You don’t have to cover reservoirs as they’re just man-made lakes. The coverings are required to reduce evaporation and to prevent treated water from forming various chemicals. Of course, most reservoirs store untreated water for that very reason.
If CA had had a come-to-Jesus moment during any of the firestorms in the last 20/30 years they could be much further along now than they are. They did it in response to earthquakes - CA buildings and bridges are much better equipped to handle the big one than, for example, the Middle East where a mild earthquake kills 100k
Even basic things like mandating new roofs be non flammable would make a difference over time. Combine that with intelligent and effective fire breaks and other taming methods and you can reduce the impact - not remove it, but reduce it. Nobody really cares if a fire burns 100,000 acres if it stays mostly in the wilderness. But 12,000 structures isn’t a wildfire anymore, that’s just a city burning.