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I’m pretty sure once you cede that you’re willing to use the energy required to move a container ships worth of weight from one end of the canal to the other, the cheaper option would be to pump water to refill the locks. Water is easier to transport than ships, and while I don’t think the efficiency of the locks helps much in this context, there might be some clever optimizations… especially if you’re willing to run a ton of high pressure pipe.

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Pump it from where though? It would be easy to pump and reuse the water that already goes through the locks, except that it gets mixed with salt water, so putting it back in the lake would quickly contaminate it.

Based on the diagram of the locks and water saving basins at https://en.wikipedia.org/wiki/Panama_Canal_expansion_project... they could have a second basin D at the same level as basin C. They would pump the fresh water in C that has come down the lock chain back up into the lake. They could pump sea water into D that would be used to fill the bottom lock when a ship is acsending the chain. Then the sea water in the bottom lock would be drained to the sea when a ship descends.

Pumping sea water to the top of the lock chain or maybe the lock next to the top would save a lot of fresh water but would slowly contaminate the lake unless there were some kind of double gate to prevent sea water used to fill the top lock leaking out when a ship enters the lock from the lake.

Solar desalination, perhaps?

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