[geph2021]

correct. It's generally not feasible to shade such a large area over water. But evaporation does take its toll on reservoir water. There was a proposal not long ago to shade the canal carrying water from Northern California to Southern California. Reducing evaporation was one of the touted benefits[1].

  A study by the University of California, Merced gives a boost to the idea, estimating that 63 billion gallons of 
  water could be saved by covering California’s 4,000 miles of canals with solar panels that could also generate 13 
  gigawatts of power. 

1 - https://www.pbs.org/newshour/economy/solar-panels-built-over...