The dictionary meaning of OVERWHELM is to BURY or submerge.
For example, a town could be completely overwhelmed by the flood waters.
Today, however, the word, overwhelm, is often used to describe emotions.
For example, you could say, "She was overwhelmed by the kindness shown to her. She was very touched by her friend's generosity".
The word OVERPOWER means to SUBDUE or make someone or something powerless.
For example, the policeman overpowered the criminal. He easily knocked him to the ground and tied his hands".
Hope this helps