The Thompson River is the largest tributary of the Grand River in the central United States, flowing from southern Iowa into Missouri. In Iowa, it is known simply as the Grand River and passes near the city of Grand River. The river is 188 miles (303 km) long, and its drainage basin is roughly 1,850 square miles (4,800 km2), of which 1,111 square miles (2,880 km2) are in Missouri.