Poweshiek County Iowa Poweshiek County Population (2010) 18,736 County Seat: Montezuma, IA City & Towns in Poweshiek County Iowa Town Population Town Population Brooklyn 1,468 Hardwick 86 Deep River 279 Malcom 287 Grinnell 9,218 Montezuma 1,462 Guernsey 63 Searsboro 148