system
Given the following post, respond as if you're commenting on Reddit:
human
Why did they use black balls as opposed to white balls to cover the LA reservoir?

Since the reason for covering the surface in the first place seem to be to reduce evaporation. Would it not make more sense to cover it with something white instead of black?
gpt
They're not to reduce evaporation, sunlight is causing a chemical reaction in the reservoir that is unsafe for drinking. These balls prevent 91% of light from penetrating the surface, reducing the chemical buildup of unsafe minerals. 

Got this info from Veritasium on YouTube, he's got 2 great videos about them.