Wikipedia served as the source for the population information. Note that the population increased between 1900 and 1910.

Waterloo is the county seat of Black Hawk County, which was established on February 17, 1843. The county was named for Chief Black Hawk of the Fox and Sac Tribes. In 1851, the county seat was situated at Cedar Falls. In 1855, the county seat was moved to Waterloo as a more central location for the county. Black Hawk County received two Carnegie grants for the towns of Waterloo and Cedar Falls. Waterloo received additional Carnegie funding so that libraries could be built on both sides of the Cedar River.

(Source: Pratt, LeRoy G. “The Counties and Courthouses of Iowa.” Mason City, Ia; Klipto Printing & Office Supply Company, 1977, p. 35.)