Question: Which river that streams westward from Pittsburgh, Pennsylvania, to Cairo, Illinois is the northern boundary of Downtown Louisville?
Context: The Lima River ("Lima" in Portuguese and Limia in Spanish) is a river, which streams westward from Galicia in Spain) to Portugal, with a length of 108 km . East Nashville is an area east of downtown Nashville, Tennessee across the Cumberland River.  The area is mostly residential and mixed-use areas with businesses lining the main boulevards.  The main thoroughfare is Gallatin Avenue and Ellington Parkway with smaller arteries interconnecting the neighborhoods.  Some of these smaller arteries include Main Street, Shelby Avenue, Porter Road, Riverside Drive, Eastland Avenue, McFerrrin Avenue, and Woodland Street in no significant order.  Ellington Parkway, which parallels Gallatin Avenue and Main Street, bypasses I-24 and I-65 and connects Briley Parkway and downtown Nashville and many other secondary streets along the way.  The Cumberland River confines most of the area with a semicircle design on the south, southwest and east.  Since East Nashville has no real boundaries on the west and north, many times it is different.  Some would say that Ellington Parkway creates a boundary on the west and northwest, while Cahal Avenue and Porter Road in the north, in the confines of zipcode 37206.  Many would now state that I-65 and I-24 create the western border and Briley Parkway as the northern boundary, which is called Greater East Nashville. West Aurora Public School District 129 is a unit school district in southeastern Kane County, Illinois.  The present "West Side" school district covers the city of Aurora, Illinois with an eastern boundary of the Fox River, a southern boundary of the Kane County and Kendall County line, a western boundary of Aurora's city limits, and a northern boundary of the village limits of North Aurora, Illinois.  Students from Aurora, North Aurora, Montgomery, Illinois and Sugar Grove, Illinois attend West Aurora schools. Ridgefield Township was a township that existed in Bergen County, New Jersey.  The township was created in 1871, when Hackensack Township was trisected to form Palisades Township in the northernmost third, Englewood Township in the central strip and Ridgefield Township encompassing the southernmost portion, stretching from the Hudson River on the east to the Hackensack River, with Hudson County to the south.  Much of the area had been during the colonial area known as the English Neighborhood.  As described in the 1882 book, "History of Bergen and Passaic counties, New Jersey," "Ridgefield is the first township in Bergen County which the traveler enters in passing up the Palisades.  His first impressions are much like those of old Hendrick Hudson in speaking of a wider extent of country: "A very good land to fall in with, and a pleasant land to see."  The valley of the Hackensack invited early settlers in the seventeenth century, and the valley of the Overpeck Creek, a navigable arm of the Hackensack, also attracted settlers quite as early in this direction.  Sloops and schooners can pass up this creek nearly to the northern boundary of the township.  Ridgefield is bounded on the north by Englewood, on the east by the Hudson, on the south by Hudson County, and on the west by the Hackensack River.  The southern boundary is less than two miles in extent, and the northern less than four, and the length of the township from north to south does not exceed four miles.  Bellman's Creek, forming part of the southern boundary, the Hackensack, the Overpeck, the Hudson, with more than a dozen other smaller streams and rivulets, bountifully supply the whole township with water.  From the western border of the Palisades the land descends to the Overpeck, forming a most beautiful valley, with the land again rising to a high ridge midway between the Overpeck and the Hackensack.  From this long ridge, extending far to the north beyond this township, it took its name of Ridgefield. <br><br>The New York, Susquehanna and Western, formerly the Midland Railroad, the Jersey City and Albany Railroad, and the Northern Railway of New Jersey—all running northward through the township— afford ample railroad accommodations.  The Susquehanna enters the township at Bellman's Creek, and the Northern at about one hundred feet south of the creek, and at a point north and east of the Susquehanna.  The Albany road in this locality is not yet constructed, diverging at present from the track of the Susquehanna between Little Ferry and Bogota stations.  It has, however, an independent line projected and now under construction to New York City. <br><br>Early Settlements.  Ridgefield embraces the earliest settlements in the ancient township of Hackensack, antedating even the organization of that township in 1693, and of the county of Bergen in 1675.  There seems to have been no town or village compactly built, like the village of Bergen, but there were settlements both of Dutch and English in and about what was subsequently known as English Neighborhood prior to 1675.  The Westervelts, the Zimcrmans, the Bantas, and the Blauvelts, all coming from Holland, settled in the middle of the seventeenth century in that locality.  The ancestors of Jacob P. Westervelt, now of Hackensack Village, with himself, were born in English Neighborhood.  His father was born there in 1776, and was the son of Christopher Westervelt, who was born there certainly as early as 1690, and he was the son of the original ancestor of this family, who came from Holland and settled on Overpeck Creek, within the present limits of Ridgefield township, probably about 1670." Illinois is in the midwestern United States.  Surrounding states are Wisconsin to the north, Iowa and Missouri to the west, Kentucky to the south, and Indiana to the east.  Illinois also borders Michigan, but only via a northeastern water boundary in Lake Michigan.  Nearly the entire western boundary is the Mississippi River, except for a few areas where the river has changed course.  Illinois' southeastern and southern boundary is along the Wabash River and the Ohio River.  Whereas, its northern boundary and much of its eastern boundary are straight survey (longitudinal and latitudinal) lines.  Illinois has a maximum north-south distance of 390 miles (630 km) and 210 miles (340 km) east-west.  Its total area is 57918 sqmi . 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. Little Walker Caldera is a depression in the eastern Sierra Nevada that is adjacent to the Sweetwater Mountains.  The caldera is very large, measuring about 18 km in diameter.  The Little Walker Caldera is named for the Little Walker River; it is the river that is little, not the caldera.  U.S. Route 395 in California crosses the Northern boundary of the caldera, just west of the Devil's Gate Pass.  California State Route 108 follows more of the northern boundary of the caldera. Downtown Louisville is the largest central business district in the Commonwealth of Kentucky and the urban hub of the Louisville, Kentucky Metropolitan Area.  Its boundaries are the Ohio River to the north, Hancock Street to the east, York and Jacob Streets to the south, and 9th Street to the west.  As of 2000, the population of Downtown Louisville was 2,575. The Pittsburgh to St. Louis Main Line was a rail line owned and operated by the Pennsylvania Railroad in the U.S. states of Pennsylvania, Ohio, Indiana, and Illinois.  The line ran from Pittsburgh, Pennsylvania west via Steubenville, Ohio, Columbus, Ohio, Dayton, Ohio, Indianapolis, Indiana, Terre Haute, Indiana, and Vandalia, Illinois to East St. Louis, Illinois.  In addition to its east end in downtown Pittsburgh, where it met the Main Line and Pittsburgh to Chicago Main Line, junctions included the Columbus to Chicago Main Line at Columbus, the C&X Branch (to Cincinnati) at Xenia, the Columbus to Indianapolis Main Line via Bradford (a freight bypass of Dayton) at New Paris, the Richmond Branch and Fort Wayne Branch at Richmond, the Louisville Branch and I&F Branch at Indianapolis, and the Peoria Branch at Farrington, Illinois. U.S. Route 60 (US 60) in the state of Kentucky parallels the Ohio River (the northern boundary of the state) from Cairo, Illinois, to Louisville, and then takes a direct eastward route (near Interstate 64, I-64) to rejoin the Ohio River at Kenova, West Virginia.
Answer: Ohio River