Riverdale Township was organized in 1869, and named after the Watonwan River.

The population density was 8.7 people per square mile (3.4/km2).

The racial makeup of the township was 96.45% White, 0.59% Asian, 2.96% from other races.

There were 126 households, out of which 33.3% had children under the age of 18 living with them, 63.5% were married couples living together, 4.8% had a female householder with no husband present, and 23.8% were non-families.

16.7% of all households were made up of individuals, and 9.5% had someone living alone who was 65 years of age or older.