In their final game before the MLS playoffs, Portland scored three times in the first half to effectively end the contest by the break, while Ryan Johnson and Will Johnson scored within five minutes of each other in the second stanza to wrap up the rout.
The victory took the Timbers a point ahead of Real Salt Lake at the top of the Western Conference standings with 57 points, while third-placed Los Angeles Galaxy (52 points) cannot overhaul Portland as they have only one game left in the regular season.
The Western Conference championship is Portland's first major title since they joined the MLS in 2011, while they will also compete in the playoffs for the first time this season.
Diego Valeri was the star of Portland's first-half blitz on Saturday as he scored the opening two goals and set up Rodney Wallace for the third.
The Argentine midfielder opened the scoring in the 16th minute, slotting home from just inside the penalty area after Wallace's effort was blocked by Chivas goalkeeper Timothy Melia.
Valeri's second goal was a sublime chip from outside the box just before the half-hour mark, which looped over Melia and into the top left corner of Chivas' net.
It was 4-0 with 18 minutes remaining when Ryan Johnson tapped home after Melia saved Wallace's initial shot, while Will Johnson's direct free-kick in the 76th minute completed Portland's triumph in front of the vocal travelling Timbers fans.
