An exceptionally wet winter last year helped the Los Angeles Department of Water and Power refill Silver Lake after nearly two years in which the decommissioned reservoir sat empty. But keeping the lake full presents its own challenges.

The water level drops about 5 feet every year due to evaporation and seepage, engineering bureau project manager Nadir Shah told the city’s Board of Public Works on Friday.

To compensate for this, DWP pumps 418 acre-feet of water into the Silver Lake Reservoir and its smaller neighbor, Ivanhoe, yearly. That’s enough to satisfy the demands of more than 800 households over the same time period, based on Metropolitan Water District calculations.

Continuing to fill the reservoirs—now simply man-made lakes—with drinking water would be a waste of an increasingly precious resource.

But water officials agreed to keep both bodies of water full when planning the piping project that required Silver Lake to be emptied in 2015.

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Given the amount necessary to keep the reservoir complex topped off, DWP is going to find new sources of water for the task.

It will rely largely on groundwater, which it plans to begin pumping from a well northeast of the reservoir complex sometime this spring. The agency is also working with the public works department to build an $8 million stormwater capture system expected to redirect 159 acre-feet of water into the reservoir annually (about 38 percent of what’s needed to keep the lake full).

The public works board approved preliminary plans for the new system on Friday. Expected to be complete by 2021, it would source water from dozens of catch basins surrounding the reservoir complex.

Since neither the groundwater nor the stormwater will be treated, the water quality in the reservoirs won’t be as pristine as in years past. To prevent algae from covering the surface of the lakes (and to avoid unpleasant smells), DWP will install aeration and filtration systems to mix up the water and keep it from stagnating. Those should be in place by fall 2020.

The agency is also developing a master plan to transition the Silver Lake Reservoir into its new role as a scenic body of water and public space.