“Riverdalian” is the name of an inhabitant of Riverdale, in the borough of the Bronx. The name “Riverdalian” has been cited in print since at least 1871.
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Wikipedia: Riverdale, Bronx
Riverdale (population: 47,850, according to the 2000 U.S. Census) is an affluent, upper middle class, and wealthy residential neighborhood in the northwest portion of the Bronx in New York City. Riverdale contains the northernmost point in New York City.
 
Old Fulton NY Post Cards
12 September 1871, New York (NY) Evening Express, “Athletic Sports Up the Hudson,” pg. 1, col. 7:
The young gentlemen appeared to enjoy the whole affair hugely, and the Riverdalians were happy in winning the majority of the matches.
   
Google Books
Prayers and Music for the Holy Day Services
By Charles E. Shulman
New York, NY: Riverdale Temple
1959
Pg. II:
The music is modern by the composer, the late Frederic Jacobi, a former Riverdalian and former conductor of the Metropolitan Opera who was our musical advisor.
 
Google Books
The Beautiful Bronx (1920-1950)
By Lloyd Ultan (Bronx County Historical Society)
New Rochelle, NY : Arlington House
1979
Pg. 37:
To a Riverdalian, it might mean his mansion; to a person from Edgewater Park on Throgs Neck, it might mean his weatherproofed bungalow; ...
   
Norwood News
March 11 - 24, 1999
School Board Votes to Cancel 650 Seats
By MATTHEW COREY
(...)
A motion to codify the Riverdalian’s 10-Point Plan for a Riverdale-Kingsbridge Academy failed 4-5 along the same factional lines.
 
Google Books
Where You Left Me
By Jennifer Gardner Trulson
New York, NY: Gallery Books (Simon & Schuster)
2012
Pg. 29:
On the one-year anniversary of Doug’s death, Michael attended his first day of kindergarten at Riverdale Country School, a private school in the Bronx just north of Manhattan. Doug had graduated from Riverdale in 1979 and decreed that his children would continue the Gardner legacy at his alma mater. Although he good-naturedly permitted me to explore other schools in the city, he’d already decided his children would be Riverdalians.
 
The Riverdale Press
Riverdalians warn Home
not to seek rezoning
By Adam Wisnieski
Posted 1/31/13
The Hebrew Home at Riverdale wants to create New York City’s first continuous care retirement community south of it’s North Riverdale campus, but nearby residents are concerned that the size of the proposed building may set a precedent for future developers to build big in Riverdale.