Thomas broke a scoreless tie in the 41st minute, blasting a header from the penalty box off a corner kick from Powers.
Powers doubled the advantage with a goal of his own just 6 minutes later when Edson Buddle fed him a pretty through ball between two Galaxy defenders and the Colorado midfielder notched his fourth goal of the season.
The Rapids (9-7-7) came into the contest one point out of the fifth and final Western Conference playoff spot.
The Galaxy, meanwhile, fell to 10-9-3. They held the West's third position with 33 points starting Saturday's play.
Ray Liotta sues skin care company over use of likeness
Millions of Getty images now available for free via embed tool