About the Puget Sound Institute

Physical environment
Depth
Because of its glacial origins the Sound is deep, averaging 70 m, compared to an average of just
6 m for the shallow, muddy Chesapeake. The deepest spot in the Sound, offshore of Point
Jefferson in Main Basin, is 286 m. If the tallest building in Seattle, the Columbia Center, had
been built on this spot just 1 m would be visible above the water’s surface at low tide. Puget
Sound is deep by estuarine standards, but if we look north into the Strait of Georgia we can find
waters up to 650 m.
Surface area
The surface area of the Sound is about 2,632 km 2 , although this number varies a bit depending
on whether the tide is high or low. If every resident of Seattle was in their own boat on the
Sound, and the boats were spread out evenly, there would be about 60 m between each of them.
Volume
The volume of water in Puget Sound is about 168 km 3 . This is substantially larger than the
Chesapeake Bay and Long Island Sound, which both have a volume of about 68 km 3 . By this
measure it could be argued that Puget Sound is the largest estuary in the continental United
States, but of course the whole Salish Sea is much bigger, and the separation of its parts is more
a matter of national boundaries than ecosystem function.
Rivers
The annual average river flow into the Sound is about 1,174 m 3 s -1 , and a third to a half of this
comes from the Skagit River flowing into Whidbey Basin. It would take about 5 years for all the
rivers flowing into the Sound to fill up its volume, which suggests, correctly, that rivers alone do
not play a dominant role in circulating water through the Sound. This is also apparent in the
salinity of the Sound, which averages about 28.5 parts per thousand, compared to about 34 for
the nearby Pacific. This means that the Sound is roughly 83% seawater. Even as far south as
Budd Inlet near Olympia it is still two-thirds seawater. The sum of rivers entering the
Chesapeake is about twice that of those entering Puget Sound, and they would fill the Bay in just
a year. Because of the stronger river forcing, and because it is shallower, the Chesapeake is
about 50% seawater, with salinity varying smoothly from oceanic to fresh over its length. 3
Tides
Tides in the Sound are large, with ranges between 3 and 4 m. The tides are forced by the tidal
variation of sea level at the mouth of the Salish Sea–the seaward end of the Strait of Juan de
Fuca. However the tidal range actually increases as you move landward, and the biggest tidal
range is at the extreme southward end. In addition high tide occurs about 1 to 2 hours later in
Olympia than it does at Admiralty Inlet. The tides bring in about 8 km 3 of water each high tide,
removing it roughly 12.4 hours later. The tides are what cause the strongest currents in the
3
Banas et al. (2015) calculates how different rivers influence different parts of the Sound.
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