decrease. This reduces the sediment available, de-
(~2 cm/year; Brown et al. 1977) and now may be
spite lower water temperatures that increase its
in the range of about 1.0 mm/year (Savage and
density, transport capacity and potential for move-
Plafker 1991).
ment of sediment into and within ERF.
Long-term responses of the physical system to
Peak tidal flood heights across ERF, in the Eagle
these changes in elevation, and hence base level
River and at the coast are delayed 20 to 40 min-
relative to sea level, are unknown. An increase in
utes relative to the peak predicted at Anchorage
flooding volume and duration may lead to an
(Lawson et al. 1996). The height is also generally
increase in pond sedimentation (e.g., Ovenshine
0.5 m or more greater than the Anchorage datum,
et al. 1976a); however, the increased tidal prism
reflecting, at least in part, the funneling of water
(depth and volume) over the Flats might provide
out of Cook Inlet into the narrower Knik Arm
more energy for erosion as water is funneled into
(i.e., Syvitski et al. 1987). Tidal flooding of the
gullies during ebb tide. These responses remain
Bread Truck, C, A and Racine Island areas may be
under investigation.
Tidal dynamics and river hydrology control
the amount of sediment that enters and drains
Table 3. Transport processes.
from ERF, as well as the locations and rates of
Summer
Winter
erosion and sedimentation. The amount of sedi-
ment transported in tidal flood waters is also a
Currents (river)
Ice floes
suspended load
freeze-on
primary factor determining scour and sediment
bedload
freeze-in
transport during ebb (Tables 2 and 3). Flooding
saltation
ice shove
tides transport large volumes of sediment onto
Wind currents (pondmudflats)
Frazil ice
the Flats during the summer, but the volume de-
suspended load
anchor ice
sedload
freeze-on
creases substantially during the fall (Lawson et
Gravitational slope processes
freeze-in
al. 1996). Transport changes seasonally as glacial
Currents (tidalgully)
input to Knik Arm decreases at freezeup and am-
flood
bient air temperatures drop. During winter
ebb
Ground water
months, sediment and water discharge in the Knik,
piping
Matanuska, Susitna and Eagle river catchments
Table 4. Depositional processes.
Morphological
unit
Summer
Winter
Ponds
Suspension sedimentation
Ice entrapment and in-situ melting
settling-out
vegetation trapping
Gullies
Suspension sedimentation
Ice growth entrapment and in-situ melting
settling-out
Ice cover confined settling out
Bedload deposition
Sediment gravity flows
Slumping
Mudflats
Suspension sedimentation
Ice freeze-on and in-situ melting
settling-out
Snow filtering and in-situ melting
vegetation trapping
Ice growth entrapment and in-situ melting
Ice cover confined suspension settling
Marshes
Suspension sedimentation
vegetation trapping
Levees
Suspension sedimentation
Ice freeze-on and in-situ melting
settling-out
(sedimentorganics)
vegetation trapping
Snow filtering and in-situ melting
Ice growth entrapment and in-situ melting
Ice cover confined suspension settling out
6
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