Before and After: The Rain’s Impact on Three California Reservoirs

Folsom Lake on Oct. 26, 2015. Right: Folsom Lake on Jan. 14, 2017.

Folsom Lake on Oct. 26, 2015. Right: Folsom Lake on Jan. 14, 2017. (Images provided by Planet Labs)

Amazing the difference a little rain makes. Wait — a little? No — a lot.

Virtually all of California is enjoying its wettest winter in five years. In fact, current statistical reports on rainfall and the water content of the Sierra Nevada snowpack show that so far, we’re in the midst of one of the wettest California rainy seasons on record.

All the precipitation has transformed a state that suffered through five years of severe drought. One of the most visible effects: high levels of the state’s major reservoirs.

Below are comparative views of three of those massive storage facilities: Folsom Lake, on the American River about 25 miles northeast of downtown Sacramento; Shasta Lake, the state’s largest reservoir, just north of Redding; and San Luis Reservoir (bottom), near the town of Los Banos in the San Joaquin Valley.

We’ve chosen satellite images from San Francisco-based Planet Labs that illustrate the reservoirs near their drought low points and as they’ve appeared this month. Additional notes on each of the reservoirs below.

Instructions: Each image contains a slider bar (a gray vertical line with arrows on each side) in the center. Click on the center of the bar and drag it right or left to see views of each reservoir a) near its drought low point and b) after recent rains.

Folsom Lake is the state’s 11th-largest reservoir with a capacity just under 1 million acre-feet. The reservoir was filled for the first time in 1955. In late 2015, the drought reduced Folsom to its lowest level ever. In wet years, reservoir managers are often required to release water to maintain space for potential floodwaters. In fact, the volume of water in Folsom has dropped 40 percent over the past two weeks even as heavy runoff continues to cascade down the American River. See current Folsom Lake storage information.

Shasta Lake, California’s largest reservoir at 4.5 million acre-feet, captures the flows of three major Northern California rivers: the Sacramento, the McCloud and the Pit. The lake, behind Shasta Dam, was filled in the late 1940s. It reached its all-time low during the drought of 1976-77. Its lowest point during the recent drought was recorded late in 2014. See current Shasta Lake storage.

San Luis Reservoir, with a capacity of about 2 million acre-feet, is a joint state-federal facility built in the 1960s to help manage supplies from two sources: the State Water Project’s California Aqueduct and the federal Central Valley Project. It’s a key storage point for farms in the San Joaquin Valley and for urban water districts in Southern California. It reached its drought nadir in late 2015. See current San Luis Reservoir storage.

  • Nils Christian

    Could you say anything about the rain’s impact on the ground water level, not just the reservoirs? I think the long term water situation for California mainly depends on the ability to maintain healthy ground water levels.

    • Dan Brekke

      Nils: That’s a crucial question. While it’s hard to offer a definitive answer, here are a couple things we know:

      1) The consensus among hydrologists and other experts is that aquifers in the San Joaquin Valley, especially, were seriously overdrafted before the most recent drought, and the dry conditions that prevailed over the last five years or so only made matters worse. There’s general agreement that replenishing aquifers could take decades. And some aquifers have suffered permanent damage through subsidence and will have a reduced capacity when and if they’re replenished (a state law enacted during the drought creates a sustainable groundwater scheme, but it’s not due to take full affect for another 20 years or so).

      2) Long-term overdraft aside, groundwater levels do bounce back to some extent in wet years. Looking at some U.S. Geological Survey data for wells in the Central Valley, it’s apparent that most well measurements so that groundwater levels are significantly lower than they were at the beginning of the drought.

      Thanks for bringing this up — we need to do more work on it.

      • Cat Erbacher

        Great information! Thank you!

      • WTF

        “take full AFFECT”? Affect? Really?

        • Dan Brekke

          Yes! No! Sorry! What can I say? It was late. Thanks for pointing it out.

        • dzerres

          relax, we’re talking about drought and aquifers here, and not submitting an english thesis. Worry that the President doesn’t know the difference between precedent and president – that’s yuuuuuge.

      • Nils Christian

        Thanks for a good answer. I hope to also read more about this in the time to come.

        • Philips

          A good pal suggested me to try working for this company on the web link provided, due to the fact he is generating five figures on a monthly basis by it. I cannot believe how super easy that was. Are you looking to win extra cash, this is for you

      • Hillary Clintub

        I wonder what it would take to bury those reservoirs to make sort of “artificial aquifers”, if you will. Anything to prevent that water from escaping through surface evaporation would give it more chance to seep back into the real aquifers. I’ve seen those small reservoirs where they use black floatie balls like they use in kids’ ball pits to help against evaporation, but they need something like that on a huge scale IMO. It couldn’t be so solid that it wouldn’t let precipitation in, though. Seems like engineers could design some kind of mats to accomplish the purpose and could be manufactured by the square mile. Or maybe they could just find some kind of plant. Water lilies? Algae?

    • Jeffrey Whitaker


  • Jose Vasquez

    Probably no information on ground water levels because they are not as easily photographed.

  • Rah-Ben

    Clearly the pictures on the left show more water. More bogus news from liberal sources. #AlternativeFacts

  • DrToad

    This would be more useful if the before-and-after satellite photos had framed the same portions of each reservoir.

    • MrsP93

      But they do – quite precisely
      The images match completely where not submerged!

      • Hillary Clintub

        No, only the first one does. The others just show spliced photos.

        • Tony Pomeroy

          Slide the bar in the photo left or right, then you will be able to get the whole effect.

          • Hillary Clintub

            Thanks. Another poster also pointed that out.

    • 1.991km

      I thought the same thing, then realized you have to slide the center bar back and forth.

    • Language lover

      I stared a long time at the Shasta photo, trying to figure out how they could possibly be showing the same area (where was the road?). I realized the “slide” feature only when I got to the 3rd set of photos, but in my opinion, the “slide” obscures the parts of the photos that you want to compare. You can’t see the same area on both photos at once.

    • dzerres

      use the “slider” arrow-button – the photos cover the same territory but the first one may be twisted a little.

    • Dan Brekke

      We might not have been clear enough about what the images are and show. Each image covers exactly the same area. You compare the before and after view by dragging the slider bar to the left or right. I’ve added instructions to that effect above.

    • David

      I’m having the same issue in Internet Explorer – the far-left of one matches the far-right of the other, so there’s very little overlap…
      In Chrome it works perfectly!

  • sezwhom

    A tad misleading on Folsom. The lake was filled to just 41 percent capacity – 80 percent of its historical average…on Wednesday. They’re easing up on releases from the dam into the lower American River which should allow the lake to rise again. Remember, with Folsom, it’s more about snow melt than rain. It’s also much smaller (about 1/4 the size) than Shasta. Ironically, the latter gets most of it water from rain and not snow.

    • Dan Brekke

      You’re correct. I tried to call out the changes in the Folsom water level in my note on the photo.

  • Robert Jay Scovel Jr.

    should be compared using the same month

  • Chris Wunz

    Winter snowpack has a larger influence on long-term (read: summer) conditions than rain does, especially this early in the year. People always ask me what the summer fire season will be like because of this or that weather occurrence in the winter or spring. I always tell them I’ll let them know in September…

  • rshimizu12

    California needs to build more reservoirs. Part of the problem is that a lot of the water bond money is being wasted for not water storage uses.

    • David

      What do you base your opinion on (that we need more), and what is the money being spent on other than reservoirs?
      Fact is, reservoirs are expensive, take a long time to approve and build, ‘waste’ a lot of water due to evaporation, displace other productive / ecological land uses, and off-gas a lot of methane…
      storing it in the ground, and not using as much, may be more cost effective than reservoirs…Seems we should decide through a transparent benefit/cost model that includes ecological costs…

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