Final answer:
The Main River passes through Frankfurt and connects to the Rhine River, which is essential for transport and industrial activities in central Europe.
Step-by-step explanation:
The river that passes through Frankfurt and empties into the Rhine River is the Main. The Rhine is a critical waterway for Germany and central Europe, playing a significant role in transportation of goods and industrial activity.
The Rhine-Main-Danube Canal connects the Main to the Danube, extending navigation from the North Sea all the way to the Black Sea. The city of Frankfurt, situated on the Main River, benefits from its position along this vital transport route.