The initial plan to protect the reservoir called for splitting it into two parts with a dam in the middle and then installing two covers over it at a cost of more than $300 million.

They’re turning the state’s reservoirs into giant ball pits.

The floating devices not only protect water from dust, chemicals and wildlife, they also stop it from evaporating away.

The balls, which have been in use in Los Angeles on a smaller scale since 2008, now cover all of the city’s reservoirs, a gently-bobbing line of defense against harmful algae blooms and other chemical reactions brought on by direct sunlight, as well as a barrier to cut down on the loss of water due to evaporation. Covering the reservoir, which is 90 feet deep, could have cost as much as $400 million, Harasick said.The whole batch of balls cost $34.5 million, or about 36 cents each.

Officials recently released 96 million floating “shade balls” into the 75-acre Los Angeles Reservoir in Sylmar, California.

Some 20,000 of the 4-inch orbs of polyethylene were released Monday into a Los Angeles reservoir, where they will prevent precious potable water from evaporating, among other things.

After four years of record low water inflows, and limited snow melts off the Sierra Nevada, the city of Los Angeles is having to become creative about water storage and conservation. The Upper Stone, Elysian, and Ivanhoe reservoirs all contain shade balls.

Mr Harasick said the balls are mesmerising to watch as they roll around the surface of massive city water supply storage.

“As the drought continues, it has never been more important to focus on innovative ways to maintain the highest quality drinking water for our 4 million residents”, said council member Mitchell Englander. “Our water system has significantly changed since William Mulholland built it more than 100 years ago”, underlines Marcie Edwards, general manager at LADWP. The balls are expected to save 300 million gallons a year from evaporation in Los Angeles alone.