Donovan collected a loose ball that had been stripped away from teammate Robbie Keane and unleashed a shot from outside the penalty area that found the net early in the contest.
Galaxy goalkeeper Jaime Penedo preserved with the win with a key save on Jordan Harvey in the 82nd minute as Los Angeles (12-9-4) grabbed second place in the Western Conference.
The Whitecaps (10-9-6) fell into the sixth in the West.
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