Today’s the first day of summer. Do you know how full your reservoir is?

The map below shows fluctuations since 2010 in California’s 30-largest reservoirs. It updates monthly, using data from the CA Department of Water Resources.

Grey borders mark the maximum capacity of each reservoir, while the blue shows fluctuating storage levels over time. All reservoirs are scaled relative to maximum storage capacity (with the largest — Shasta Lake — at 30 pixels). Click on each reservoir to see the change in capacity since 2010, and view changing percentages over time by mousing over the reservoir pop-out to the left of the chart.

Source: California Department of Water Resources

First the good news: El Niño served California pretty well this winter. At the end of the rainy season last month, the Sierra Nevada snowpack looked a good deal more robust than it’s been in years. A series of March storms helped refill many of Northern California’s largest reservoirs to near-capacity levels, which will be critical in abating short-term drought impacts this summer.

The less-good news: California still hasn’t shaken it’s long-running drought. Many of the smaller reservoirs in the south remain below half capacity, and the Sierra snowpack is less than 20 percent of its historical average, (as measured since 2003), according to NOAA’s SNODAS model.

The following graph, produced by data journalist David Kroodsma, uses figures from the National Snow and Ice Data Center to show the estimated volume of water in California’s snowpack on a given day as compared to a ten-year average (since 2003). At the end of March 2011, the state’s snowpack held more than 37 million acre-feet of water. But by the end of March 2015, there was only about one million acre-feet, or 8 percent the historical average, the lowest on record. This year, snowpack levels have risen but are still far below average as California head’s into another long, hot summer.

Major Bay Area water sources


More on California’s water sources

  • Kent H

    Please, please update the graphic with latest available data!

  • cowboybob

    this is great. Lake Shasta is near capacity of 4.5mil, Lake Oroville is near 3.4mil, the small reservoirs are not near capacity, but that is ok because they are small, can’t compete with the monsters. Remember city folks, conserve 25%, which saves 4% of state water so we farmers can grow more alfalfa for export and makes us money to give to Gov Brown as legal campaign contributions. Gov Brown is the greatest. .


Victor Powell

Victor Powell is a Berkeley-based web developer, specializing in data visualization. He runs the consultancy Setosa.

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