Which city enjoyed a long reign as the steel capital of the country due to its location at the confluence of three rivers

Pittsburgh, Pennsylvania, is the right answer.

Explanation:

Pittsburgh is a city located in Pennsylvania, the U.S. This city is well known for the three rivers (the Allegheny, Monongahela and Ohio) that flow here. In Pittsburgh, the steel industry developed rapidly after 1830 and a milestone in the steel production was achieved in the year 1875 when a new process called Bessemer process was introduced by Edgar Thomson Works to produce steel. Very soon Pittsburgh became the steel capital of the world.

the event that was most responsible for ending the great depression is world war 2.