Background information Edit
Long before Star Trek IV: The Voyage Home established that Kirk was from Iowa, Stephen E. Whitfield and Gene Roddenberry had stated in their book The Making of Star Trek that Kirk had been "born in a small town in the State of Iowa." However, they had not been more specific.
Note that while The Making of Star Trek gave no specific year for Kirk's birth, since the time frame of the original series had not been locked down when the book was written, it did state that Kirk was thirty-four years old, matching the age given in TOS: "The Deadly Years". Given the assumption of the Star Trek Chronology that "The Deadly Years" occurred in 2267, that would mean Kirk's birth year was 2233. The Chronology also assigned Kirk a birthdate of March 22, which is the birthdate of William Shatner. 2233 is confirmed in Star Trek, since the timeline does not change until the Narada appears, and his mother, Winona Kirk, was pregnant with him at that time.
In March of 1985, Riverside was looking for a theme for its annual festival. Steve Miller, who was a member of the Riverside City Council, had read The Making of Star Trek and suggested that Riverside proclaim itself the future birthplace of Kirk. This motion passed unanimously.  The council contacted Roddenberry for permission to be designated as the official birthplace of Kirk. Roddenberry agreed, and designated Riverside as Kirk's birthplace.
Since 1985 the town has held an annual "Trek Fest" in June, a themed Star Trek festival based on an original series episode. The festival features a costume contest and a parade with Star Trek themed floats, including the classic bridge, and a float based on that year's theme. The city also showcases a Constitution-class "U.S.S. Riverside" statue (which doubles as a parade float), a "birthstone" marking the future birthplace of James T. Kirk, and a memorabilia museum, named "The Voyage Home", which houses props from the TV series and the Invasion Iowa reality show.
In 2006, William Shatner, the iconic actor who portrayed Kirk, starred in a reality mini-series entitled Invasion Iowa which took place in Riverside, Iowa, since it had been declared as the future birthplace of Kirk. The series had Shatner recruiting Riverside residents as actors for a film he was directing; however, it was all an elaborate joke on the people of Riverside.
In 2008, Walter Koenig was the Grand Marshal for the parade, judge for the costume contest, and held a Q/A and autograph session with the attending patrons. In 2009, Nichelle Nichols and George Takei joined Koenig, participating in the same roles as he had done in 2008. At the local Riverside Casino and Hotel, the trio held a mini convention answering questions, signing autographs, and holding photo-ops. In 2010, Barbara Luna was welcomed to the small town, along with Koenig, and even stayed at the hotel, dining with fans. James Cawley was scheduled to appear, but a representative from his production company addressed fans stating his minor injuries in a car accident preventing him from attending but looked forward to it.
The people of Riverside also made world headlines in 2009 when the town was given a preview of the Star Trek film. This film marked the first official canon reference to Riverside. While Star Trek established that the alternate reality version of Kirk lived in Riverside, there is still no official link to the prime version of Kirk.