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 th

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.