A surveyor's line was drawn "from the end of Rock Island west to the Missouri river " which essentially puts Henry, Lee, Louisa, Van Buren and Des Moines counties in what they called Des Moines. The rest - north of that line was called Dubuque. This is for the year 1836. Various Maps of Muscatine County and surrounding counties:
1836 Map
1837 Map
1838 Map
1838 Map