Figure 1. Flume configuration (9-m long, 1.2-m wide) for (a)
longitudinal distribution density experiments and (b) permeability
experiments. (a) shows the change in jam quantity from a single jam to
multiple jams. (b) shows the change in permeability for a single logjam
from a more permeable jam to a less permeable jam with coarse
particulate organic matter additions. Data were collected at sensors
above and below the experimental reach. However, we use the “below
jam(s)” sensor data for results and analyses because they best
represent the combined effects of logjam distribution density and
permeability changes.
A list of all flume runs is included as Table 2. We implemented tracer
experiments with sodium chloride (NaCl) to characterize surface
transient storage flow paths via fluid conductivity. We added 360-g of
NaCl dissolved in 3.78 L of water to the flume at the injection point
(Figure 1) via a pulse injection. The ability to resolve transient
storage depends in part on the relative travel times through the mobile
portion of the channel and transient storage zones, as well as the
exchange rates between them. Given our expectation of short transport
times (on the order of minutes) in this flume-scale system, we deemed a
pulse injection sufficient to load even some the slower flow paths with
salt. The first 3-m of the flume served as a mixing zone. The
experimental section where jams and fluid conductivity sensors were
placed spanned from 3.0 to 7.5 m (Figure 1). No jams or sensors were
placed in the last 1.5-m of the flume (from 7.5 to 9.0 m) to minimize
interference from backwater effects near the sediment screen. We
measured specific conductivity at 10-s intervals during the tracer tests
using Onset Computing HOBO U-24 fluid electrical conductivity loggers
(data loggers). Data loggers recorded fluid conductivity measurements
for 30 minutes following the pulse to allow ample time for the flume to
return to background solute concentrations. Sensor placement was uniform
across all flume trials. We deployed one data logger upstream of the
NaCl injection point to measure background fluid conductivity, and two
data loggers 0.3-m upstream and downstream of the logjam(s) in the
monitoring reach, at 3 m and 7.5 m from the top of the flume (Figure 1).
We only report data from the downstream sensor for clarity as it
represents both the above and below logjam effects. The upstream sensor
data are included in the Supplemental Information. The measured instream
conductivity signals represent the combined effects of surface transient
storage in the channel (e.g., backwaters, eddies) and subsurface
transient storage via hyporheic exchange.
We set high and low discharge values that ranged over a factor of 10
(0.001 m3/s and 0.01 m3/s). Initial
test runs suggested that tracer was flushed too quickly to measure with
accuracy above 0.01 m3/s. Discharge readings were
obtained from a flow meter with ±0.2% accuracy. Discharge for all flume
runs was fully turbulent, allowing relaxation of Reynolds number scaling
(Peakall et al., 1996). We ran replicates for all flume trials. A
digital elevation model for the flume was constructed using structure
from motion. Images were captured at regular downstream intervals with a
camera mounted at a consistent elevation and we used ground-control
points along the flume bed for additional adjustment. Images were
processed using Agisoft Metashape Professional. The resulting digital
elevation model had a resolution of less than 1 mm.