Well, it’s not exactly a dam, but rather a coffer dam, or a door, or the thing that holds back the Delaware River at the former Philadelphia Naval Shipyard. Take a look at this photo:
The coffer dam of a graving dock at the former Philadelphia Naval Shipyard.
Click on the photo for a larger version, and count the numbers on the left, indicating feet above the bottom. Then imagine all the water that thing is holding back. If it lets go, you’re going to get deluged by the full force of the river. Thanks to a friend, I was able to check out the laying of keel blocks in this graving dock. More posts to follow, hopefully, showing the vessel on the blocks, which is an amazing thing in its own right.