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.
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