Question: The biggest tributary of the Mississippi runs through what Indiana county?

Context paragraphs:
Downtown Indiana Historic District: Downtown Indiana Historic District is a national historic district located at Indiana in Indiana County, Pennsylvania.  The district includes 86 contributing buildings and 1 contributing site in the central business district and surrounding residential areas of Indiana.  The district includes notable examples of buildings in the Italianate, Second Empire, and Queen Anne styles.  Notable buildings include the Federal-style William Houston House (c. 1805), Clawson Hotel (c. 1850-1874), Thomas Sutton House, Calvary Presbyterian Church, Zion Lutheran Church, First United Presbyterian Church, and First Methodist Episcopal Church.  The contributing site is Memorial Park, established as a burial ground in the early 19th century.  Located in the district and listed separately are the Silas M. Clark House, James Mitchell House, Old Indiana County Courthouse, Indiana Borough 1912 Municipal Building, Indiana Armory, and Old Indiana County Jail and Sheriff's Office.

Ohio River: The Ohio River, which streams westward from Pittsburgh, Pennsylvania, to Cairo, Illinois, is the largest tributary, by volume, of the Mississippi River in the United States.  At the confluence, the Ohio is considerably bigger than the Mississippi (Ohio at Cairo: 281,500 cu ft/s (7,960 m/s); Mississippi at Thebes: 208,200 cu ft/s (5,897 m/s)) and, thus, is hydrologically the main stream of the whole river system.

Punxsutawney Area School District: Punxsutawney Area School District is a midsized, rural/suburban public school district located in Jefferson County, Pennsylvania and Indiana County, Pennsylvania.  Punxsutawney Area School District encompasses approximately 295 sqmi .  In Indiana County, Canoe, Banks, and North Mahoning Townships are part of district boundaries.  Punxsutawney, Worthville, Big Run, and Timblin are Jefferson County boroughs that are served.  The townships of Bell, Gaskill, Young, Perry, Porter, Ringgold, Olver, McCalmont, and Henderson are also part of the district.  According to 2000 federal census data, it serves a resident population of 22,055 people.  According to District officials, in school year 2007-08, Punxsutawney Area School District provided basic educational services to 2,589 pupils.  Punxsutawney Area School District employed: 213 teachers, 203 full-time and part-time support personnel, and 14 administrators.  Punxsutawney Area School District received more than $21.1 million in state funding in school year 2007-08.

Indiana County–Jimmy Stewart Airport: Indiana County–Jimmy Stewart Airport (IATA: IDI, ICAO: KIDI, FAA LID: IDI) (Indiana County Airport or Jimmy Stewart Field) is a county-owned public airport two miles (3 km) east of the borough of Indiana, in Indiana County, Pennsylvania.  The airport is about 65 mi northeast of Pittsburgh and is in the Pittsburgh Combined Statistical Area.  It is classified as a business service airport by the Pennsylvania Bureau of Aviation.

Old Indiana County Jail and Sheriff's Office: Old Indiana County Jail and Sheriff's Office is a historic jail and Sheriff's office located at Indiana, Indiana County, Pennsylvania.  It consists of two buildings built in 1887-1888.  They are brick buildings in the Italianate-style.  The jail is attached to the rear of the Sheriff's Office, also known as the Sheriff's House.  They are connected to the Old Indiana County Courthouse by a second floor pedestrian bridge.

Silas M. Clark House: Silas M. Clark House, also known as Clark Memorial Hall and History House, is a historic home located at Indiana, Indiana County, Pennsylvania.  It was built in 1869-1870, and is a three-story, brick building with a cross-gable roof in Italian Villa-style.  It features round headed windows, a central tower, and arched entryway.  It housed the local chapter of the American Red Cross from 1918 to 1930.  The building has housed the Indiana County Historical and Genealogical Society since 1951.

Old Indiana County Courthouse: The Old Indiana County Courthouse is a former courthouse located in Indiana, Indiana County, Pennsylvania.  The courthouse was built between 1869 and 1870 and designed by local architect James W. Drum.  It was the second courthouse to serve the county, with the first demolished in 1868.  The final cost of the project was $150,000.  A dedication ceremony took place on December 19, 1870.  Former Governor of Pennsylvania William F. Johnston spoke at the ceremony.

Magnet, Indiana: Magnet is an unincorporated community along the Ohio River in Union Township, Perry County, in the U.S. state of Indiana.

Harmony Area School District: Harmony Area School District is a diminutive, rural, public school district located in Indiana County, Pennsylvania and Clearfield County, Pennsylvania.  The district is one of the 500 public school districts of Pennsylvania and one of nine public school districts in Clearfield County.  It serves residents in: Cherry Tree Boro in Indiana County; as well as Westover Boro, Burnside Township, and Chest Township in Clearfield County.  Harmony Area School District encompasses approximately 86 sqmi .  According to 2000 federal census data, Harmony Area School District served a resident population of 2,576.  By 2010, the district's population declined to 2,343 people.  The educational attainment levels for the School District population (25 years old and over) were 85.5% high school graduates and 6.8% college graduates.  In 2009, Harmony Area School District residents’ per capita income was $12,775, while the median family income was $31,413.  In the Commonwealth, the median family income was $49,501 and the United States median family income was $49,445, in 2010.

Indiana County, Pennsylvania: Indiana County is a county located in the central west part of the U.S. state of Pennsylvania.  As of the 2010 census, the population was 88,880.  Its county seat is Indiana.  Indiana County compromises the Indiana, PA Micropolitan Statistical Area, which is also included in the Pittsburgh-New Castle-Weirton, PA-WV-OH Combined Statistical Area.

Supporting paragraphs: Magnet, Indiana Ohio River
Answer: Perry County