v.1. To deepen some body of water by digging and removing material from its bottom: They'll have to dredge up the river, or else the larger ships won't be able to pass through. Because the stream was hard to navigate, the farmer paid someone to dredge it up.
2. To raise something from the bottom of a body of water: The workers dredged up a car from the bottom of the lake. They dredged a lot of garbage up from the bottom of the canal.
3. To bring something back into discussion or importance, especially something unpleasant: I wish you hadn't dredged up that issue with our boss. I had been content to forget about the accident until my neighbor dredged it up by asking me questions about it.