sediment during the fall through early spring,
doned point bars are common along the length of
when glacial discharge is limited primarily to
the active channel and are especially evident in
ground water sources (e.g., Lawson 1993). Samples
photographs of the southern reaches of the Flats
from along the coast in Knik Arm in January,
(Fig. 5c). Channel changes are a natural progres-
March and November of 1994 indicated that sedi-
sion resulting from the erosion and recession of
ment remains in suspension through the winter,
the outer banks of meander bends, deposition of
but in lesser quantities than during the summer.
sediments as point bars in the inner parts of each
The continued presence of suspended sediment
bend, and a general downstream migration of the
during winter is important, because it is the only
channel (e.g., Allen 1982). At some locations, a
source available for deposition in ERF during that
breaching of tight meander loops causes their
time.
abandonment.
Expanding the above discussion, we note that
Channel changes remobilize bank sediments,
aerial photographs from 1950, 1967 and 1993 re-
which may be transported either downstream or
veal changes in the channel pattern entering ERF.
upstream, depending on the timing of collapse.
Two main channels entered the Flats in 1950, but
In the idealized river model, they would move to
by 1967 the southern channel was abandoned and
the next point bar and be redeposited; however,
the northern channel diverged northwest of the
redeposition will be controlled by the interaction
Route Bravo bridge. The two primary channels
among tidal and river processes, particle grain
entering the Flats today are partially braided and
size, and water properties affecting density (e.g.,
have a divergent pattern characteristic of an allu-
temperature and salinity) and flocculation. Sedi-
vial fan (Fig. 5c). Both characteristics indicate a
ments may therefore be redeposited in the ponds
significant decrease in channel gradient near the
and mudflats during flooding tides, or they may
bridge, leading to a reduction in carrying capac-
be transported towards the Knik Arm during the
ity and the deposition of sediments in transport,
ebb cycle.
as well as a fundamental change in the channel
Gullies in the mudflats have also lengthened
configuration (e.g., Leopold et al. 1964). In addi-
and deepened during the last 40 years, in some
tion, the 1993 image shows a well-defined change
cases extending over 200 to 300 m. A significant
in surface texture where these channels merge
extension of gullies toward the ponds and marshes
into a single meandering channel (Fig. 5c). This
is particularly evident in Figures 5b (1967) and 5c
textural change probably bespeaks a change in
(1993). Headwalls recede during flood or ebb flow,
physical characteristics such as elevation, gradi-
as water is funneled into and out of the ponds
ent or the grain size and mineralogy of the sub-
and mudflats through the drainage system. The
strate materials.
cause of gully expansion is not proven. But the
In addition, landforms in this part of ERF are
tectonic activity during and following the 1964
mainly levees, vegetated marshes and abandoned
Alaskan earthquake or large-scale river channel
channels and point bars, except in the center of
changes caused by flooding can cause the gully
the Racine Island area (Fig. 5c). Here, an aban-
expansion. We must further quantitatively ana-
doned gully within the center of an intertidal pond
lyze historical aerial photographs and deposits in
exists, while surrounding it, the deposits are rela-
the alluvial landforms to find its cause.
tively featureless and probably consist of overbank
Tributary channels with dendritic patterns,
deposits from Eagle River flooding. In contrast,
called vegetated drainageways, drain water from
the northern two thirds of ERF have landforms
the mudflats and ponds into the gullies (Fig. 6). A
more typical of tidal flats near river mouths else-
second set of channels intercepted by the active
where in this region (e.g., Ovenshine et al. 1976b).
tributary system unconformably crosses other
Both the tributary channel and the meander-
landforms, including ponds (such as C and Bread
ing main channel deepen downstream, becoming
Truck) where they lie below the pond water sur-
incised by over 5 m. Near-vertical scarps and evi-
face. Their pattern is irregular and unrelated to
dence of recent slumping characterize river banks
the active gully drainage system. We interpreted
that are actively eroding. Aerial photographic
these secondary channels as relict drainages that
analyses reveal that, historically, banks have re-
reveal significant changes to the ERF drainage
ceded several tens of meters at some locations.
system in the past. The cause of such change is
The relatively tight meander loops in the lower
unknown, but may also be related to larger scale
section of the river have significantly changed
forces such as river avulsion, flood-induced chan-
over the last 40 years. Meander scars and aban-
nel migration, or earthquake-induced subsidence.
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