2000
Knik Arm
S
)
1000
0
20 Sep '93
21
22
23
24
25
Figure 26. OBS measurements from Knik Arm during late September.
while the TSS samples from the river yielded
not always relate directly to the weight of solids
much lower values (less than 100 mg/L) (Fig.
present.
26).
We attempted to calibrate OBS sensor read-
Further samples are clearly needed to deter-
ings of backscatter to suspended sediment con-
mine if the relationship of sediment concentra-
centrations using the TSS measurements. In a
tion for the river and tidal sources is representa-
few events, correlations were good (Fig. 25a).
tive of the entire year, especially because of the
However, in others, there is not a simple rela-
inherent seasonal variability in the Eagle River's
tionship between the two values. For example,
glacial sources. Sediments in Knik Arm waters
secondary peaks in OBS readings were observed
are, however, also derived from glacierized
during ebb tide at the Parachute site without
basins feeding the Susitna, Knik, Matanuska and
correspondingly high TSS values (Fig. 25b). Re-
other rivers that drain into Cook Inlet and thus
gression analysis of this relationship produced
the tidal source may also vary seasonally.
low correlation coefficients. A possible explana-
tion for the secondary peaks is that, if extremely
fine material is in transport during ebb, it can
EROSION DYNAMICS AND
cause a large amount of optical backscatter, but
GULLY HYDRAULICS
add little to the total weight of suspended solids.
This problem remains under investigation.
Longitudinal cross sections of the gullies, sur-
veyed along their central channel or thalweg,
SEDIMENT SOURCES
have several characteristics in common. Each of
the nine gullies in the study area has moderate
The suspended sediment concentrations
to relatively steep slopes downstream of an
measured during August and September sug-
eroding headwall (Fig. 27). The headwall region
gest that the tidal flood waters of Knik Arm
is characterized by a nearly vertical face ranging
have greater concentrations of sediment in sus-
from 1.2 to over 2.0 m in height, at the base of
pension than do those in the Eagle River. TSS
which is a depression or plunge pool scoured by
measurements preceding, during and following
water flowing over the headwall (Fig. 27). This
tidal flooding cycles at both the Bread Truck and
plunge pool ranges in depth from about 0.5 to
Parachute gully sites are consistently higher
over 2.0 m.
than those in the Eagle River and are similar in
Channel gradients are steepest near the Eagle
range to those of Knik Arm. TSS data for the Ea-
River (0.01 to 0.02 m/m). The steep bed slope
gle River reveal concentrations ranging from 23
probably results from the large range in tidally
to 275 mg/L for 25 August to 20 September.
controlled variations in water level and dis-
Concentrations in two samples from Knik Arm
charge in the river proper. Upstream of this
at the coastal site were much higher: 1116 and
steep section, the gradient becomes shallower
1273 mg/L. OBS turbidity data for 20 to 24 Sep-
(0.003 to 0.007 m/m), but then progressively
tember at the coast also were consistently high,
steepens with distance to the base of the head
28
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