The Thompson River is the largest tributary of the Grand River

Grand River (Missouri)

The Grand River is a river that stretches from northernmost tributary origins between Creston and Winterset in Iowa approximately to its mouth on the Missouri River near Brunswick, Missouri....

 in the central United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, flowing from southern Iowa

Iowa

Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...

 into Missouri

Missouri

Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

. The river is 188 miles (302.6 km) long, and its drainage basin

Drainage basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

 is roughly 1850 mi2, of which 1111 mi2 are in Missouri.

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

Greenfield, Iowa

Greenfield is a city in Greenfield Township, Adair County, Iowa, United States. The population was 2,129 at the 2000 census. It is the county seat of Adair County. The current courthouse, which sits in the middle of the Greenfield Square, was built in 1891 and is listed on the National Register of...

. The river initially flows east then bends south, passing Macksburg

Macksburg, Iowa

Macksburg is a city in Madison County, Iowa, United States. The population was 142 at the 2000 census. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area. -Geography:Macksburg is located at ....

 and Davis City

Davis City, Iowa

Davis City is a city in Decatur County, Iowa, United States. The population was 275 at the 2000 census.-Geography:Davis City is located at along the Thompson River...

 and crossing Interstate 35

Interstate 35

Interstate 35 is a north–south Interstate Highway in the central United States. I-35 stretches from Laredo, Texas, on the U.S.-Mexico border to Duluth, Minnesota, at Minnesota Highway 61 and 26th Avenue East. Many interstates used to have splits or spurs indicated with suffixed letters , but I-35...

. It then flows into Missouri, where much of its course is channelized. The Weldon River joins at the town of Trenton

Trenton, Missouri

Trenton is a city in Grundy County, Missouri, United States. The population was 6,006 in 2009. The population according to the 2000 census was 6,216, indicating a 3.4% decrease in population from 2000 to 2009. It is the county seat of Grundy County...

, the largest settlement along the Thompson River. Below Trenton, the Thompson meanders south for another 25 miles (40.2 km), joining the Grand River just north of Utica

Utica, Missouri

Utica is a village in Livingston County, Missouri, United States. The population was 274 at the 2000 census.-Geography:Utica is located at ....

 in Livingston County.