Possible Clues:

• Big engineering project
• Where you'll find locks made from tin and aluminium?
• Body of water, lake beneath miraculous site
• Spot for locks
• Passage where there's a lot of barging
• Waterway in N America, in western US state
• Only heads can actually negotiate a lock on this?
• One can always find a lock here
• Amsterdam feature
• Mailer is missing from All-American passage synonymous with Venice, for one