Final answer:
The confluence of the Ohio, Monongahela, and Allegheny rivers occurs in Pittsburgh, Pennsylvania, a city historically known for its steel industry.
Step-by-step explanation:
The Ohio, Monongahela, and Allegheny rivers converge in the city of Pittsburgh, Pennsylvania. This confluence made Pittsburgh a historically significant industrial hub, particularly for the steel industry, due to the industrial advantages provided by the rivers' transportation capabilities. Furthermore, the unique location of Pittsburgh has resulted in nearly 450 bridges in the greater Pittsburgh area to span these waterways.