The Puyallup River ( pew-AL-əp) is a river in the U.S. state of Washington. About 45 miles (72 km) 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 948 square miles (2,460 km2) in Pierce County and southern King County.The river's watershed is the youngest in the Puget Sound region, having been formed from a series of lahars starting about 5,600 years ago. The valley's 150,000 residents are at risk from future lahars. For ... Source: Wikipedia