Of course they could turn it off, but then the army of carp just waiting outside the parasitic barrier would take its chance and colonise the lake. Hence, no turn-off, at least not for both barriers at the same time.
Open the outer door, enter the middle area, wait for the inner door. (lather, rinse, repeat for n > 2)
http://en.wikipedia.org/wiki/Barrage_%28artillery%29#Moving_...
So there would be a number of active zones at a time, each one slowly moving downriver, pushing back any invading carp in front of them.
You could switch off the electrodes in any section of the river, and then when you switch them back on again later, the pattern sweeps any occupying invaders back out.