The Sounders boosted their points total to 48, assuring that they'll retain the Western Conference and Supporters Shield leads, regardless of any other results this weekend.
Dempsey scored his 11th goal to move within two of the Sounders' season record. Fredy Montero and Eddie Johnson each had 13 goals in 2012.
Portland moved into fifth place in the Western Conference, a point ahead of Vancouver. The top five will earn playoff spots.
Powell opened the scoring in the 51st minute, second-half substitute Urruti scored in the 75th and Wallace made it 3-0 in the 79th, slipping behind the defense and converting a pass from Darlington Nagbe.
Real Salt Lake's Javier Morales equalized in the 36th minute of a 1-1 draw with the San Jose Earthquakes.
Sam Cronin opened the scoring for San Jose in the 14th minute with a left-footed strike into the far-left corner of the goal.
The Montreal Impact downed the Columbus Crew 2-0 after recently acquired designated player Ignacio Piatti scored.
The Argentine midfielder took a feed from Felipe Martins inside the Columbus area, beat Michael Parkhurst and drilled in a shot in the 40th minute. Piatti added another goal in extra time with a sliding shot on a loose ball.
Newly signed designated player Jermaine Jones came off the bench in the 65th minute for New England.
The Chicago Fire ended Dallas FC's 10-game unbeaten streak when Robert Earnshaw scored from a late header for a 1-0 win.