The canal is higher elevation than the sea. They have to pump fresh water to elevate the ships to canal level, and then dump fresh water into the sea to lower the ship on the other site. You can't use sea water for those operations because you would discharge salt water into the lakes and rivers which would damage those resources.
But why? Why can't you pump in salt water from the sea and then dump salt water back into the sea. To me that always seemed like the logical methodology. Also, why is the canal higher than the sea? I thought one side was higher that was why there had to be a raising and lowering depending on which way you were traveling. I guess the film I remember watching didn't explain the whole thing very well.
2 reasons that I can think of:
Salt water is more corrosive to pipes and pumps. If they use fresh water, they can get longer usage out of their system before needing to replace parts of it.
There are different animals in the Pacific and Atlantic, using fresh water would kill off most salt water animals before they could make it across the canal and become an invasive species.
1 they could use non-corrosive parts. 2 I don't see what pumping water from one side to the other has to do with cross contamination of species. The water could be filtered through the pipes. But I know what you are saying.
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