during snowmelt runoff and the second in early
high. Rates ranged from 0.1 to 4.9 m during sum-
August during the peak glacial melt season. In
mer 1992, 0.4 to 6.3 m during winter 199293, 0.0
to 9.8 m during summer 1993, 0.0 to 2.3 m in win-
contrast, waters of Knik Arm ranged from about
ter 199394, and 0.0 to 2.6 m in summer 1994. Gul-
1000 to 2800 mg/L from May to October and
appear to be the primary source of new sediment
lies are progressively extending into the mudflats
in ERF. A range in TSS values similar to that of
toward the pond complexes of the Bread Truck,
Knik Arm characterized the eight gully sites,
C, A, D, and C/D. Two gully headwalls, one on
indicating this causal relationship. During any
the western side of Bread Truck and the other
given tidal cycle, TSS values steadily increased
near the pond complex between Bread Truck and
with the flooding tide, but progressively
C ponds, are advancing at a rate sufficient to
decreased at a slower rate during the ebb. Sea-
cause increased drainage of those ponds within
sonally, TSS values in gully and Knik Arm waters
the next 1530 years if the average rates persist.
increased from spring to fall; the cause of this
Surveyed longitudinal profiles of tidal gully thal-
increase is unknown and under study.
wegs revealed unstable, nonequilibrium gradi-
ents that reflect their progressive elongation into
Sedimentation
the mudflats and ponds by headwall erosion.
Sedimentation ranges from several millime-
Headwalls have nearly vertical faces that vary
ters per year on levees, 1015 mm on mudflats,
from 1 to 2.5 m in height. Above them, gradients
and up to 2040 mm per year in ponds. If the high
within drainageways range from 0.001 to 0.004
sedimentation rates measured in ponds are char-
m/m across the mudflats and into the ponds,
acteristic, natural sedimentation would aid in in-
being slightly lower in slope than within the tidal
filling of dredged areas and may provide a meth-
gullies below the headwall. Tidal current velocity
od of burying and thereby naturally avoiding WP
data from three gullies indicate that sediment
ingestion by waterfowl.
transport and channel erosion are potentially
Eagle River provides access for tidal waters to
greatest during the ebb cycle.
inundate the innermost reaches of the Flats.
Increases in water level and reduced or halted
White phosphorus transport
flow result from tidal damming by incoming
There is some area-specific evidence for the
floodwaters that may lead to localized increases
movement of white phosphorus via ice-rafted
in depositional rates. Preliminary analyses indi-
sediments, resuspension in ponds (measured in
cate that sedimentation in the northern two-
sediment traps), and ebb tide gully discharge
thirds of ERF is tidally dominated, whereas the
water (captured in plankton nets). Concentra-
southern one-third appears to be river-dominated.
tions found to date are too low to indicate actual
Sediments deposited by the Eagle River at the
particle movement, but WP is moving locally in
head of the Flats where it enters ERF proper has
some form. In the case of ice-rafted WP, contamin-
apparently formed an alluvial fan with four radi-
ated pond bottom sediments can freeze onto the
ally spreading channels of Eagle River distribut-
ice bottom during its growth and subsequently
ing water after entering the tidal flats.
be uplifted and transported during tidal inunda-
At the exit of Eagle River into Knik Ann, 6
tion. Ice and water erosion and transport of WP
remain to be quantitatively assessed.
hours of bathymetric profiling of Knik Arm off-
shore of ERF revealed the existence of two sub-
marine channels. The primary channel is 100200
Remediation
m wide and generally parallels the coast. The sec-
ondary channel is located about 11.2 km off-
the result of sedimentation and burial, erosion
shore and characterized by erosional features.
and pond drainage, abrasion during transport,
Large intertidal bars occur north of the Eagle
and other causes needs to be considered. The
River mouth, and west of the primary channel.
inherent complexity of this dynamic environ-
These bars, as well as a zone adjacent to the coast-
ment makes it extremely difficult to predict what
line south of the Eagle River mouth, are potential
effects potential remedial measures for white
depositional repositories for WP.
physical system. Because the physical conditions
Erosion
and processes vary widely across ERF, each spe-
Erosion and recession rates of headwalls and
cific area slated for WP remediation needs to be
lateral walls of tidal gullies are also relatively
evaluated and ranked in terms of the suite of
48
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