Stewart's 16th home run of the season, for three runs in the top of the ninth off Joel Hanrahan, tied the game at 5. Helton's first home run since June 15, for two runs off Sean Gallagher (2-0) in the top of the 10th, gave the Rockies the lead.
But the golden chance to trim the Rockies' deficit in the National League Wild Card race slipped away when Street couldn't navigate the bottom of the 10th.
"It's human nature to take a look to your right [at the out-of-town scoreboard] every once in a while and see what's going on in relation to everybody you're chasing," Rockies manager Jim Tracy said. "You're in a wonderful position, only to have it taken away from you.
"It's not the end of the world, that's for sure. We certainly have to bounce back."
The NL Wild Card-leading Giants lost to the Braves, 3-0, on Saturday, and the Rockies lost a day in the chase. The Rockies will send rookie call-up Esmil Rogers to the mound Sunday to start the finale of a four-game series hoping to earn a split against the Pirates, who have the NL's worst record but have won four of the six meetings with the Rockies.
"Finally, I think the baseball gods finally looked down on us and said enough is enough," Pirates manager John Russell said.
A sellout crowd saw fireworks, a postgame George Thorogood concert, and a surprise ending. Actually, it would have been an unexpected ending even if the Rockies had won. They haven't in, now, 27 games when trailing after eight innings.
Instead, Alvarez left the park smiling.
Andrew McCutchen opened the 10th with a double -- one that was ruled a homer before umpires saw replay that showed the ball hit the top of the wall in right-center. With two out, Street missed inside on a 3-2 fastball against Garrett Jones, who homered earlier in the game off Randy Flores for his 16th of the season and his fourth against the Rockies.
Street could be second-guessed for using a changeup instead of a fastball on the 1-0 pitch to Alvarez, but he said the pitch felt good to him earlier in the inning.
"It's pretty unreal," said Alvarez, who has 10 homers since being called up in June. "I can remember one other time hitting a walk-off home run. Obviously, here on a Saturday night, a packed house, in the big leagues, nothing can beat that maybe except Game 7 of the World Series."
That's a ways from reality for the Pirates. Helton, who has actually been to a World Series, nonetheless had a thrilling Saturday. Since his last homer, it had been 61 at-bats and a stint on the disabled list with a lower back strain.
"It's been a long time, but the odds were in my favor I was going to hit one sooner or later," Helton said.
Rockies starter Jorge De La Rosa threw shutout ball through the first five innings. But Chris Snyder hit his first home run for the Pirates, after 10 with the D-backs, for three runs in the sixth and a 3-1 Pirates lead.
"I was a little wild almost all game, then I made that mistake to Snyder," De La Rosa said. "I started to throw a changeup, but I threw a fastball right across the middle."
The Rockies scored two runs, one earned, in six innings against Pirates starter Ross Ohlendorf, who struck out six against one walk.
Actually, Stewart's home run should have given the Rockies the lead. The Pirates led, 4-2, with one out and the bases loaded in the eighth. Rockies reliever Joe Beimel fanned Jones, but the ball bounced away from catcher Miguel Olivo and Delwyn Young scored from third.