Why is there anything downstream? I mean, it shouldn't be the case because then the water which could be used for the channel locks, is wasted being sent there. If there is anything going downstream from the lake indeed, solution is even simpler - close it off with a dam - but i don't think it's the case. I can't see anything on the map that is being fed from the lake apart from channel locks.

And river that feeds the lake is of course, not artificial - Chagres river - but nothing is gonna happen to it.