Portland (8-8-10) moved into fifth place in the Western Conference, a point ahead of Vancouver (7-6-12). The top five will earn playoff spots.
Powell opened the scoring in the 51st minute. Diego Valeri threaded a ball into the box and the unmarked Powell headed it past goalkeeper David Ousted.
Urruti, a second-half substitute, scored in the 75th, taking a pass from Michael Harrington and blasting a shot that hit the crossbar and fell into the net behind Ousted.
Wallace made it 3-0 in the 79th, slipping behind the defense and converting a pass from Darlington Nagbe.