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).