Table 2. Dates of river ice break-up and freeze-up. (After NOS 1994, p. T-21.)
Ice Break-up
Ice Freeze-up
Years of
Place
Waters
Earliest
Average
Latest
Earliest
Average
Latest
Record
Susitna
Susitna River
4/12/41
May 1
5/10/46
10/19/33
Nov 1
11/14/36
1933-46
Kasilof
Kasilof River
3/27/41
Apr 13
4/29/46
11/13/45
Dec 3
12/24/48
1937-47
Kenai
Kenai River
3/18/52
Apr 2
4/14/51
11/23/51
Dec 10
12/26/37
1937-52
Anchorage
Ship Creek
2/16/44
Mar 29
4/17/42
11/10/35
Nov 24
12/10/36
1915-53
that the ice cover fails to increase in extent, even de-
lar, sediment-laden sails. Smith (2000) reported that
creasing in some years (LaBelle et al. 1983). Then the
brine ice cores melted from stamukhi contained a mean
ice extent and thickness both increase through late
salinity of only 1.07 ppt but a sediment concentration
January and February, reaching maximums by mid-
of 25 g/L, and more than 50% of the sediment par-
ticles by weight were greater than 63 m in diameter
February to early March.
During colder winters the ice may extend into the
(sand-sized).
Lower Inlet as far south as Anchor Point on the east
side and Cape Douglas on the west side. The thick-
3.1.4 Estuarine and river ice
ness of the ice pack varies between 0.5 and 2 m, ap-
A significant portion of Cook Inlet's ice is fresh-
parently limited by the residence time of ice floes in
water ice that forms in the rivers and estuaries, espe-
the Inlet. Blenkarn (1970) reported that most of the
cially in Knik and Turnagain Arms. Estuarine ice is
ice in Cook Inlet is fine- to medium-grained with a
similar in life cycle to sea ice, but it is significantly
bulk salinity of 46 ppt. He referred to previous stud-
stronger and tends to remain more securely in place
ies, which indicated that most of the ice found in Cook
as fast ice. In the Upper Inlet, where the wind and
Inlet is less than 30 days old, older ice having been
water stresses are greater, estuarine ice is commonly
transported out of the Inlet on the tides to the open
entrained in the moving pack ice, where it is more of a
ocean or having melted in transit. Based solely on
danger to vessels and shoreline structures because of
Anchorage freezing degree-days (FDDs) over 30 win-
its strength. River ice is generally unaffected by the
ters, Blenkarn calculated that sea ice could grow to a
tides, wind, and waves, at least until spring break-up.
maximum thickness of 0.66 m during the coldest 30-
At that time a considerable amount of river ice, pieces
day period. Utt and Turner (1992) conducted a similar
of which can sometimes be 2 m thick (Brower et al.
FDD analysis and found that the maximum ice thick-
1988), is discharged into the Inlet. Table 2 shows spring
ness for the coldest 30-day period between 1931 and
break-up and fall freeze-up data for several rivers in
1989 was 0.79 m.
the region.
Various rules of thumb for Cook Inlet ice condi-
tions have been published. For example, substantial
3.2 General marine ice environment
ice formation is said to begin after the Anchorage daily
The ice cover in Cook Inlet is seasonal, forming in
mean temperature drops below 20F (6.6C) for the
the fall and disappearing completely each spring. Typi-
season (Blenkarn 1970) or when the seawater tem-
cally formation first occurs in October but does not
perature cools to 1.1C (Poole and Hufford 1982).
cover a significantly large area until late November.
According to the NWS, ice will completely clear from
By December about half the Inlet area north of the
Cook Inlet approximately 21 days after the mean daily
Forelands is normally covered with new ice and pan-
air temperature in Anchorage has risen above 0C for
cake ice (up to 10 cm thick) and thin, first-year ice
the season (Schulz 1977b). However, the ice cover can
(3070 cm thick). Much of the ice cover is brash ice
vary significantly from year to year in terms of dates
that has been broken by tidal movement and further
of onset and clearing, thickness, concentration, and
thickened by compaction, fingering, rafting,
extent of coverage.
hummocking, and ridging (Fig. 9). It ranges in con-
Table 3 shows the variability in dates of onset and
centration primarily from open (1/10, or 10% ice cov-
clearance for the 17-year period ending with the 1985-
erage for a given area) to close pack (7/10 to 8/10).
86 season. The date of the first significant ice occur-
The area south of the Forelands is normally still ice
rence and the springtime date of ice clearance are de-
free in December. In late December or early January
fined as the first and last occurrences for the season,
of most years, a relatively warm period occurs, such
13
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