Down the courses of the Chagres and other smaller jungle rivers flow billions of gallons of rainwater annually, into Gatun Lake, the immense reservoir created as part of the canal. It's stored there. When a ship enters the lock at sea level valves are opened and the lock is filled by gravity with water from the lake. Water is then released in steps down to the sea, where it mingles with the ocean. No recycling, no pumping.
All water flows downhill to the sea.
The catch of course is that an enormous amount of water goes into the sea. Thirty to 40 ships transit the canal daily, and each one requires 52 million gallons of fresh water to get it across. During our passage there were actually four large ships in the double locks. Just ahead and to our left, a monstrous automobile transport vessel with a capacity of 6,500 cars. Behind us, two freighters, one Japanese and one Panamanian.