Table 4. Temperature and salinity of Eagle
sharply increase as the tidally dominated water
River and Knik Arm.
mass subsequently fills the gully (Table 4).
As ebb occurs, the decrease in turbidity takes
20 August
18 September
place more gradually than at the Bread Truck site
Temperature Salinity
Temperature Salinity
(C)
(C)
until the water elevation drops below bank
Site
(ppt)
(ppt)
height. Then it decreases rapidly, as does temper-
Eagle River
6.0
0
4.0
0
ature and salinity, to levels less than Knik Arm,
Knik Arm
12.9
--
10.3
6.8
suggesting that river water is a larger component
of the ebb discharge at the Parachuate site.
high tide of 10.33 m illustrate several important
In contrast, the 1819 September tidal event is
aspects of tidal flats hydrology (Fig. 20 and 21).
characterized by lower suspended sediment
As described for 18 August at the Bread
concentrations, higher salinities and lower water
Truck site, the water level on 2021 August rises
temperatures (Fig. 21). Several other important
rapidly as flooding begins, but slows as water
aspects are also evident. At Bread Truck, the tem-
spills into the vegetated drainageway above the
perature does not decrease rapidly as the tidal
gully headwall and floods the mudflats. Coinci-
waters rise. An initially lower temperature with-
dent with this rise, water temperature, salinity
in the plunge pool during September than in
and turbidity change but in different ways at
August may account for a temperature similar to
the Bread Truck than at the Parachute site.
that of water in Knik Arm (Table 4). Both turbidi-
At the Bread Truck gully site, where the
ty and salinity increase rapidly, however, as
flooding threshold is lower, water temperature
flooding takes place. The rapid drop in salinity
shows little variation until the approximate
just before peak flood level may indicate incom-
time when water height is sufficient to overflow
plete mixing of Knik Arm and Eagle River waters
the gully. Temperature then rapidly declines to
within the flood. During ebb, water temperature
near that of water in Knik Arm, signaling the
and salinity gradually decline as the freshwater
arrival of tidal flood waters (Table 4). Small
component becomes more important, but the tur-
temperature variations (tenths of a degree Cel-
bidity values exhibit a series of rapid fluctuations
sius) during the remainder of the tidal cycle
that nearly equal those during peak flood.
may result from local mixing of tidal and fresh-
The Parachute gully record has a more gradu-
er water masses. Coincident with the tempera-
al and sustained fall in turbidity beginning at
ture drop, turbidity increases sharply and
about the same time as at the Bread Truck site.
remains high through flooding; however, just
The source of this difference from Bread Truck is
after this rise, OBS values decrease sharply, per-
unknown, but we speculate that the sustained
haps reflecting an initial override of fresher
values at Parachute represent discharge of sedi-
water by the intruding more saline tidal flood-
ment transported from within the Flats. The
waters.
pulses of suspended sediment in the water col-
As ebb takes place, turbidity values drop
umn may result from local erosion, failure and
rapidly within about 30 minutes, indicating ei-
collapse of the gully walls. But this argument is
ther that waters inundating the Flats have lost
less convincing for the Bread Truck site; the high-
their suspended sediment through deposition
ly erratic peaks are more likely caused by vegeta-
in ponds and mudflats, or that the mixing of
tion that caught on the probe.
river and tidal water masses has diluted the
Of further note are the significantly higher sa-
concentration of solids. With time, the turbidity
linities in the Parachute flood and ebb waters in
continues to decrease as water elevation de-
September than in August. These values proba-
clines; both sediment concentration and salinity
bly represent the seasonally reduced contribution
decrease to levels present before the flood.
of Eagle River water to flooding of the Flats.
At the Parachute gully site, the drop in tem-
Similarly, turbidity values at both sites were less
perature accompanying the initial encroach-
than in August and correlate with reduced con-
ment of flood waters is greater than at Bread
centrations of suspended sediment in both the
Truck. This reduction is believed to be attribut-
river and Knik Arm. This reduction is also a re-
able to a higher proportion of river water in the
sponse to the seasonal drop in sediment dis-
first flood waters rising to the gully sensors be-
charge from the glacierized basins that flow into
cause of the more inland location of the gully
the Eagle River and the rivers entering Knik
mouth. Salinity, turbidity and temperature
Arm.
23
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