What had been a defensive contest changed complexion late when Riley was marking Houston's Ricardo Clark on a corner kick and beat him to the ball, but his header found its way past Bill Hamid into the D.C. net.
Clark made it 2-0 Houston 9 minutes later on a blast into the far upper corner and the Dynamo (1-0-0) extended their home unbeaten streak to 26 games -- three behind Real Salt Lake's all-time MLS record.
Hamid made four saves to keep United (0-1-0) in the game.
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Exploding whale video goes viral on Internet