A historic reservoir in Central California, which is also a major supplier of water to Silicon Valley, is dangerously low this season. The San Louis Reservoir, located inland from Monterey Bay, is the largest off-stream reservoir in the world; now, it’s only 17% full.

The low level is making water officials in Silicon Valley nervous, since they draw billions of gallons from it every year. This year, the water level in the San Louis Reservoir was at the lowest since August of 1989. The low level also causes higher temperatures in the water, which promotes the growth of algae

Back then, California experienced a major drought, which didn’t end until 1992. A record dry spring this year accounts for the present low level.  When full, San Louis holds two million acre feet of water, enough to supply 10 million people for a year.

Although the district has a year’s supply of ground water to make up the difference, another dry year in 2014 will challenge the system and pit farmers and cities against each other for water use and rights. Further drought could lead to water rationing for Silicon Valley and nearby towns.