Along with the Shield, the winner gets top seeding in the Western Conference, the right to open against the lowest-qualifying team in the conference semifinals, the right to play the deciding game at home during the aggregate-score playoff series, and the right to play MLS Cup on its home pitch if it advances that far.
Other Saturday games will determine the third-, fourth- and fifth-place finishers in the West, as well as whether Vancouver or Portland will miss the playoffs with a sixth-place finish.
The Whitecaps eliminate the Timbers with a win over Colorado, a Portland loss to Dallas or if both teams tie.
Dallas clinches third place with a win over Portland, while a Dallas draw or loss gives third to Real Salt Lake.
After their high-noon showdown, the Sounders and Galaxy will have to wait a few hours to learn their playoff-opening opponents, as the Timbers visit Dallas at 5:30 p.m., and the Rapids visit Vancouver at 7 p.m.
The fourth and fifth finishers in each division will meet in a single play-in match for the right to face the top seed in the conference semifinal series. The second and third seeds meet in the other semifinal.