5 OCEANOGRAPHY
tide. The U.S. Army Corps of Engineers excavates
5.1 Bathymetry
about 172,000 m3 of silt from the Port's maneuvering
The bottom topography of Cook Inlet is, in gen-
area each year to maintain 35 ft MLLW level
eral, extremely rugged, with dramatic changes in el-
(USACE 1993).
evation. It has many relatively deep locations adja-
Between the Forelands the water reaches a depth
cent to shoals (Fig. 16). However, the depth of the
of 124 m (68 fathoms). The channels to the east and
Inlet is generally less than 73 m (40 fathoms). Near
west of Kalgin Island attain depths of 73 and 121 m
the mouth of the Inlet, the bottom drops away steeply
(40 and 66 fathoms), respectively. Similar depths are
to a general depth of 183366 m (100200 fathoms)
not found elsewhere north of the 50-fathom isoline
near the Barren Islands.
(NOS 1997). South of the 50-fathom contour, the bot-
Significant portions of Knik and Turnagain Arms
tom drops away steadily such that the three entrances
are exposed at low tide. Several extensive tidal flats
to Cook Inlet have depths exceeding 183 m (100 fath-
are located in the Head and Upper Inlet (Fig. 17). The
oms).
largest of these are the Susitna Flats, near the mouth
of the Susitna River; Potter Flats, south of the city of
Anchorage; Chickaloon Flats, east of Point Posses-
5.2 Currents and tides
Throughout the year the strong Alaska Current
sion; and Palmer Hay Flats, at the northern end of Knik
trends northwesterly along the Canadian and Alaskan
Arm.
coastlines, bringing southern waters to the Cook Inlet
Of the many rivers that discharge into the Inlet, three
region. This marine water enters the Inlet via the
contribute about 70% of its total freshwater input.
Stevenson and Kennedy Entrances and mixes to vary-
These are the Knik, Matanuska, and Susitna Rivers,
ing degrees, depending on the season and meteoro-
and all find their way into the Upper Inlet via the
logical conditions, with the fresh water draining from
Susitna Lowland. These rivers are the outwash drain-
the land and mountains surrounding the Inlet. The in-
ages for large glaciers, and consequently each trans-
coming ocean water is relatively clear and saline com-
ports a heavy burden of sediment, especially during
pared to the outflowing surface drainage. During the
the summer months, when discharge volume is high.
summer months, melting snow and surface runoff
The surface waters draining into Knik Arm deposit a
cause a net freshwater outflow of more than 100,000
total of 20 million tons of sediment per year, while the
m3/s from the Inlet to the Gulf of Alaska. During the
annual sediment discharge into Turnagain Arm is only
winter months, low air temperatures and the produc-
about 3 million tons (Gatto 1976). Because of the vol-
tion of ice shrink the volume and velocity of river run-
ume of sediment delivered, the southern third of Knik
off substantially. The mean and extreme flow volumes
Arm, which serves as the shipping approach to the
for the three largest rivers emptying into the Upper
Port of Anchorage, is only about 1836 m (1020 fath-
Inlet are shown in Table 7. River sediment delivery
oms) deep mid-channel. At the entrance to Knik Arm
diminishes during the low-flow period, and the Inlet's
lies a stable shoal with a mean lower low water
outflow becomes equal to its inflow (Murphy et al.
(MLLW) depth of 8.5 m (28 ft) over which deep-draft
1972). This hydrologic exchange, superimposed on re-
traffic to and from Anchorage must pass during high
53
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