The Plum River is a tributary

Tributary

A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...

 of the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

, about 46.6 miles (75 km) long, in northwestern Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 in the United States

United States

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

. It rises in Jo Daviess County

Jo Daviess County, Illinois

Jo Daviess County is a county located in the northwest corner of U.S. state of Illinois. According to the 2010 census, it has a population of 22,678, which is an increase of 1.7% from 22,289 in 2000. Its county seat is Galena....

 and flows generally south-southwestwardly into Carroll County, where it joins the Mississippi at Savanna

Savanna, Illinois

Savanna is a city in Carroll County, Illinois, United States. The population was 3,062 at the 2010 census, down from 3,542 at the 2000 census. Savanna is located along the Mississippi River at the mouth of the Plum River. Going from north to south, the second automobile bridge between Iowa and...

. Among its several short tributaries are the East Plum River, which rises in Stephenson County

Stephenson County, Illinois

As of the census of 2000, there were 48,979 people, 19,785 households, and 13,473 families residing in the county. The population density was 87 people per square mile . There were 21,713 housing units at an average density of 38 per square mile...

and flows southwestwardly into Carroll County, and the Muddy Plum River, which flows for its entire length in Jo Daviess County.