Puget Sound's physical environment

The Puget Sound ecosystem is shaped by its physical environment. This article looks at Puget Sound's geologic history as well as dynamic factors such as the flow of its rivers and currents.

Map image of Puget Sound and surroundings. Courtesy of USGS.
Map image of Puget Sound and surroundings. Courtesy of USGS.

Summary

Oceanographers define Puget Sound as the region of marine and brackish waters extending landward from Admiralty Inlet.1 It is part of the Salish Sea, a larger system of inland marine waters that includes the Strait of Georgia and the Strait of Juan de Fuca. The deep and complex troughs that make up Puget Sound were carved by glaciers, most recently about 10,000 years ago. The Sound has remarkable patterns of water circulation that support its thriving ecosystem, and which give rise to water quality problems such as hypoxia. The circulation patterns are a consequence of the shape of the Sound and the interaction of tides and rivers. 

Puget Sound is about 161 km in length, going from Admiralty Inlet to Olympia.  Long Island Sound, also carved by glaciers, is similar to Puget Sound at 182 km.  Because of their glacial origins these systems are sometimes called fjords, and have much in common with other high latitude estuaries in both hemispheres.  At lower latitudes the most common estuarine type is a drowned river valley, meaning that the estuarine channel was originally a river valley that has since been filled in by the ocean as sea level has risen about 120 m since the Last Glacial Maximum. We refer here to all such systems as estuaries, loosely defined as any bay or channel off of the ocean that is influenced by rivers and tides.2

Chesapeake Bay, the largest estuary on the East Coast, is an example of a drowned river valley.  The Chesapeake is about 322 km long, and San Francisco Bay, a West Coast drowned river valley is 97 km long.  The length of an estuarine channel can be a region where ocean and river water mix, creating a gradual salinity variation to which the biology must adapt.

Coastline

The coastline around Puget Sound is 2,143 km (1,332 miles) long. It would take about 18 days to walk the whole shoreline if it were passable—or legal—everywhere. Note: this distance refers to Puget Sound proper and does not include the San Juan Islands or the Strait of Juan de Fuca. 

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 km2, 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 km3. This is substantially larger than the Chesapeake Bay and Long Island Sound, which both have a volume of about 68 km3.  By this measure you could say that the Sound is the largest US estuary, 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 m3 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 km3 of water each high tide, removing it roughly 12.4 hours later. The tides are what cause the strongest currents in the Sound, peaking around 2.2 m s-1 in Admiralty Inlet, 3.4 m s-1 in Tacoma Narrows and over 3.8 m s-1 in Deception Pass.4

While tidal currents are quite apparent to boaters, their importance to Puget Sound water quality is primarily because of the turbulent mixing they cause.  In terms of the residence time of water in the Sound, the important currents are the persistent ones. Tidal currents mainly move water back and forth, over a distance called the tidal excursion.  The tidal excursion in Admiralty Inlet is about 20 km, and in Main Basin it is about 1.5 km.  However, if you put a current meter at any place in the Sound (or any other estuary) you will find that after averaging over many tidal periods the mean is not zero, but instead there is a persistent inflow of deep water and outflow of shallower water.  This pattern is called the “estuarine circulation” or the “exchange flow” and it is a characteristic of every estuary in the world.  In Puget Sound the estuarine circulation turns out to be very large, and exerts a profound influence on water properties.

Circulation

The strength of the estuarine circulation at Admiralty Inlet is estimated to be 20,000-30,000 m3 s-1, or about 20-30 times the total of all the rivers entering the Sound.  This flow comes in through the deeper part of Admiralty Inlet, and then spills down into Main Basin and Hood Canal.  At “hot spots” of tidal turbulence, like Tacoma Narrows, this dense ocean water is mixed with less dense river water, and the mixture rises to the surface.  This provides the energy to keep the exchange flow going throughout the year, pulling ocean water into the deep Sound and expelling slightly fresher surface water back to the Pacific.5

We can calculate the “residence time” of water in any of the basins of Puget Sound as the ratio of the basin volume to the volume transport of the exchange flow coming into the basin.  The result is that the average residence time in Puget Sound is about two months.  It is shorter, more like a month, in Whidbey Basin and South Sound.  Hood Canal has the longest residence time, 2-4 months.  This is primarily because tidal currents, and hence tidal mixing, are relatively weak in Hood Canal.  This residence time is long enough for biogeochemical processes to use up the dissolved oxygen in the deep water there, leading to a severe hypoxia problem almost every fall.6

The deep and shallow waters of the Sound are kept separate from each other by “stratification.”  The shallow waters tend to be fresher and warmer, and hence less dense, than the deep waters, and so the water forms horizontal layers.  Anyone swimming in the Sound or our local lakes will be familiar with a thin layer of warm water near the surface; this is an example of stratification.  The stratification in the Sound is created by the incoming branch of the exchange flow (which makes the deep water dense), and rivers and sunshine (which make the surface water less dense).  In Puget Sound the variation of density is mostly controlled by salinity.  Tidal mixing destroys the stratification, and indeed there is very little stratification near the energetic sills.  The actual density difference between surface and deep waters is surprisingly small, being about 0.5 kg m-3 in Main Basin.  This is just 0.05% of the density of seawater, but it is enough to resist tidal mixing, which effectively isolates the deep water from the surface. Hood Canal, with weaker mixing, develops much stronger stratification, about 5 kg m-3, and Dana Passage, where mixing is intense, is more like 0.25 kg m-3. In addition to being colder and saltier and lower in oxygen, the deep waters have high concentrations of nutrients such as nitrate.  It is the places and times where this deep water is brought to the surface that are especially favorable for phytoplankton blooms.  Stratified waters can support “internal waves” which are wave-like undulations of the density surfaces.  These waves can routinely be 50 m high and several km long in the Sound.  Sometimes from a boat or plane you can see the subtle surface signature of these underwater giants as lines of alternating smooth and rough water. These are where the horizontal convergence of the internal wave velocity field near the surface has concentrated or excluded small wind waves.7


Footnotes

1. The facts in this article refer to this definition of Puget Sound, not the Puget Sound watershed or region as defined
by Water Resource Inventory Areas. See the Geographic Boundaries section of the Puget Sound Fact Book for more information.

2. Data regarding the shape, area, and depth of Puget Sound are nicely summarized in Ebbesmeyer et al. (1988),
relying in part on McLellan (1954). The author confirmed many of the numbers using more modern bathymetry from
Finlayson (2005).

3. Banas et al. (2015) calculates how different rivers influence different parts of the Sound.

4. By far the best references on tides in the Sound and Salish Sea are the excellent NOAA reports by Mofjeld and
Larsen (1984) and Lavelle et al. (1988).

5. Observations of the exchange flow at Admiralty Inlet are given in Geyer and Cannon (1984), and observations of
tidal mixing there are reported in Seim and Gregg (1994).

6. The exchange flow and residence times are estimated in Cokelet et al. (1991), Babson et al. (2006), and Sutherland et
al. (2011).

7. Stratification numbers were estimated by the author using observations from the Washington State Department of
Ecology. Data may be downloaded from http://www.ecy.wa.gov/apps/eap/marinewq/mwdataset.asp.

References

Babson, A. L., M. Kawase, P. MacCready (2006): Seasonal and interannual variability in the circulation of Puget Sound, Washington: A box model study.  Atmosphere-Oceans, 44, 29-45.

Banas, N. S., L. Conway-Cranos, D. A. Sutherland, P. MacCready, P. Kiffney, and M. Plummer (2015) Patterns of River Influence and Connectivity Among Subbasins of Puget Sound, with Application to Bacterial and Nutrient Loading. Estuaries and Coasts, 38(3), 735-753, DOI 10.1007/s12237-014-9853-y.

Cokelet, E. D., R. J. Stewart, and C. C. Ebbesmeyer (1991): Concentrations and ages of conservative pollutants in Puget Sound. Puget Sound Research '91, Vol. 1, Puget Sound Water Quality Authority, 99-108.

Ebbesmeyer, C. C., J. Q. Word, and C. A. Barnes (1988): Puget Sound: a fjord system homogenized with water recycled over sills by tidal mixing. Hydrodynamics of Estuaries: II Estuarine Case Studies, B. Kjerfve, Ed., CRC Press, 17-30.

Finlayson, D. P. (2005): Combined bathymetry and topography of the Puget Lowland, Washington State. University of Washington, (http://www.ocean.washington.edu/data/pugetsound/)

Geyer, W. R. and G. A. Cannon (1982): Sill processes related to deep water renewal in a fjord. J. Geophys. Res., 87, 7985-7996.

Lavelle, J. W., H. O. Mofjeld, E. Lempriere-Doggett, G. A. Cannon, D. J. Pashinski, E. D. Cokelet, L. Lytle, and S. Gill (1988): A multiply-connected channel model of tides and tidal currents in Puget Sound, Washington and a comparison with updated observations. NOAA Tech. Memo. ERL PMEL-84, Pacific Marine Environmental Laboratory, NOAA.

McLellan, P. M. (1954): An area and volume study of Puget Sound. UW Dept. Of Oceanography Tech. Report, 21, 39.

Mofjeld, H. O. and L. H. Larsen (1984): Tides and Tidal Currents of the Inland Waters of Western Washington. NOAA Tech. Memo. ERL PMEL-56, Pacific Marine Environmental Laboratory, NOAA.

Seim, H. E. and M. C. Gregg (1994): Detailed observations of a naturally occurring shear instability. J. Geophys. Res., 99, 10 049-10 073.

Sutherland, D. A., P. MacCready, N. S. Banas, and L. F. Smedstad (2011) A Model Study of the Salish Sea Estuarine Circulation.  J. Phys. Oceanogr., 41, 1125-1143.

About the Author: 
Parker MacCready is a professor at the University of Washington School of Oceanography and an editor at the Encyclopedia of Puget Sound.