A barge filled with scrap iron is in a canal lock. If the iron is thrown overboard, what happens to the water level in the lock? What if it is thrown onto the land beside the canal?

The water level will remain unchanged if the iron is thrown overboard.
When a barge filled with scrap iron is present in a canal lock, it displaces a volume of water equal to its overall weight. As the iron is thrown overboard, the position of the iron are changed within the barge. This does not affect the overall weight of the barge. As the weight of the barge is not affected, the volume of the water displaced remains the same. This will not affect the water level in the lock.
The water level will decrease if the iron is thrown onto the land beside the canal.
The overall weight of the barge decreases as the iron is thrown onto the land beside the canal from the barge. This in turn decreases the weight of the water displaced by the barge. In accordance with Archimedes’ principle, the weight of the water displaced is equal to the buoyant force. The buoyant force in turn is proportional to the volume of the water displaced. The decrease in the buoyant force will be due to the decrease in the volume of the object. In order to minimize volume, the water level will decrease.