12,000 years ago may be affecting processes as
5-ft (1.5-m) sections of 0.75-in. (2-cm) diameter
well.
steel pipe were mounted vertically in large cans
The glacial sources for the Eagle River modify
filled with concrete, and the eyehooks mounted
the discharge seasonally, mainly in response to
on the platform's wood frame were lowered over
variations in climate, and daily, mainly in re-
them. Foam filled the inner space of the deck
sponse to diurnal changes in ablation and melt-
framework and provided flotation so that the
water runoff. Maximum discharge occurs during
platform could move vertically up or down the
and following the spring runoff of snowmelt, but
steel pipes during tidal inundation and ebb. The
may remain high throughout the summer, de-
platforms allowed us to establish monitoring sta-
pending upon meteorological factors affecting
tions without setting stakes or foundations in the
meltwater production on the glacier surface (e.g.,
ordinance-bearing mudflat sediments and, at the
Rthlisberger and Lang 1987, Lawson 1993).
same time, keep the data loggers and related de-
Rapid, hourly fluctuations in discharge may re-
vices dry even under the highest flood levels.
sult from heavy rain in the river's watershed.
An ISCO Model 3700 water sampler, also
The glacially derived sediment within runoff
mounted on each platform, obtained 500-mL wa-
from the watershed also varies seasonally and
ter samples at specific intervals through the flood
annually; within ERF, this component mainly
and ebb cycle. The Total Suspended Sediment
affects the amount of sediment in suspension.
concentration (TSS) of each sample was analyzed
Sediment in suspended transport from the Eagle
using standard vacuum techniques with 45-mm
River is one potential source of material deposit-
glass microfiber filters. The TSS data were later
ed within the ponds and mudflats.
used to calibrate the D&A Optical Backscatter
Tidal flood waters are another major source of
Sensor (OBS) turbidity readings for sediment
suspended sediment. These sediments are like-
concentrations measured at each site.
wise of glacial origin, being derived from glacier-
Water quality data were recorded from mid-
ized watersheds feeding major rivers that enter
August to the end of September using 2-minute
Knik Arm and Cook Inlet, including the Susitna,
sampling intervals at the gully sites and 10-
Knik and Matanuska rivers. It is not known
minute intervals at the river site. A Seabird
whether the seasonal and annual variability of
SBE19 Profiler or CTD (ConductivityTempera-
tureDepth) profiler was moored at the coast in
sediment discharge from these glacial rivers is
Knik Arm from mid- to late September to mea-
evident within the tidal waters of Knik Arm. Lim-
sure salinity, temperature, depth and turbidity
ited measurements of Knik Arm in November
during several of the tidal cycles (Fig. 11). At the
1993 and March 1994 indicated that sediment is in
same location, a Seabird SBE26 Wave and Tide
suspension through the winter, but in smaller
Gauge measured tidal height variations and wa-
quantities than during the summer. The contin-
ter temperature from mid-August to late Septem-
ued presence of suspended sediment during win-
ber (Table 1). ISCO water samples were acquired
ter is important, as it would then be the most
at 1/2-hour intervals through the flood and ebb
important if not the only source of sediment for
cycles of August, but this was changed in Sep-
deposition during that time. The Eagle River, as
tember to a 10-minute interval about peak tide.
with other glacial rivers, transports an insignifi-
Water samples were also taken by hand from
cant amount of material in suspension during the
Knik Arm at the wave and tide gauge recorder
fall through the early spring when glacial dis-
during the September inundation cycle.
charge is limited to groundwater sources (e.g.,
Lawson 1993).
The interaction and combination of tidal and
PROCESS OBSERVATIONS
river sources of water and sediment control the
The activity and relative importance of ero-
amount of material available for deposition in
sional and depositional processes depend upon a
ERF, and may also affect the locations and rates
number of factors, including those that are exter-
of erosion. The amount of sediment within flood
nal to the immediate Flats environment. External
waters is a primary factor determining their cap-
factors include tidal forces, glacially derived run-
acity to erode and transport additional sediments
off and sediment, climate, and tectonic and earth-
during ebb.
quake activity. In addition, a slowly rising sea
A preliminary analysis of aerial photographs
level and isostatic rebound following occupation
of ERF suggests that sedimentation of the land-
of the region by glaciers that retreated about
ward third of the area (southern one-third) is
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