The Blues sent former No. 1 overall pick Erik Johnson, forward Jay McClement and a first-round pick to the Avalanche for winger Chris Stewart, defenseman Kevin Shattenkirk and a second-round draft choice.
All four players involved are former first-round picks.
"I obviously was not expecting this," Johnson told the St. Louis Post-Dispatch in a text at 2:40 a.m. CT, just hours after the Blues beat the Sabres in Buffalo. "It is tough to leave St. Louis, where I made so many great friends on and off the ice and played for a first-class organization. I am thrilled to be joining the Avalanche organization that has a terrific young team that I can't wait to be a part of."
Johnson, 22, has five goals and 14 assists in 55 games this season. At 6-foot-4 and 232 pounds, he's a physical force on the blue line and at the 2010 Vancouver Olympics helped the United States win a silver medal.
He was the highly touted top pick in the 2006 draft, but a 2008 preseason golf cart accident that resulted in two torn knee ligaments interrupted his development. He had surgery in November 2008 and missed the entire season.
McClement, 27, is a solid defensive forward. He had 16 points (six goals, 10 assists) in 56 gams with the Blues this season. He was the team's first pick (57th overall) in the 2001 draft.
"We are excited to be adding a top young defenseman in this league, a player who will have an impact on our blue line for several years to come," Avalanche general manager Greg Sherman said in a statement. "In addition, Jay McClement brings six years of NHL experience and will complement our young hockey club. He will also have an important role on our penalty-killing unit."
Stewart, 23, was the Avalanche's top pick in the 2006 draft (18th overall), and led the team last season with 28 goals and 64 points in 77 games.
He has 13 goals and 17 assists in 36 games this season, but missed 21 games because of a broken hand and has scored just twice since returning Jan. 14.
Shattenkirk, 22, ranks second among league rookie defenseman with 26 points (seven goals, 19 assists) and was invited to participate in the skills competition during All-Star Weekend last month. He was drafted 14th overall by the Avalanche in 2007.
"We're very excited to acquire a young emerging power forward in Chris Stewart and a top young NHL defenseman in Kevin Shattenkirk," Blues general manager Doug Armstrong said in a statement. "These are two solid young players who will help us now as we continue to grow and improve our team. I would like to thank Erik and Jay for their service to the St. Louis Blues and wish them nothing but the best of luck."
The Avalanche have lost 10 straight entering Saturday and are next to last in the Western Conference with 57 points. The Blues are a step above the Avalanche, with 61 points.
On Friday, the Blues shipped their captain, defenseman Eric Brewer, to Tampa Bay for defenseman Brock Beukeboom, an unsigned draft pick, and a third-round pick in the 2011 draft.
Information from ESPN.com's Pierre LeBrun and The Associated Press was used in this report.