In a typical string of solar panels design, you'll get tons of volts but not a lot in amps - current cell maximums top out at ~11A and the connective MC4 wiring can't handle too much more current than that, so what you end up with is like a 1,000V 10A string on one MPPT connection into the inverter.
14 gauge wire is basically all you need to carry 10A safely for an extended period of time regardless the voltage. It doesn't matter that you are carrying 120KW.
The proof of this is in EV charge cables. Those bad boys can carry up to 350kW. Yet the cables are often thinner than you might expect. How do they do this? It's by using high voltages (around 900V) which cuts back the amps to around 300->400.
Tesla's chargers peak (or used to) around 600V which has required them to have much beefier cables to handle the high current.
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?
While having 12kV going through panels is not safe for many reasons, having 300V at 10-20A is completely normal.
12kV is impractical because these voltages can jump quite far, and you need a lot of insulation for them to be safe.
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.
And they're never installed in config chains that high voltage - cell spacing between cells is typically very low and only capable of withstanding ~5kV before internal arcing occurs.
Also, your typical 72-cell solar panel is only 32V.