I get that. I'm pretty skeptical that 120 KW going through the last panel doesn't cause damage, and in the "multiple miles chained together" (to quote op) many MW going through the panel at the end of the chain doesn't run into some sort of issue in the real world. Even small percentage losses turn into a lot of heat or other issues at that scale.
If it doesn't why not just make the whole 100 mile stretch of canals discussed a single very long daisy chain of panels, and still address the concern of the OP in terms of extra material for conductors?
And so ultimately, despite my details being wrong about currents, the notion of stringing miles of panels together is probably not a good basis for rejecting the panels over the canals.
In reality, you shouldn't string a lot of panels together for other reasons. An obstructed panel doesn't just not produce energy. It acts as a resistor, _wasting_ energy that flows through it.
You would have short strings of panels and small string inverters built every ~100 meters along the channel. Inverters will also connect to the high-voltage bus cable, likely buried in a trench along the channel. And most channels in California are not gravity-fed, so they already need pump stations along the way and have power distribution systems for them.
If it doesn't why not just make the whole 100 mile stretch of canals discussed a single very long daisy chain of panels, and still address the concern of the OP in terms of extra material for conductors?