The Puyallup River ( ) is a river in the U.S. state of Washington. About long, it is formed by glaciers on the west side of Mount Rainier. It flows generally northwest, emptying into Commencement Bay, part of Puget Sound. The river and its tributaries drain an area of about in Pierce County and southern King County.

The lower, northwestern half of the river's watershed is a complex amalgam of glacial and tectonic features dating back to the Pleistocene, as well as more recent (Holocene) changes caused by a series of lahars which flowed down from Mount Rainier between 5,600 and 800 years Before Present. The valley's 150,000 residents are at risk from future lahars. For this reason, the United States Geological Survey has installed a lahar warning system.

Course

The upper watershed of the Puyallup River flows off of Mount Rainier through the South Cascade Range of Washington towards the lowlands surrounding Puget Sound. The Puyallup River begins from two forks, the North Puyallup River and the South Puyallup River. The North Puyallup River flows from the toe of Puyallup Glacier, while the South Puyallup River flows from Tahoma Glacier. The two streams flow through the western part of Mount Rainier National Park, joining just outside the park boundary and forming the Puyallup River proper, which then flows generally north and northwest until it reaches the edge of the foothills near the town of Electron. A third significant tributary, the Mowich River, flows down from the Edmunds and North Mowich Glaciers, on the northwestern slopes of Mount Rainier, and joins the Puyallup River slightly more than 7.3 kilometers (4.5 miles) northwest of the fork. The river is dammed at Electron Diversion Dam shortly below the Mowich confluence. The dam diverts a portion of the Puyallup River into a long flume, which runs for several miles to Electron, where the water is passed through turbines in a hydroelectric powerhouse before being returned to the river. The Puyallup River passes through a steep and narrow gorge between Electron Dam and the powerhouse.

The main valley flows north from Electron along a lateral meltwater channel (a meltwater channel formed along the edge of a glacier) and into a larger subglacial meltwater channel (formed beneath the glacier), collecting the Carbon River on the way to Alderton and Sumner. and the Duwamish Valley continuing north along the purely glacial channel towards Seattle. At present, the Puyallup River follows the Lower Puyallup River Valley northwest past the cities of Puyallup and Fife, and through the Puyallup Indian Reservation, before emptying into Commencement Bay at the Port of Tacoma, part of the city of Tacoma.

Natural history

The upper watershed (upstream of Electron) of the Puyallup River was created by the advance of alpine glaciers on Mount Rainier, limiting its maximum possible age to approximately 500,000 years when Mount Rainier was first formed. It is characterized by deep glacial valleys carved through the low mountains and rugged foothills of Washington's South Cascade Range.

The lower watershed (downstream of Electron) is much younger than the upper watershed: a combination of glacial and tectonic features less than 20,000 years old as well as changes caused by the Osceola Mudflow and younger lahars between 5,600 years Before Present and today. The main valley was originally part of a much larger lateral meltwater channel network formed during the Vashon Glaciation, which drained large portions of the Puget Sound Area as well as most of Washington's Cascade Range south into the Chehalis River prior to the unblocking of the Strait of Juan de Fuca around 14,800 years Before Present. After the retreat of the Cordilleran Ice Sheet allowed Puget Sound to connect with the Pacific Ocean through the Strait, the channel network fragmented into a collection of large, independent river valleys draining into Puget Sound. The Puyallup River's portion consisted of small section of lateral meltwater channel between Lake Kapowsin and Orting, a primary subglacial channel running north from Orting through the Duwamish Valley to Lake Washington, and a channel fork which split off at Sumner to follow a tectonic fault associated with the Tacoma Fault Zone. As both sea level and land level recovered from the glaciation, the valley flooded and became a saltwater embayment of Puget Sound which extended through the Puyallup and Duwamish Valleys from Kenmore, Washington, on what is now Lake Washington, to Orting at the foothills of the Cascade Range.

See also

  • List of crossings of the Puyallup River
  • List of Washington rivers

References

  • Puyallup River Basin
  • Mount Rainier Volcano Lahar Warning System
  • US Corps of Engineers Water Management
  • NW River Forecast Center Puyallup @ Orting
  • NW River Forecast Center Puyallup @ Puyallup
  • NOAA Advanced Hydrologic Prediction Service Puyallup @ Puyallup
  • Trends in Streamflow and Comparisons with Instream Flows in the Lower Puyallup River Basin, Washington United States Geological Survey