When a harbor becomes filled with silt, you dredge it to make it navigable again. You scoop out the mud with a big shovel, which is also called a dredge, and make a channel deep enough for ships.
Dredges have been at work in the Hudson River for years getting rid of industrial toxins, dredging up the contaminated sediment and loading it into barges to be carried away. If you've dropped your gold watch or your car rolled into a pond by mistake, there are dredges that can be dragged across the bottom to recover things that have fallen in. You can dredge up memories, too, which usually entails reminding people of unpleasant ones they’d rather forget.