These shade balls will save LA 300 million gallons of water per year.

Shade balls are small plastic spheres floated on top of a reservoir for environmental protection and to slow evaporation.

August: Shade balls put into Los Angeles Reservoir during 2015 California drought.

Two months later, 400,000 black hollow plastic shade balls were placed in Ivanhoe, which remains in use, to reduce the likelihood of the sunlight-fueled bromate.

Mt Baldy is the first ski resort in the world to install hollow plastic shade balls in the reservoir to reduce water evaporation.

The shade balls used in the Los Angeles project are made of high-density polyethylene (HDPE) with carbon black additive to protect the plastic from ultraviolet radiation.

In February 2013 LADWP contracted with Glendora, California-based manufacturer XavierC LLC to supply an additional 6.4 million hollow plastic shade balls for reservoirs.

Workers on Monday unleashed 400,000 of the hollow, 4-inch "shade balls" down a slope to cover the surface of the Ivanhoe Reservoir, which provides water to parts of downtown, central and south Los Angeles.

Irfan Khan/Los Angeles Times via Getty Images Two days ago, Los Angeles mayor Eric Garcetti released a final batch of so-called "shade balls" into the LA Reservoir.

In 2014 and 2015, the Los Angeles Department of Water and Power deployed 96 million shade balls on its largest reservoir in response to the United States Environmental Protection Agency's surface water treatment rule, which requires large reservoirs to be covered.

Each of the 'shade balls' costs 36 cents, and they work by floating on the surface of the water and blocking the sun's rays.

Los Angeles mayor Eric Garcetti joined officials yesterday to release the final 20,000 shade balls as part of the region's $34.5 million water quality protection project.

Councilman Mitch Englander added: "In addition to cutting back on the need to chemically treat our water to prevent natural occurrences like algae, these shade balls are a cost-effective way to reduce evaporation each year by nearly 300 million gallons, enough to provide drinking water for 8,100 people for a full year."