Effects of Hyporheic Exchange Flows on Egg Pocket Water Temperature in Snake River Fall Chinook Salmon Spawning Areas, 2002-2003 Final Report. Page: 32 of 164

                                
3.2 Hydraulics

The hydraulic gradients between the river and the riverbed (shallow hyporheic zone) suggested the
potential for predominantly small upwelling and downwelling vertical exchange. During the spawning
and early incubation periods, 10 of the 14 sites had a mean hourly difference in head pressure (dh in cm;
hyporheic - river) within a +2.0 cm range. During the late incubation period, 9 of the 14 sites had a mean
dh within a +2.0 cm range (Figures 6 and 7). Within each site there was little change in mean dh between
time periods. Between the spawning and early incubation periods the difference in mean dh was less than
1.0 cm at all sites, with an average change of 0.4 cm. Between the early and late incubation periods the
difference in mean dh was less than 1.0 cm at 11 of the 14 sites, with an average change of 0.4 cm for
those sites. Study sites 149.2, 156.8, and 198.8 exhibited a change in mean dh of 1.7 cm, 2.1 cm, and
1.5 cm, respectively, between the early and late incubation periods. While mean dh did not change much
between time periods, the range in dh changed between the low, stable discharge period and the variable
discharge period (Figure 7).
Each of the lower, middle, and upper segments of the study area included sites exhibiting both
upwelling and downwelling potential. Sites within the lower segment had a mean dh ranging from -
0.1 cm (+0.7 cm SD) to 1.6 cm (+0.4 cm SD) during the spawning period, from -0.9 cm (+0.6 cm SD) to
1.6 cm (+0.3 cm SD) during the early incubation period, and from -0.6 cm (+0.7 cm SD) to 3.2 cm
(+0.9 cm SD) during the late incubation period (Figures 6 and 7). Within the middle segment, the range
of mean dh among sites was much larger, ranging from -1.0 cm (+0.4 cm SD) to 4.7 cm (+0.5 cm SD)
during the spawning period, from -1.5 cm (+0.7 cm SD) to 4.6 cm (+0.4 cm SD) during the early
incubation period, and from -1.0 cm (+0.5 cm SD) to 4.7 cm (+0.8 cm SD) during the late incubation
period (Figures 6 and 7). Sites within the upper segment also had a large range of mean dh, ranging from
0.3 cm (+0.7 cm SD) to 3.4 cm (+0.6 cm SD) during the spawning period, from 0.2 cm (+0.6 cm SD) to
3.7 cm (+0.6 cm SD) during the early incubation period, and from -0.3 cm (+1.6 cm SD) to 3.1 cm
(+1.2 cm SD) during the late incubation period (Figures 6 and 7). Tests for differences in mean dh among
all sites resulted in indications of significant differences for nearly all sites in all time periods (Tables 5
and 6). However, many of the differences in mean dh were less than 1.5 cm, which is approaching the
pressure transducer uncertainty error of+ 1.4 cm.
The vertical hydraulic gradients (VHG) between the river and shallow hyporheic zone, as well as
between the river and deep hyporheic zone, showed little relationship to changes in river discharge at
most sites. At the large temporal scale, most sites exhibited small effects of river discharge on VHG
between the low, stable discharge periods (spawning and early incubation) and the variable discharge
period (late incubation) (Figure 8). Sites 149.2, 156.8, and 198.8 indicated marked changes in shallow
hyporheic zone VHG between the low, stable discharge periods and the variable discharge period. At all
three sites, the upwelling potential increased. During the low, stable discharge periods, 12 of the 14 study
sites indicated small upwelling potential between the river and deep hyporheic zone (Figure 8). As
discharge increased and became more variable during the late incubation period, only site 156.8 showed a
marked change (increase) in VHG between the river and deep hyporheic zone.

3.7