It seems like it would be more effective to simply put all the solar panels in a field, and construct a cheap shade over the entire canal no? The supports in the picture are massive and don't look cheap. Also I would imagine the solar array uses more copper than a similar capacity array just built in a field. You can't daisy chain multiple miles of solar panels together, so you need an extra power line to run alongside the whole thing.

Put the solar panels in a field: The solar array uses less copper. The shade supports don't have to hold up solar panels: Shade supports cost less.

The best reasoning they give is that California has insane permitting requirements, and it takes 1/6 the time to build on developed land compared to undeveloped land.