4 July 2001, New York Times, pg. F4:
"A lot of people would argue that there is no difference between a float and an ice cream soda," said Robin Weir, a food historian, who has studied and written about ice cream. "The float is a little simpler, usually just ice cream and soda pop." Mr. Weir thought the term dated from the early 1900's. So did Laura E. Quarantiello, the author of 'The Root Beer Book: A Celebration of America's Favorite Soft Drink" (Tiare Publications, 1977). She said she thought the float originated in the New York area.
The earliest reference in print Mr. Weir could find was in "The Dispenser's Formulary," a collection of more than 2,000 soda fountain recipes, dating from 1915, and he was surprised it was not earlier. The ice cream float in that book consisted of a fruit ade with soda water or ginger ale and topped with sherbet and ice cream, no root beer.
7 August 1903, Ottumwa (Iowa) Daily Courier, pg. 5, col. 1:
"Root Beer Cream Float" touches the spot - Ayres & Taylor.
7 June 1906, Trenton (NJ) Times, pg. 5?, col. 6:
New Drinks at Our Fountain
A Few of the New Ones:
Root Beer Float, 5c.
The name will give you some idea of it, but it takes a taste to give you full appreciation.
Philadelphia Grocery Co.
3 August 1909, Evening News (Ada, Oklahoma), pg. 1, col. 6:
Root Beer Float