Hydrology

The United States Geological Survey (USGS) defines Puget Sound as a bay with numerous channels and branches; more specifically, it is a fjord system of flooded glacial valleys. Puget Sound is part of a larger physiographic structure termed the Puget Trough, which is a physiographic section of the larger Pacific Border province, which in turn is part of the larger Pacific Mountain System.^[16]
Puget Sound is a large salt water estuary, or system of many estuaries, fed by highly seasonal freshwater from the Olympic and Cascade Mountain watersheds. The mean annual river discharge into Puget Sound is 41,000 cubic feet per second (1,200 m³/s), with a monthly average maximum of about 367,000 cubic feet per second (10,400 m³/s) and minimum of about 14,000 cubic feet per second (400 m³/s). Puget Sound's shoreline is 1,332 miles (2,144 km) long, encompassing a water area of 1,020 square miles (2,600 km²) and a total volume of 26.5 cubic miles (110 km³) at mean high water. The average volume of water flowing in and out of Puget Sound during each tide is 1.26 cubic miles (5.3 km³). The maximum tidal currents, in the range of 9 to 10 knots, occurs at Deception Pass.^[1]

Evening on Puget Sound by Edward S. Curtis, 1913

The size of Puget Sound's watershed is 12,138 sq mi (31,440 km²).^[2] "Northern Puget Sound" is frequently considered part of the Puget Sound watershed, which enlarges its size to 13,700 sq mi (35,000 km²).^[17] The USGS uses the name "Puget Sound" for its hydrologic unit subregion 1711, which includes areas draining to Puget Sound proper as well as the Strait of Juan de Fuca, the Strait of Georgia, and the Fraser River.^[18] Significant rivers that drain to "Northern Puget Sound" include the Nooksack, Dungeness, and Elwha Rivers. The Nooksack empties into Bellingham Bay, the Dungeness and Elwha into the Strait of Juan de Fuca. The Chilliwack River flows north to the Fraser River in Canada.
Tides in Puget Sound are of the mixed type with two high and two low tides each tidal day. These are called Higher High Water (HHW), Lower Low Water (LLW), Lower High Water (LHW), and Higher Low Water (HLW). The configuration of basins, sills, and interconnections cause the tidal range to increase within Puget Sound. The difference in height between the Higher High Water and the Lower Low Water averages about 8.3 feet (2.5 m) at Port Townsend on Admiralty Inlet, but increases to about 14.4 feet (4.4 m) at Olympia, the southern end of Puget Sound.^[1]
Puget Sound is generally accepted as the start of the Inside Passage.^[19][20]