The frantic effort over the last few days to lower water levels at Oroville Dam after the structure’s two spillways became damaged is part of a larger drama playing out as California rapidly shifts from extreme drought to intense deluges.
Large swaths of the region are on track to experience their wettest winter on record, with many areas having already surpassed their average precipitation for an entire year.
And all that water is putting new strains on the network of dams, rivers, levees and other waterways that are essential to preventing massive flooding during wet years like this one.
The biggest danger zone lies in the Central Valley at the base of the Sierra Nevada, whose tall peaks can wring the skies of huge amounts of rain and snow. The area is essentially one giant floodplain that would be easily transformed into an inland sea without man-made flood control. At 400 miles long and 40 miles wide, it has only a tiny bottleneck from which to drain — a one-mile opening at the Carquinez Strait at San Pablo Bay — before water heads into the San Francisco Bay.
LIVE UPDATES: Oroville reservoir level continues to drop amid new rain storms
“You got this big bathtub — water doesn’t flow out of it very quickly,” said Jeffrey Mount, senior fellow at the Public Policy Institute of California and former director of the UC Davis Center for Watershed Sciences.
As the site of the nation’s tallest dam and the main storage for the State Water Project that sends water to the Southland, Lake Oroville has commanded national attention as the crippled spillways forced the evacuations of more than 100,000 downstream. But smaller water systems are also under intense pressure.
Sixteen reservoirs, ranging from small to the biggest in the state, were above 90% full as of Wednesday morning.
Among them is the Don Pedro Reservoir, the sixth-largest in California and located near Yosemite National Park. As of Wednesday afternoon, it stood at an elevation of 827.4 feet, just shy of its 830-foot capacity, the Turlock Irrigation District said in a statement. The district continued to make releases to the Tuolumne River, which flows through Stanislaus County and into urban centers such as Modesto.
Forecasters predict about 4.7 inches of precipitation could fall in the watershed over the next six days. Although the irrigation district said it does not anticipate an overflow, it advised residents of Stanislaus and Merced counties to register for emergency notifications.
Even as rain began to fall Wednesday, Croyle said the storms forecast over the next few days will not be enough to test the integrity of the Oroville Dam or its two damaged spillways. He said the public “won’t see a blip in the reservoir” levels, now dropping about 8 inches an hour.
Officials at the dam said their biggest worry wasn’t the weather but the damage done to the dam’s already compromised main spillway during days of sustained pounding from heavy releases of water. When the emergency spillway began to fail Sunday, officials sent massive amounts of water down the main spillway — despite the damage — in a desperate effort to reduce the water level in the reservoir.
“It’s holding up really well,” Croyle said of the main spillway. But he added continued mass water releases could be causing hidden damage to the rocky subsurface adjacent to the concrete chute.