dominated by the Eagle River. Landform types
that, historically, banks have receded several
and distribution differ here, being mainly levees
tens of meters at some locations.
and vegetated marsh floodplains. These riverine
Significant changes in the relatively tight me-
landforms overlie and cover older marshes, a
ander loops in the lower section of the river
part of which is apparently exposed in the center
have taken place over the last 40 years. Land-
of Racine Island (Fig. 13c). In the northern two-
forms, such as meander scars and abandoned
thirds of ERF, landforms are similar to tidal flats
point bars, are common along the length of the
near river mouths elsewhere in this region (e.g.,
active channel and are especially evident on
Ovenshine et al. 1976b).
photographs of the southern reaches of the Flats
Historical aerial photographs and ground ob-
(Fig. 13c). These landforms result from changes
servations also indicate that ERF is physically
in the channel location over time. Channel
changing over both short- and long-time scales.
changes are a natural progression resulting from
Large-scale changes resulting from riverine pro-
the erosion and recession of the outer banks of
cesses over the last 40 years include abandon-
meander bends, deposition of sediments as
ment of a primary channel within the southern
point bars within the inner parts of each bend,
part of the Flats, and channel migration of vari-
and a general downstream migration of the
ous amounts from near the Route Bravo bridge
channel (e.g., Allen 1982). At some locations, a
north to the river mouth at Knik Arm.
breaching of tight meander loops causes their
Aerial photographs from 1950, 1967 and 1993
abandonment (Fig. 13c).
reveal that the primary river channel (of three)
Erosion associated with channel changes
was abandoned sometime between 1950 and
remobilizes bank sediments, which may be
1967 (Fig. 13). By 1967, two channels diverged
transported either downstream or upstream, de-
from Eagle River north of the Route Bravo
pending upon the capacity of the river and tidal
bridge, the primary avenue of discharge now ap-
forces. In the idealized river model, they would
pearing to be the eastern course. Our initial re-
move to the next point bar and be redeposited.
view of these photographs does not reveal the
Redeposition will also be controlled by tidal and
cause of this change; however, it is clearly impor-
river processes and by the materials' fine grain
tant to understanding the hydrology and sedi-
size, and therefore they may also be redeposited
mentology of the Flats.
within the ponds and mudflats, as well as with-
The two primary channels entering the Flats
in Knik Arm.
today are partially braided and have a divergent
Gullies have also progressively lengthened
pattern characteristic of an alluvial fan (Fig. 13c).
and deepened during the last 40 years, in some
Both suggest that the channel gradient signifi-
cases extending over an estimated 200 to 300 m.
cantly decreases near the bridge, leading to a re-
A significant extension of gullies toward the
duction in carrying capacity, deposition of sedi-
ponds and marshes is particularly evident on
ments in transport, and a fundamental change in
photographs covering the period of 19671993
the channel configuration (e.g., Leopold et al.
(Fig. 13b and c), but the amount of recession
1964). In addition, the 1993 color infrared image
cannot yet be quantified. Recession is in re-
shows a well-defined change in texture and color
sponse to erosion during ebb flow from the
where these channels merge into a single mean-
ponds and mudflats into the gullies (Table 2).
dering channel (Fig. 13c). This textural change
Erosion measurements in 1992 indicated that
may indicate a physical change, such as in ele-
this recession is continuing at variable rates of
vation, or in the grain size and mineralogy of the
up to 5 to 10 m per year (Lawson and Brockett
substrate materials. The predominant types of
vegetation reflect these physical differences and
The cause of gully expansion is unclear, but it
thus the apparent change from river dominated
may be a response to external forces such as
to tidally dominated physical processes.
earthquake activity, including the 1964 Alaskan
Both the tributary channel and the meander-
earthquake when this area subsided about 0.6 m
ing main channel progressively deepen down-
(Small and Wharton 1969), the eustatic rise in
stream, essentially being incised over 5 m during
sea level, or large-scale river channel changes
low tide. Near-vertical scarps and evidence of re-
caused by flooding. Further quantitative analy-
cent slumping indicate that the river banks are
ses of historical aerial photographs and sedi-
actively eroding. A qualitative assessment of the
mentological analyses of deposits within the
1950, 1967 and 1993 aerial photography suggests
alluvial landforms are required to determine