Puget Sound

Puget Sound is a complex estuarine system located on the northwest coast of Washington state. An estuary is a place where freshwater from rivers meets saltwater from the sea.

This system is part of the larger Salish Sea. The Salish Sea includes Puget Sound, the Strait of Georgia, and the Strait of Juan de Fuca. Puget Sound is the second-largest estuary in the United States. Only the Chesapeake Bay in Maryland and Virginia is larger. The Sound connects to the open Pacific Ocean through the Strait of Juan de Fuca. This connection happens through three main entrances. The largest is Admiralty Inlet. The other two are Deception Pass and the Swinomish Channel.

The geography of the Sound is defined by several deep basins. These basins are separated by shallower underwater ridges called sills. A sill is a submarine terminal moraine, which is a pile of debris left by a glacier. The four main basins are Hood Canal, Whidbey Basin, South Sound, and the Main Basin. The Main Basin is further divided into the Central Basin and Admiralty Inlet. Significant sills include the one at Admiralty Inlet and the one at the Tacoma Narrows. The Tacoma Narrows sill sits about 40 feet below the surface. These sills act as barriers that check the flow of water between the different basins.

Glacial activity shaped the landscape of the Sound millions of years ago. During the Vashon Glaciation, a massive ice sheet called the Puget Lobe covered the region. This ice was about 3,000 feet thick near Seattle. The ice reached almost to the current border between the U.S. and Canada. About 14,000 years ago, the ice began to retreat. As the glaciers melted, they eroded the land and created deep valleys. This process formed the fjord system of flooded glacial valleys we see today. The melting ice also left behind vast amounts of glacial till. This is a mixture of unsorted soil, gravel, and rocks.

As the ice retreated, several large lakes formed in the landscape. The first was Glacial Lake Russell, which stretched from Seattle to the Black Hills. Sediments from this lake created the Lawton Clay, a blue-gray clay. Later, Glacial Lake Bretz formed in the region. Eventually, the melting ice allowed marine waters to flood the lowlands. This created the modern Puget Sound. The weight of the massive ice sheets actually depressed the land. After the ice melted, the land began to rise again in a process called post-glacial rebound. Because the land rose at different rates than the sea level, the basin filled with both fresh and salt water.

The Sound is also shaped by tectonic forces. It sits part of the Cascadia subduction zone. This is a place where the Juan de Fuca Plate is being pushed under the North American Plate. This movement can cause earthquakes. A massive magnitude nine earthquake occurred on January 26, 1700. Today, smaller earthquakes still happen as oceanic rocks fracture under stress. The Seattle Fault and the Tacoma Fault also run through the region. These faults can cause the ground to buckle or shift.

The water in the Sound is a mix of salt and fresh water. Rivers from the Olympic and Cascade Mountains flow into the system. The mean annual river discharge is about 2,100 cubic feet per second. The tides in the Sound are of the mixed type. This means there are two high tides and two low tides each day. The tidal range increases as you move south. At Port Townsend, the difference between high and low water is about 14 feet. In Olympia, at the southern end, it increases to about 16 feet.

Many different species live in these waters. Marine plants like eelgrass and various types of kelp are very important. Bull kelp forms large canopies near the surface.

These plants provide habitat for animals like the harbor seal. Many fish species also rely on the Sound. Sockeye salmon are a notable example of the life found here.

The complex mix of deep basins, flowing tides, and mountain runoff makes Puget Sound a unique maritime environment.