relatively steep near their mouths, but are reduced
are parts of former drainageway and gully sys-
in the lower and mid-reaches. Channel gradients
tems that are now inactive and therefore relict.
then steepen in the uppermost reaches up to a
Their fragmentary presence is a sign of signifi-
nearly vertical headwall that ranges in height
cant changes to the drainage system in the past.
from 1 to 2.5 m. Incipient drainageways from
The cause of such change is unknown, but events
ponds and mudflats into tidal gullies have very
such as river avulsion, channel migration or earth-
low gradients. Several sharp increases in gradient
quake-induced subsidence may cause abandon-
(knickpoints) occur in each gully, marking chan-
ment of drainageways and gullies.
nel adjustment as the tidal gully progressively
Chemical analyses of a limited number of sedi-
extends into the mudflats and ponds.
ment trap and plankton net samples indicate that
Aerial photographs from 1950, 1967 and 1993
WP undergoes suspension, transport and redepo-
reveal changes in the distributary channel pattern
sition in ERF, as well as transport through tidal
entering ERF (Fig. 7). Three main channels en-
gullies into the Eagle River and Knik Arm. Tidal
tered the Flats in 1950, but by 1967 the western-
and wind-driven currents can scour pond bot-
most channel was abandoned and the eastern and
toms, resuspending and transporting WP, whether
middle channels diverged just northeast of the
as individual particles or sorbed to sediment. Ero-
Route Bravo bridge. These two primary channels
sion and scour of vegetated drainageways and
were characterized in 1967 by partial braiding
gullies can also entrain sediment containing WP
and a divergent pattern characteristic of an allu-
and transport it in suspension or as bedload. In
vial fan (Fig. 7b). Sediments in the fan area are
addition, pond and mudflat sediments can freeze
relatively featureless overbank materials depos-
to the base of the ice cover during its growth, and
ited by Eagle River flooding. Only the center of
subsequently be rafted during tidal inundation.
the Racine Island area (Fig. 6) is characterized by
Bathymetric profiling offshore of ERF shows no
intertidal ponds and an abandoned gully. The
evidence of deposition of ERF sediments as a delta
westerly channel was abandoned in September
at the mouth of the Eagle River. WP entering
1995, during flooding of the Eagle River as chan-
Knik Arm is potentially diffused by tidal and
nel scour shifted the easterly channel to the north-
river currents, but there are several sites where
east.
WP-contaminated sediment might be deposited
The northern two-thirds of ERF has landforms
(Lawson et al. 1996). These sites include intertidal
typical of tidal flats near river mouths (e.g.,
bars and nearshore deposits where waterfowl may
Ovenshine et al. 1976b) that consist mainly of
feed.
levees, vegetated marshes and abandoned chan-
Previous investigations suggest that natural at-
nels and point bars. Significant changes in the
tenuation of WP contamination is possible, but
relatively tight meander loops in this lower sec-
that further work is needed to assess the length of
tion of the river have also taken place over the last
time over which this may happen and to identify
40 years. Channel changes are a natural progres-
specific locations where natural attenuation is the
sion resulting from the erosion and recession of
remedial method of choice. The relative impor-
the outer banks of meander bends, deposition of
tance of erosion and deposition processes varies
sediments as point bars in the inner parts of each
from area to area in response to tidal and river
bend, and a general downstream migration of the
hydrology; however, changes in the system over
channel (e.g., Allen 1982). Meander scars, aban-
seasons, years and decades are possible.
doned meander loops, and point bar deposits are
Pond sedimentation rates appear to be suffi-
common along the length of the active channel
ciently high in some areas to bury WP, thereby
and are evidence of such changes in the past.
reducing the exposure risk for waterfowl natu-
Tributary channels with dendritic patterns,
rally. The drainage system is also changing as
herein called vegetated drainageways, drain wa-
tidal gullies expand across mudflats into ponds.
ter from the mudflats and ponds into the tidal
Certain ponds are likely to drain in 20 years or
gullies (Fig. 3 and 8). A second set of channels,
less. Drier conditions will permit in-situ degrada-
intercepted by the active tributary system,
tion and natural remediation of WP contamina-
unconformably crosses other landforms, includ-
tion. Erosion will, however, continue to release
ing ponds (such as C and Bread Truck). Their
WP from mudflat and pond sediments over time,
pattern is irregular and unrelated to the active
while ice and water transport mechanisms will
gully drainage system. These secondary channels
move WP into tidal gullies, the Eagle River and
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