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2015 Puget Sound Factbook Book | v3.0
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 m 3
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
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).
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