The Thompson River is the largest tributary of the Grand River in the central United States, flowing from southern Iowa into Missouri.

[6] It rises in the agricultural lands of Adair County, Iowa, a few miles northeast of Greenfield.

The river initially flows east then bends south, passing Macksburg and Davis City and crossing Interstate 35.

Below Trenton, the Thompson meanders south for another 25 miles (40 km), joining the Grand River just north of Utica in Livingston County.

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