Rivera was drafted by the Seattle Mariners in the second round (49th overall) of the 2001 Major League Baseball Draft as a 17-year-old. He played most of the 2004 season with the High-A California League Inland Empire 66ers but played with both the Triple-A and Major League clubs in September. He made his Major League debut September 22, 2004, playing in two games for the Mariners that season as a late-inning replacement. Rivera began 2005 with the Double-A San Antonio Missions. He was promoted to the Mariners on May 15, but optioned back to San Antonio on May 20 without appearing in a game. He was again promoted to the Mariners on May 30 when starting catcher Miguel Olivo was sent down to Triple-A. On May 31, Rivera had his first Major League start and base hit, going 3 for 5 at the plate. Rivera shared catching duties with Pat Borders in June and was optioned back to San Antonio when Olivo returned. He played with the Triple-A Tacoma Rainiers in August and returned to the Mariners in September. In 2005, he had 12 starts at catcher and 19 hits in 48 at-bats at the Major League level.

Rivera was one of the final players cut from the roster in 2006 spring training and began the year at Triple-A. He was back with the Mariners by mid-April and served as a back-up to Kenji Johjima for the remainder of the season, batting .152 in 99 at-bats over 35 games. During the off-season, the Mariners weighed the benefits of allowing the young catcher to develop his hitting in the minor leagues against his providing limited offense in a back-up role in the Majors. In 2007, Rivera played for the Mariners Double-A affiliate, the West Tenn Diamond Jaxx, batting .214 in 91 games. He was granted free agency in November.

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