As a freshman for the 2003 Wolverines, he was the regular kicker for the team from the start, making four point after touchdowns in the first game of the season. Beginning his streak of many game-winning kicks for the Wolverines, he made the game-winner in the largest comeback in Michigan history, capping a 21-point 38-35 comeback victory against Minnesota with a 33-yard field goal with 50 seconds left in the Battle for the Little Brown Jug. Rivas was involved in a controversial national story in the October 4, 2003 when Michigan head coachLloyd Carr changed the punting gameplan in the middle of the game by asking Rivas to line up as a punter and carry the ball or kick while rolling to his right behind a wall of blockers. After it worked twice, the Iowa Hawkeyes blocked the third attempt; the resulting great field position led to Iowa taking the lead, breaking a 20-20 tie en route to a 30-27 win. As a sophomore for the 2004 Wolverines team, his season-best 4-4 field goal performance came in a September 11, 2004 28-20 loss to Notre Dame in the Michigan – Notre Dame football rivalry game. He posted a 4th-quarter field goal to bring the Wolverines to within 24-20 in their October 9, 27-24 win against Minnesota. He made the go-ahead 35-yard field goal with 2:45 remaining against Purdue in an October 23, 2004, 16-14 win. He also went 3-3 on field goals and 4-4 on extra points in the October 30, 2004, overtime 45-37 win against Michigan State in the Paul Bunyan Trophy game. In the season-ending 38-37 loss to Texas in the 2005 Rose Bowl, Rivas was 3-3 on field goals and 4-4 on extra points. Following the season he was recognized as an honorable mention All-Big Ten selection. As a junior for the 2005 Wolverines, he was involved in many close games. The team's first five conference games were all decided in the final 24 seconds of regulation or in overtime. After missing a 27-yard potential game-winner in the final minute, he made the winning field goal in overtime against Michigan State. In the October 8, 2005 23-20 loss to Minnesota, Rivas missed two late chances to break a 20-20 tie: a 42-yard field goal late in the third quarter and a 34-yard kick with 8:27 left in the game. In the October 15, 27-25 last-play win against Penn State, he gave the team a 21-18 lead on a 47-yard field goal with 3:45 remaining. Rivas posted 15 points in an October 29, 2005, 33-17 win against Northwestern. At the end of the season, Rivas was named an honorable mention All-Big Ten selection for the second-straight year. As a senior for the 2006 Wolverines, he never made more than two field goals in a game yet still managed to score a total of 93 points, one shy of his career-high set back in 2004. That year Michigan won all eleven its victories by at least seven points. At the conclusion of the Big Ten schedule, he was selected as a 2006 first team All-Big Ten Conference selection. Rivas set a new Michigan Wolverines football record for career scoring, surpassing Anthony Thomas' record of 336 set back in 2000. He also broke Remy Hamilton's field goal record of 63, set back in 1996, with 64 career field goals, and J. D. Carlson's PAT record of 137, set back in 1991, with 162. He additionally tied Mike Gillette's record from 1998 of 13 career 40-yard field goals.
Professional career
He signed with the Tampa Bay Buccaneers on May 3, 2007 as an undrafted free agent, but was released on June 2, 2007. Rivas signed to the Florida Firecats of the af2 league on April 25, 2008, for the 2008 and 2009 seasons. In 2009, he played in nine of the first ten games, but he did not appear in any of the last six games. During his final af2 season he completed 41 of 58 point after touchdown conversions and missed both of his field goal attempts. He played for the Tampa Bay Storm in the reincarnated Arena Football League in 2010 and participated in the Arena Bowl with the team. He also serves as an assistant football coach and physical education teacher at Berkeley Preparatory School, where he runs the offseason football conditioning program.
