The L.A. Department of Water and Power plans to start draining 400 million gallons of water from the reservoir late this month or early next month.

The DWP says it needs to remove all of the water to replace a huge water pipe that’s buried underneath the reservoir. Of course, during a drought, that’s far too much water to pour down the drain. So the DWP will install portable filtration units to purify the water and then put it back into the city’s drinking water system.

The reservoir will be empty for about a year. After that it will be refilled with water that’s not suitable for drinking. The 66-inch steel pipeline that’s being replaced will connect to a new reservoir – called Headworks – that’s being built on the other side of the Hollywood Hills, near Griffith Park.

The DWP disconnected the Silver Lake Reservoir from the city’s drinking water system two years ago. That was after the federal government ordered the phase out of all open-air reservoirs because of exposure to runoff and other pollution.

The reservoir was last drained seven years ago because the water became contaminated with bromate, a carcinogen that forms when naturally occurring chemicals mix with sunlight and chlorine. To prevent more contamination, city officials dumped thousands of floating black plastic balls into the reservoir to help shade the water.

The latest draining comes as the city and neighborhood groups debate the future of the 110-year-old landmark. Some residents are even pushing to turn the reservoir into a swimming lake.

Meanwhile, it’s expected to take several weeks to empty the reservoir once the DWP pulls the plug.