Steve Miller, a Riverside councilman, knew this before anyone else. Steve was a Trekkie, and he read in Gene Roddenberry's book, Making of Star Trek (1968) that Kirk would be born in a small town in Iowa. The book didn't name the town. Miller thought, "Why not Riverside?" At the next council meeting (March 25, 1985) Miller proposed that Riverside declare itself the Future Birthplace of James T. Kirk. The motion passed unanimously.
Riverside quickly altered its town slogan from "Where the best begins" to "Where the Trek begins," and changed its annual summer festival from River Fest to Trek Fest. Miller jabbed a stick into the ground behind the town barber shop (he owned the property) and declared that it was the future birth spot. An engraved monument was eventually placed on the spot for present (and future) fans. Later, a bench was added for contemplation along with a Shuttlecraft-shaped donation box for upkeep.
Miller's inspiration became official Star Trek canon in May 2009, when Riverside was identified as Kirk's hometown in the Star Trek reboot movie. The entire town was invited to a special secret preview screening of the film, a day before its official release. It wasn't screened in Riverside, but in Iowa City, which was the nearest town with a movie theater.
Star Trek fans, as finicky stewards of series factoids, sometimes wonder why the March 22, 2228 date on the Riverside monument differs from the March 22, 2233 date usually accepted as Kirk's birthday. The answer is that the 2233 date wasn't promoted until eight years later -- 1993 -- with the publication of the book, The Star Trek Chronology: The History of the Future. Take your pick which to believe, but both agree that March 22 is the day, which also happens to be William Shatner's real birthday.