Table 2. Summer erosional processes.
and minimum volume of hourly, daily and sea-
sonal discharges, the lag between precipitation
Morphological
and the resultant increase in runoff, and long-
unit
Processes
term trends in annual discharge of the basin (e.g.,
Ponds
Currents
Tidal currents
Gurnell and Clark 1987, Lawson 1993). Water
--wind
Debris impacts
--tidal
(e.g., logs)
quality parameters, including suspended solids,
Wind waves
show similar, but not directly related, variations
Wind currents
with time.
Ducks and other
The USGS recorded discharge from the Eagle
bottom-feeding
organisms
River between 1966 and 1981 (USGS 1981). Dur-
ing this period, daily discharge averaged 14.7
Gullies
Currents
m3/s. The maximum mean discharge during the
--tidal
--piping
--runoff
--sapping
height of the glacial melt season (JulyAugust)
Overland flow
Slope mechanisms
averaged 42.5 m3/s, with peaks ranging from
--sheet
--slump
about 65 to 76 m3/s. Rain-induced floods during
--rill
--block collapse
Wind waves
--sediment flow
peak glacial runoff exceeded 105 m3/s. Unfortu-
nately, a new station to record discharge of the
Mudflats
Currents
Wind waves
--wind
Debris impacts
Eagle River could not be installed in 1993, and we
--tidal
(e.g., logs)
therefore do not have current hydrologic records
Overland flow
Rain drop impact
for the Eagle River.
--sheet
We were, however, able to establish instru-
--rill
mentation in mid-August to measure basic water
Marshes
Currents (rare)
quality parameters, suspended sediment concen-
Levees
Currents
Debris impacts
trations and water level changes through Sep-
--tidal
Wind waves
tember. These data show diurnal variations in
--river
water height, conductivity and temperature typi-
whether the gullies are expanding in response to
cal of a glacially fed river (Fig. 15) (e.g., Lawson
such external forces.
1993). Sediment concentrations were generally
Tributary channels with a dendritic pattern
low from 25 August to 20 September, ranging
feed water into the gullies (Fig. 14). These shal-
from 23 to 275 mg/L and decreasing steadily
low, vegetated channels cross the mudflats and
through September. Additional water quality and
head in the ponds. A second set of channels that
sediment transport data will be acquired during
is intercepted by the active tributary system is
1994.
also common (Fig. 14). These channels, however,
unconformably cross other landforms, including
TIDAL HYDROLOGY AND INUNDATION
ponds (such as C and Bread Truck) where they lie
below the water surface. Their pattern is irregular
Erosional, transport and depositional process-
and unrelated to the dendritic channel and gully
es are complexly related to tide height, Eagle River
system. These secondary channels are probably
discharge, and the length of time of inundation.
relict drainage and indicate significant changes to
Although sufficiently high tides can flood all of
the drainage in the past. The causes of such
ERF, lower high tides may be supplemented by
changes are unknown, but may be related to larg-
the river's discharge and flood more area to a
er scale forces such as flood-induced channel mi-
greater depth than the predicted tide height indi-
gration or earthquake-induced subsidence.
cates. During spring snowmelt runoff, precipita-
tion-induced flooding or other high discharge
events, tidal waters moving up the channel block
GLACIER RUNOFF AND SEDIMENT YIELD
the flow of the river water, causing it to slow and
Runoff from the Eagle River is characteristic of
pool, and eventually can reverse flow within the
a watershed occupied partly by glaciers. Glaciers
Flats proper. Some overflow of the tidal water by
cover about 13% of the watershed area, yet it
fresh river water may also take place. The flood-
takes only a few percent glacier cover to signifi-
ed area is increased and runoff during ebb is pro-
cantly modify the runoff and sediment yield of a
longed. Similarly, increased flooding can result
drainage basin (e.g., Lawson 1993). Glaciers mod-
from storm-driven waters moving into the Flats
ify peak discharges, the timing and maximum
from Knik Arm and Cook Inlet.
Pond sedimentation results mainly from set
16
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