Think of Puget Sound as a water-filled valley between the Olympic and Cascade mountains, carved by powerful glaciers that advanced and retreated during four different Ice Ages stretching back hundreds of thousands of years.
During those formative years measured in geologic time, the land mass that now is home to more than 3.6 million people was buried under ice 1.5 miles thick.
Puget Sound as we know it today was formed about 13,000 years ago at the end of the last Ice Age when an ice dam across the Strait of Juan de Fuca broke loose, allowing the marine waters of the Pacific Ocean to rush into Puget Sound and mix with the river waters from the two mountain ranges to form the treasured body of water named by British explorer Capt. George Vancouver in 1792.
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Viewed more closely, Puget Sound consists of four distinct, yet interconnected, basins: Hood Canal, the area south of the Tacoma Narrows, the central basin between Commencement Bay in Tacoma and Admiralty Inlet and the areas south of Skagit Bay around Whidbey Island. The basins are separated by underwater shelves, or sills, that average about 150 feet in depth. They act as water circulation barriers and pollution traps, reducing the flushing action between Puget Sound and the Pacific Ocean.
It takes the entire Puget Sound about five months to flush itself and receive a new supply of water. The flushing time for South Sound is about two months; for Budd Inlet, it's 2.4 days. However, anywhere from 50 percent to 70 percent of the water that flows out of Puget Sound on each outgoing tide - about 5 percent of all the water in Puget Sound - returns on the incoming tide.
The constant recycling of water means that pollution in the water is returned to Puget Sound, too.
PATH OF POLLUTION:
A particle of pollution dumped in Budd Inlet can take four to six months to leave South Sound and more than a year to make its way out to sea, according to research by the National Oceanic and Atmospheric Administration.
Heavier particles of pollution such as heavy metals and pesticides sink in the water and attach to the bottom sediments, creating toxic hot spots, especially in urban bays.
Of Puget Sound's 1.8 million acres of submerged land, about 5,700 acres is highly contaminated.
By the numbers
Puget Sound holds about 168.6 billion cubic feet of water, or about 45 trillion gallons.
The water surface of Puget Sound covers about
3,200 square miles, which is about five times the land surface of Thurston County.
Puget Sound has 2,354 miles of shoreline.
The deepest place is 930 feet north Seattle near Edmonds.
The shallowest spot between high and low tide is the bottom of Budd Inlet in Olympia - 6 feet.
The fastest current is 11.5 miles per hour at Deception Pass south of Anacortes.