Cook Inlet is a large tide-dominated estuary in southern Alaska. Highly turbid streams enter the upper inlet, providing an excellent tracer for circulation in the lower inlet. MSS 4 and 5 images both can be used in this area to plot sediment and pollutant trajectories, areas of (probable) commercial fish concentration, and the entire circulation regime

Burbank, D. C.

Sharma, G. D.

Wright, F. F.

English

NASA Technical Reports Server

Paper M 8 
ERTS-1 OBSERVATIONS OF SEA SURFACE CIRCULATION AND SEDIMENT 
TRANSPORT, COOK INLET, ALASKA 
F. F. Wright, G. D. Sharma and D. C. Burbank, Institute of Marine Science, University 
of Alaska, Fairbanks, Alaska 
ABSTRACT 
Cook Inlet is a large tide-dominated estuary in southern Alaska. 
Highly turbid streams enter the upper inlet, providing an excellent tracer 
for circulation in the lower inlet. MSS 4 and 5 images both can be used 
in this area to plot sediment and pollutant trajectories, areas of (probable) 
commercial fish concentration, and the entire circulation regime, 
INTRODUCTION 
Cook Inlet is one o€ the largest estuaries in  North America. It extends 
almost 200 nautical miles (360 Km) northeastward from the Gulf of Alaska and 
is over 30 nautical miles (54-Km) wide through much of i ts  extent (Figure 1). 
Depths are typically less than 20 fathoms (40 m). The hinterland is predomin- 
antly mountainous and glaciation is still extensive . Deglaciation is occurring 
around the upper inlet and tremendous quantities of 'glacial flour' (largely 
si l t  and clay-sized mineral grains) enter the inlet. Cook Inlet lies a t  about 
latitude 60° North, and the climate could be regarded a s  subarctic. There is, 
however, strong maritime influence and temperatures are never low enough to 
completely inhibit runoff. Circulation of the inlet's waters is dominated by 
tidal effects. In the upper inlet, tidal ranges greater than 30 feet (10 m) are 
not uncommon, and currents in  excess of eight knots (14 KPH) are often 
encountered. Despite i t s  size and extreme tidal and climatic influences, Cook 
Inlet appears to behave a s  a model high latitude estuary, and a n  understanding 
of the regime here should be useful in all estuarine circulation and mixing 
problem areas. 
ERTS imagery is particularly well suited for the study of Cook Inlet 
circulation. The highly turbid runoff waters can very readily be distinguished 
from clean sea water on both MSS Bands 4 and 5. This information, of course, 
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Original photography may be  ha^^$ 
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1 
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https://ntrs.nasa.gov/search.jsp?R=19730019615 2020-03-17T08:51:53+00:00Z
FIGURE' 1. Cook Inlet, Alaska, showing bathymetry and major tributary rivers 
(from Sharma and Burrell, 1970). 
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Page 2 
could also be obtained from conventional aerial photography, but the size 
of the inlet and the speed of the tidally-controlled mixing processes make 
it virtually impossible to obtain a truly synoptic survey. Standard oceano- 
graphic procedures , limited by vessel and sampling speeds, are hopeless to 
obtain an instantaneous view of the inlet. Even ERTS requires two frames to 
cover the entire inlet, but these do give us a true picture, clearly defined 
by the turbid fresh waters , of the instantaneous circulation pattern. 
Circulation in Cook Inlet is of far more than academic interest. By 
coincidence, the majority of the turbid waters originate from Knik Arm in the 
vicinity of the city of Anchorage, Alaska's population and industrial center. 
Offshore petroleum production also is largely confined to the upper inlet. 
Thus tracing the turbid waters permits us to follow in detail the trajectory 
of potential pollutants from these sources. As well, Cook Inlet supports a 
major salmon fishery. Migrations of the salmon are known to be related to 
water movements but little systematic study has been made of this in the 
inlet area. Significant fisheries for herring, shrimp, tanner and king crab 
are found in the normal sea waters of the lower inlet. An understanding of 
the circulation patterns will be important to both environmental protection 
and resource developmknt and management in the entire Cook Inlet area. 
DIRECT OBSERVATIONS OF COOK INLET CIRCULATION 
Although Cook Inlet has been important a s  a gateway to the Alaskan 
interior since Russian times , there was relatively little navigation or systematic 
study of the area until quite recently. Mariners, in fact, avoided the inlet as 
much as  possible because of i ts  dangerous waters. The Alaska Railroad, for 
example, circles the head of Cook Inlet, but i t s  major termini were a t  Seward 
and Whittier on the Gulf of Alaska. Anchorage, now the population and in- 
dustrial center of the state, developed during and after World War I1 as a 
staging point between the coast and the interior. It was not until successful 
petroleum development in  the 1950's that there was significant regular commerce 
in the upper inlet. Aside from basic tide height and current observations, and 
rather superficial bathymetric survey, oceanographic studies of Cook Inlet were 
non-existant until required for petroleum related engineering projects . The 
first large-scale studies of hydrography, sedimentation, and chemistry were 
conducted by the University of Alaska in  the later 1960's (Kinney, Groves and 
Button, 1970; Sharma and Burrell, 1970). These data, with much additional 
biological and engineering information, have recently been compiled by Evans 
and others (1972). 
In cooperation with the ERTS program, the University of Alaska is presently 
conducting an extensive series of cruises in Cook Inlet, These cruises have 
been scheduled to coincide with ERTS orbits over the inlet, but the only 
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useable imagery obtained t 
observations . One cruise eptember 1972) was conducted with the 
University's major vessel, 
data a s  well as suspended 
as collected when there were no surface 
During this cruise , complete hydrographic 
nt values were obtained for the entire water 
Additional cruises h en  
based on Homer in the lower inlet, 
striated to the surface w 
) permits a much.larger area to but her speed (over four times that of 
be covered while the satellite is actually overhead. 
Sea surface observations ("ground truth") obtained during the Cook Inlet 
cruises have included measurement of water temperatures , salinities and 
suspended sediment load (Figure 2). These data, though clearly not 'synoptic' 
in  the normal oceanographic sense, give us an understanding of the general water 
types present in the inlet. It has been found that, during the active runoff 
season, the water types can be easily characterized,, Runoff tends to be low 
in  salinity and very, very high in  suspended load. Values greater than 100 mg/l 
suspended material characterize the upper inlet; the highest value reported was 
over 1700 mg/l from the Anchorage area, Sea water, of course, tended to have 
salinities ranging downwards from 32 o/oo and was relatively transparent, 
Water temperature shows a very decided seasonal effect, Sea water tended 
to be 10-12°C regardless of se on, while runoff ranged from warm 
sea in later summer (12-16oC) considerably colder in the winter 
(4-8' Q). Suspended sediment concent tions greater than 2-4 mg/l have 
proven to be the mbst reliable criterion for rapidly distinguishing the two 
water masses of the inlet. 
ERTS - 1  INIAGERY OF COOK INLET 
The normally cloudy conditions of the Alask n cbast have severely 
interfered with ERT8 observations of Cook Inlet, Only on one occasion in  
1972 were useable imag 
compression of orbital t 
successive days, 3-4 November, Th these data, particu- 
larly the MSS 4 and MSS 5 Bands, are extremely good, and several grey shades 
within the turbid waters can be distinguished with the naked eye. These 
gradations appear to be correlable with relative concentrations of suspended 
material in the surface water layers, There are indications that turbid waters 
somewhat below the actual water surface can be detected by comparison of 
the two bands. Unfortunately, these subtle distinctions are not found on the 
enlarged prints. 
to the high latitude 
entire inlet on two 
A comparis~n of the s u ~ ~ e n d e ~  load informat~on from the Septemb~r crui 
and the November ERTS image (Figures 2 and 3)  makes clear the utility of the 
ERTS observations. The h y ~ r o g r ~ p ~ ~  data show a somewhat confused pattern 
of suspended sediment d i s ~ ~ u t i o n ,  reflecting the fact that it required five 
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154" 1 5 3 O  152" 150" 
FIGURE 2. Suspended sediments in  the surface waters of Cook Inlet, September 
1972.  Area shown in Figure 3 is outlined. 
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FIGURF: 3 .  ERTS Image of Lower Cook Inlet, MSS 4 Band, November 4, 1972.  
1320 
days to collect all  the stations shown. The ERTS image, on the other hand, 
reflects accurately the conditions at one instant. It was just after low slack 
water in  the lower inlet, and the flood is commencing. A very distinct mass 
of clear sea water is seen advancing up .the eastern side of the inlet, extending 
then almost as far north as Cape Ninilchik, while turbid waters dominate the 
surface to the west. This pattern very clearly reflects the influence of Coriolis 
effect upon the circulation of Cook Inlet. This is the tendency of moving 
particles to be deflected by the earth's rotation toward the right in the northern 
hemisphere. In the ERTS image of Cook Inlet we see both the incoming and 
outflowing currents controlled by this effect. This picture is corroborated by 
numerical models of inlet circulation (Matthews and Mungall, 1972). 
APPLICATION OF ERTS - 1 DATA I N  COOK INLET 
An understanding of the circulation regime is essential to the systematic 
protection and development of the resources of the Cook Inlet region. The 
local salmon fishery presents a good example of the relation of the fauna to 
the environment (Evans et a&, 1972). The fish routinely enter the inlet on 
their spawning run and proceed with the tide, dropping to the bottom to rest 
when the tide sets  against them. Apparently, also, they tend to mass for a 
time near the boundary between the clear and turbid waters before they make 
their final run to the spawning streams. The non-anadromous fisheries of the 
lower inlet -- including commercial operations for herring, shrimp, and crab, 
and sports fishing for halibut -- are, of course, limited by the extent of rela- 
tively normal sea water. ERTS imagery can locate the water mass boundary 
with precision and, with repetitive coverage, we should ultimately be able to 
predict its location a t  any stage of tide or runoff. This boundary is also 
important as  a factor in navigation. An extensive "trash line" of floating logs 
and other terrestrial debris may accumulate there and create a genuine hazard 
to navigation. As  well, emphemeral but often nasty tide rips and whirlpools 
may develop along the boundary. 
The major input of turbid waters into the Inlet is actually from the Knik and 
Matanuska Rivers, both tributary to the Knik Arm above the city of Anchorage 
(Sharma, in progress). This coincidence permits the ERTS imagery to be used 
in another way. Effectively all non-solid sewage and industrial waste from 
the city is discharged directly into the inlet where i t s  trajectory and sites of 
dispersion can easily be followed by tracing the turbid waters. Most of the 
current petroleum development is also limited to the upper inlet, so an under- 
standing of the circulation will permit prediction of the behavior of any spill. 
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As yet, too few satellite observations of Cook Inlet circulation have 
been obtained to permit more than the most superficial conclusions about the 
regime . Both additional satellite and surface observations will be required 
to generate an integrated picture, however, the potential utility of the 
satellite system in this environment cannot be doubted. The extremely turbid 
inflow at the head of the inlet provides us with a unique tool to study all 
aspects of the circulation of Cook Inlet. 
REFERENCES 
Evans, Charles D., Eugene Buck, Richard Buffler, Greg Fisk, Robert Forbes, 
and Walt Parker, 1972, The Cook Inlet Environment, A Background Study 
of Available Knowledge: Report to U S . Army Corps of Engineers, Alaska 
District, Contract DAcW85-72-C-0052, Resource and Science Service 
Center, Alaska Sea Grant Program, University of Alaska, Anchorage, 128 p. 
Kinney, P. J., J. Groves, and D.K. Button, 1970, Cook Inlet Environment 
Data: Report #70-2, Institute of Marine Science, University of Alaska, 
120 p. 
Matthews, J.B., and J.C.H. Mungall, 1972, A Numerical Tidal Model and its 
Application to Cook Inlet, Alaska: Journal of Marine Research, V. 30 (11, 
p. 27-38. 
Shanna, G. D, , and D . C . Burrell, 19 70, Sedimentary Environment and Sediments 
of Cook Inlet, Alaska: Bulletin, American Assoc. of Petroleum Geologists 8 
V. 54 (41, P. 647-654. 
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ERTS-1 observations of sea surface circulation and sediment transport, Cook Inlet, Alaska

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