California Drought Snapshot: Lake Oroville Revisited

A little more than a year ago, I went car camping to the very nice Loafer Creek Campground at Lake Oroville State Recreation Area. The lake, the main reservoir for the State Water Project and the second-largest California reservoir after Lake Shasta, was about 85 percent full at the time. If you were following the vagaries of the state's 2012-13 water season, you might have been a little troubled by the fact the rains had virtually ceased after the turn of the new year. What wasn't apparent during that March 27, 2013, visit to Lake Oroville was that the rains wouldn't return in the fall either, and that the lake would fall to just one-third full by January — low in any season, but especially alarming in that the reservoir levels here and virtually everywhere else across the state continued to decline at a time when they'd usually be filling up with runoff from fall and early-winter storms.

I drove back up to the lake on Jan. 18, the day the reservoir reached its lowest point during the current water year (which for the state Department of Water Resources runs from Oct. 1 through Sept. 30). The difference in the lake's appearance was dramatic. But when seasonal rains finally returned in early February, the lake began to rise.

One way of measuring lake level is the height of the lake surface above sea level. When full, Lake Oroville's surface is 900 feet above sea level. When I visited in March 2013, the surface level was 860 feet; when I went back in January, it stood at 701 feet. The lake's surface elevation is now at 762 feet and rising.

Last weekend, I went back up to Oroville to take a look and take a new set of pictures to show the change since January. My impressions:

I suppose this is a "glass half-full/half-empty" exercise on a grand scale, especially since the lake is at almost exactly 50 percent of its total capacity right now. On the one hand, the lake is up 61 feet from the last time I saw it and has added about 40 percent to its storage -- it's added about 500,000 acre-feet since January, enough water for about 1 million California households. More water is coming, too: Even though the forecast for the next couple of weeks and beyond looks pretty dry, and even though we're nearing the tail end of the rainy season, the snowpack will start to melt and run down the branches of the Feather River that flow into the lake.

The conventional wisdom is that half of the state's stored water is captured in the Sierra snows that wind up in streams, rivers and reservoirs. One slice of Lake Oroville history shows how dramatic an impact the snowpack can have:

Drier-than-normal water years in 2007-08 and 2008-09 reduced the reservoir's long-term storage, and after a dry start to the 2009-10 season, Lake Oroville's levels fell to a shade more than 1 million acre-feet, less than 30 percent of capacity, and lowered the surface to 665 feet above sea level by early January 2010; that's about 20 percent less water and about 36 feet lower than the level we saw this past January. Then storms began arriving, dumping more than 20 inches of rain at the dam over the next four months and building the northern Sierra snowpack. The water content of the snow in the Feather River drainage reached about 130 percent of normal by early April 2010. The reservoir, which had reached its lowest point on Jan. 11, kept rising through June 29, when it reached its high point of about 2.7 million acre-feet and elevation of 843 feet above sea level. That's a rise of 178 feet in less than six months.

So that's the glass half-full. It's normal for our reservoirs to rise and fall, often dramatically. An empty-looking reservoir can fill up in a hurry when enough rain and snow arrive. And no, I'm not addressing here the impact of how the reservoirs are operated — how much water is released, when and why. To a certain extent, water managers need to be risk-takers, basing decisions on how much water they send to farms and cities downstream on educated guesses about long-term weather and watershed conditions. A year ago, when Lake Oroville was pushing 90 percent capacity, the Department of Water Resources was willing to freely release supplies. This year, as my KQED Science colleague Lauren Sommer reports, managers are cutting deliveries to virtually nothing and conserving as much of the reservoir supplies as they can just in case the drought continues into next year.

And that brings us to the empty half of the glass for Lake Oroville: This year, the DWR estimates that the water content in the thin layer of snow in the Feather River watershed is just 13 percent of average for this time of year. Thirteen percent. So, we're not going to see any late season rise in the lake like the one in 2010. More likely, we'll see a scenario more like the one that unfolded in 2007-08, when drier-than-normal years left the lake at close to the same level we see today — 753 feet. The watershed's snowpack was also lower than normal, and though snowpack runoff gave the lake a boost, the reservoir topped out at just 760 feet and 50 percent capacity in late May. That dry rain year was followed by another, and in February 2009 the state declared a drought emergency.

None of this is meant to make a single reservoir, even a big one like Lake Oroville, seem more important than it really is. But in a state where it typically doesn't rain six months of the year, all that stored water makes it possible for 38 million people to live side by side with the nation's richest farm economy. And at the moment, Lake Oroville's water storage happens to mirror what's happening with the state's water supply picture as a whole: The Department of Water Resources' daily summary of 44 key reservoirs shows them collectively at 64.4 percent of average for today's date. Lake Oroville is at 65 percent.