The first train reached Rockwell City on August 7, 1882, and the population count doubled in the same year.

The first courthouse, built of wood, burned to the ground in 1884 and the county government moved into a nearby hotel.

With a damage path 500 yards (460 m) wide and 55 miles (89 km) long, the tornado destroyed about 80% of the homes in Pomeroy.

[3] Calhoun County is located entirely within the Des Moines Lobe of the Western Corn Belt Plains ecoregion, as defined by the United States Environmental Protection Agency (EPA).

One of the flattest regions in Iowa, the Des Moines Lobe ecoregion is a distinctive area naturally defined by Wisconsin glaciation but modified by humans for extensive agriculture.

In general, the land is level to gently rolling with some areas of relief defined by glacial features like moraines, hummocky knobs, and kettles, and outwash deposits.

The stream network is poorly developed and widely spaced, with major rivers carving valleys that are relatively deep and steep-sided.

Most of the region has been converted from wet prairie to agricultural use with substantial surface water drainage.

It first voted for the Democrats in 1932 when Franklin D. Roosevelt won in a landslide victory against Herbert Hoover.

In 2020 and 2024, Trump increased it to 70.2% and 72.7% respectively, the highest percentages for any party in the county since Warren G. Harding a century earlier.