Inflatable dam destroyed by vandals.Alameda Co Water Dist says critical water supply lost..enough for 500 families pic.twitter.com/pUwKKdYshT — Jodi Hernandez (@JodiHernandezTV) May 22, 2015

Fremont police are investigating what they believe is an act of vandalism that destroyed an inflatable rubber dam in the East Bay on Thursday morning, costing the area nearly 50 million gallons of water.

The police believe someone went to Alameda Creek on Thursday morning and damaged the dam, which is made of roughly an inch-thick rubber material. The dam was partially submerged when discovered, making it difficult to determine what caused it to deflate, said Geneva Bosques, a spokeswoman for the Fremont Police Department.

The loss of the water couldn’t come at a worse time for the Alameda County Water District, as the state suffers through an extended drought and cities and towns are under orders by the state to reduce consumption. The State Water Resources Control Board has ordered the district to reduce its water demand by 16 percent.

The damaged dam is used to capture water, where it is diverted to Quarry Lakes Regional Park. The water then percolates into the ground and becomes groundwater. The deflated dam meant the water “simply flowed down Alameda Creek into San Francisco Bay,” said Robert Shaver, general manager of the Alameda County Water District.

One of two rubber dams on Alameda Creek, it is 210 feet long and can inflate to a height of 13 feet. It was used because of its versatility, as the water flow down Alameda Creek changes dramatically with rainfall.

“This was a malicious intent,” Shaver said. “It was an incredibly senseless, wasteful, destructive act.”

The 50 million gallons could have supplied 500 families for a year, Shaver estimated. But he said the loss won’t make a significant dent in the district’s water-reduction goals because the amount is less than “one-half of 1 percent of our total demand.”

The water district serves roughly 85,000 businesses and homes, and 335,000 people in Fremont, Newark and Union City.

The Police Department doesn’t have any leads on who or what caused the dam to deflate, Bosques said.

“We are investigating it as being intentional,” she said. “We are putting a lot of police resources behind the investigation.”

Emily Green is a San Francisco Chronicle staff writer. E-mail: egreen@sfchronicle.com Twitter: @emilytgreen