The ecology of eelgrass meadows in the Pacific Northwest: A ...

the area during a period of rising sea
level.
North of the Salmon River all rock
formations are Tertiary. Eocene
formations are widespread and include
volcanic (basalts with tuffs and breccias)
and sedimentary rocks (siltstones and
sandstones) (proctor et al. 1980b). In the
01 igocene 1 irnited sedimentary formations
developed (siltstones, shales,
sandstones). In the Miocene extensive
basalt flows occurred in northern Oregon
and southwestern Washington. During the
Pleistocene, extensive erosion occurred
during coastal uplifting.
It appears that the late Pliocene and
Pleistocene were characterized by
alternating coastal submergence and
uplift, with resulting deposition and
erosion, respectively. The Puget Sound
Basin-Willamette Trough was formed in the
late pliocene (ca. 3 million years ago)
when a general north-south uplift, forming
the Olympic and Cascade Mountains, was
accompanied by a downwarp between the two
ranges (Yoshinaka and Ellifrit 1974).
Present coastal con£ igurations and
sediments in estuaries to the north are
the result of the melting of lobes of the
Vashon glacier of the Wisconsin period
(17,000 to 9,01i70 years ago) (Wiedemann et
al. 1974; Yoshinaka and Ellifrit 1974;
Jefferson 1975). As the glaciers melted,
erosion occurred with bay deposits forming
beds, filling in all river mouths
(Jefferson 19751.
Coastal submergence
occurred again, creating the general
features of the present coastline. Even
though the glaciers did not extend south
of the olympic Mountains (~umler 1969),
the glacial melting did result in a
general rise in sea level 120 to 150 m
(400-500 ft) above its recent level
(Jefferson 1975).
1.2 CLIMATE OF REGION
The climate of the Pacific Northwest
coastal region is marine (Proctor et al.
1980b). ~t is characterized as a midlatitude,
west-~oast-m~ he type, with wet
winters and dry summers. The region is
located in the Center of the prevailing
westerlies, with local winds ranging from
northwest to southwest througlmut most of
the year. The winds exchange heat with
the ocean and become nearly saturated with
moisture from evaporation. The air rises
as it reaches land, cools, and gives up
considerable moisture. Thus the climate
is moderated by the Pacific ocean and
characterized by high rainfall and a
moderate temperature range. There is
little north-to-south variation in
temperature and only a slight increase in
precipitation to the north in the region.
Two semipermanent pressure patterns, the
Aleutian Low and the North Pacific high,
control the climate. The Aleutian Low,
consisting of a series of low pressure
centers, lies over southwest Alaska and
the Aleutian Islands. Winds blow
counterclockwise around the low, bringing
a series of wet onshore frontal storms
which are often characterized by strong
winds. This pressure system dominates the
winter weather from Octcber to March. The
low shifts southward during winter,
resulting in an increased frequency of
frontal storms reaching the coast. Eighty
percent of the annual precipitation falls
during this period able 1).
The North Pacific High dominates the
summer weather of the region. As summer
approaches, the high expands over the
eastern Pacific north of 2g0 N almost to
Alaska and westward to 16g0 E. Storms are
veered northward from the region by the
high. Only 5% of the annual precipitation
falls in July and August. Winds flow
clockwise and bring predominantly fair
weather. During this time coastal winds
are from the northwest and north.
Seasonal shifts in wind direction
influence nearshore ocean currents,
resulting in upwelling which brings cooler
water to the surface near slmre in summer.
'This produces fog, reduced insolation, and
lclwer air temperatures.
Precipitation in winter varies from a high
of 32 cm (13 inches) at Tatoosh Island in
northwestern Washington to 27 cm (11
inches) at North Bend in southwestern
Oregon to 18 cm (7 inches) at Eureka in
northern California, all in December
(Proctor et al. 1980b). In summer,
precipitation varies from a low of 5 cm (2
inches) in August at Tatoosh Island, to 2
cm (0.8 inch) in July at North Bend, to 0
at Eureka in July.