I now have Seeger's American Favorite Ballads book, but it doesn't include this song. I did find it in another book, George Korson's Pennsylvania Songs and Legends (1949)

Where the Old Allegheny and Monongahela Flow
(Sung by J. J. Manners at Pittsburgh, 1947, and used by his permission. Notated by Jacob A. Evanson.)

I want to go back once more
To those hills I roamed before
To gaze upon those hills and mills
Where those mighty rivers flow

CHORUS:
I live in that city that is built among the hills
Where smoke is always pouring from the big rolling mills
And steamboats on the rivers go towing to and fro
Where the old Allegheny and Monongahela flow.

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Click to play

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Notes from Korson's chapter editor Jacob Evanson:

The source of this song has been only partially identified, but there is no question of its popularity. Manners said he wrote the verse, but learned the chorus from the Smoky City Quartet in the 1900's. Fred Schmidt, who wrote down the chorus for me in 1942, said, "I've sung it all my life. I can sing all four parts just the way they are always sung. A lot of people here on the South Side sing it. I've heard gangs sing it in beer places on corners down Sarah Street."
George J. Schwartz, an elderly steelworker, added, "I don't know where the song came from, but I learned it about 1910 when I was top tenor of the old Montouth Quartet before Montouth Borough became part of Pittsburgh. Boy, the smoke really rolled over the hills in those days from a hundred puddling mills down on the river!"
Peter Diebold, only surviving member of the Smoky City Quartet, recollected that "Where the Old Allegheny and Monongahela Flow" was a favorite number in its repertoire.
The Allegheny and Monongahela rivers meet at the Point in downtown Pittsburgh to form the Ohio River.