Question: In what region of the U.S. is the river spanned by the 31st Street Bridge in Pittsburgh?

full_context: Document 1: Washington Crossing Bridge (Pittsburgh): The Washington Crossing Bridge, commonly known as the Fortieth Street Bridge, is an arch bridge that carries vehicular traffic across the Allegheny River between the Pittsburgh neighborhood of Lawrenceville and the suburb of Millvale.
Document 2: French Creek (Allegheny River tributary): French Creek (also known as the Venango River) is a tributary of the Allegheny River in northwestern Pennsylvania and western New York in the United States.
Document 3: Allegheny River: The Allegheny River (/ ˌæləˈɡeɪni / AL - ə - GAY - nee) is a principal tributary of the Ohio River; it is located in the Eastern United States. The Allegheny River joins with the Monongahela River to form the Ohio River at the ``Point ''of Point State Park in Downtown Pittsburgh, Pennsylvania. The Allegheny River is, by volume, the main headstream of the Ohio River.
Document 4: Homestead Grays Bridge: The Homestead Grays Bridge, also known as the (Homestead) High Level Bridge, was built in 1936 and spans the Monongahela River between Homestead Borough and the southernmost tip of Pittsburgh's Squirrel Hill neighborhood. It is notable as the first bridge to incorporate the Wichert Truss, which uses a quadrilateral shape over each support, into its design. This made the truss statically determinate, so that forces in the structural members could be calculated.
Document 5: 33rd Street Railroad Bridge: The 33rd Street Railroad Bridge is a truss bridge that carries the Allegheny Valley Railroad on the P&W Subdivision over the Allegheny River in downtown Pittsburgh, Pennsylvania, United States.
Document 6: List of crossings of the Monongahela River: This is a complete list of current bridges and other crossings of the Monongahela River starting from Pittsburgh, Pennsylvania, where the river helps to form the headwaters of the Ohio River, and ending in Fairmont, West Virginia, where the West Fork River and Tygart Valley River combine to form the Monongahela.
Document 7: Cowanshannock Creek: Cowanshannock Creek (the eastern section is sometimes referred to as the North Branch Cowanshannock Creek) is a tributary of the Allegheny River in Armstrong and Indiana counties, Pennsylvania in the United States.
Document 8: Herr's Island Railroad Bridge: The Herr's Island Railroad Bridge, also known as the West Penn Railroad Bridge, is a truss bridge across the Allegheny River between the Pittsburgh neighborhoods of Troy Hill and Herrs Island (commonly referred to as Washington's Landing).
Document 9: 31st Street Bridge: The William Raymond Prom Memorial Bridge, commonly known as the 31st Street Bridge, is an arch bridge that carries vehicular traffic across the Allegheny River between the Pittsburgh neighborhoods of Troy Hill and the Strip District. The bridge passes over, but does not serve, Washington's Landing, which is connected to the mainland by the 30th Street Bridge. Sidewalks along the bridge feature viewing platforms.
Document 10: Deer Creek (Allegheny River tributary): Deer Creek is a tributary of the Allegheny River located in both Allegheny and Butler counties in the U.S. state of Pennsylvania.
Document 11: Brady Street Bridge: The Brady Street Bridge, also known as the South 22nd Street Bridge, was a steel bowstring arch bridge in Pittsburgh, Pennsylvania, which crossed over the Monongahela River at South 22nd Street. It was replaced by the Birmingham Bridge in 1976, and was demolished on May 29, 1978. The bridge's main span was a tied arch with a suspended road deck, with two through-truss side spans, carrying two traffic lanes between Brady Street on the Pittsburgh side and South 22nd Street on the south side. Approach viaducts were built at either end. The bridge was built by the Schultz Bridge and Iron Company.
Document 12: Brilliant Branch Railroad Bridge: The Brilliant Branch Railroad Bridge is a truss bridge that carries Allegheny Valley Railroad's Brilliant Branch across the Allegheny River between the Pittsburgh neighborhood of Highland Park and Aspinwall, Pennsylvania.
Document 13: 30th Street Bridge: The 30th Street Bridge, is a girder bridge that carries vehicular traffic across the Allegheny River between the Pittsburgh neighborhoods of Troy Hill and Herrs Island (known by the moniker of Washington's Landing). This is the fourth bridge that has stood on this site. A wooden 19th Century bridge was washed away during an 1882 flood. This was replaced by an arch bridge that spanned the entire width of the Allegheny River; after a 1921 fire destroyed the span across the main channel, the 31st Street Bridge was constructed as a replacement, and this bridge was again relegated to taking traffic to Herrs Island. A 1939 truss bridge then stood on this site, and it was replaced by the current structure as part of the redevelopment of the island to feature condominiums and a business park instead of warehouses and stockyards.
Document 14: Pucketa Creek: Pucketa Creek is a tributary of the Allegheny River located in both Allegheny and Westmoreland counties in the U.S. state of Pennsylvania.
Document 15: Cheat River: The Cheat River is a tributary of the Monongahela River in eastern West Virginia and southwestern Pennsylvania in the United States. Via the Ohio River, the Cheat and Monongahela are part of the Mississippi River watershed. Owing to the ruggedness of the surrounding Allegheny Mountains, the Cheat remains largely remote with few settlements or developments along its banks. Its headwaters are in the Cheat-Potomac Ranger District of the Monongahela National Forest.
ground_truth: Eastern United States
sub_queries: 31st Street Bridge >> crosses Answer: Allegheny River
where is Allegheny River located on a map Answer: Eastern United States