. We observed an increase in skew and mean arrival time in the flume results with an increase in logjam longitudinal distribution density (Figure 2). Tukey adjusted pairwise comparisons between mean arrival time and the number of jams indicated slower advection with more jams (p<0.0001). In other words, not surprisingly, more logjams in the flume result in slower movement of the tracer down the channel. Pairwise comparisons between skew and the number of jams indicate an increase in skew with more jams present (p<0.0001), meaning that more jams increase retention. Numerical modeling simulations further support this hypothesis. Increasing the longitudinal distribution density of logjams results in a greater extent of hyporheic zone (Figure 3). Mean subsurface arrival time and skew both increase with the addition of multiple logjams in the numerical model (Figure 2). We see a 14% increase in hyporheic exchange fluxes with multiple jams present compared to a single jam at low flow and a 2% increase in hyporheic exchange at high flow (Supplemental Table 5). Multiple successive jams provide more pervasive exchange by distributing the head drop at each jam, leading to distributed but shallow flow paths (Figure 3). Thus, greater logjam longitudinal distribution density facilitates more surface water storage as well as downwelling into the subsurface (Figure 4).