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10 things Southern Californians should know about the Oroville Dam crisis

Lake Oroville, the emergency spillway, and the damaged main spillway, are seen from the air on Feb. 13, 2017 in Oroville, California.
Elijah Nouvelage/Getty Images
OROVILLE, CA - FEBRUARY 13: Oroville lake, the emergency spillway, and the damaged main spillway, are seen from the air on February 13, 2017 in Oroville, California. Almost 200,000 people were ordered to evacuate the northern California town after a hole in an emergency spillway in the Oroville Dam threatened to flood the surrounding area. (Photo by Elijah Nouvelage/Getty Images)

Lake Oroville, a major source of water for Southern California, has been all over the news the last couple of days. Officials are scrambling to release enough water from behind the Oroville Dam to keep the reservoir from overflowing. The releases are also necessary to make room for even more storm runoff expected later in the week. But letting water out of the dam is also causing major erosion in the two spillways explicitly designed for this purpose, heightening the risk of flooding in communities downstream. Yesterday, evacuation orders were issued for nearly 200,000 people. 

Where is Lake Oroville?

Lake Oroville is a man-made reservoir on the Feather River in Northern California. It’s 80 miles north of Sacramento and 35 miles southeast of Chico. Oroville Dam, which was completed in 1968 to create the lake, is tallest dam in the United States.

I've been reading a lot about spillways. What are those and how do they work?

Think of Lake Oroville like a big bathtub. Your tub has a drain at the bottom and a drain at the top to keep the tub from overflowing. Lake Oroville works the same way. The drains at the top are called "spillways," and there are two of them: the main spillway and the emergency spillway.

Department of Water Resources engineers prefer using main spillway because it has a gate they can use to control the flow. The emergency spillway, is out of their control: water flows out of it when the lake gets high enough, and stops when the level drops.

What caused the current crisis?

Primarily, the incredibly wet winter we have been having. Snowpack in the northern Sierra Nevada is 224 percent of normal, and this winter is on track to be the wettest on record in California.

With Lake Oroville nearing full last week, the DWR began releasing huge volumes of water out of the main spillway to lower the level of the lake. They did this to avoid the lake overflowing and to make room for future storms.

But the water began to eat away at the concrete in the main spillway, creating a huge hole and eroding the ramp. So they stopped releasing water.

Then, it stated raining this past weekend. So DWR began releasing water again though the main, damaged spillway, but the lake kept going up, enough to cause water to flow over the emergency spillway for the first time since the dam was finished in 1968.

Then, DWR discovered that the water flowing over the emergency spillway was causing it to erode. Suddenly, officials worried the emergency spillway would fail, causing a wall of water to gush into the Feather River and flood downstream communities. So the county sheriff ordered mass evacuations.

Is Oroville Dam going to fail?

No. While there are serious concerns about the integrity of the emergency spillway -- concerns that began years ago -- the dam is separate and is structurally sound.

How bad could it get if the emergency spillway fails?

We don’t know. But a lot of water could come cascading out of the top of Lake Oroville into the Feather River below, potentially overflowing downstream levees. The only map DWR has on file is of what would happen if Oroville Dam failed – which would be much more catastrophic.

Will the flooding affect Southern California?

No. The flooding is local to the Feather and Sacramento River watersheds.

Where do things stand now?

Lake Oroville is still nearly 100 percent full but water levels are dropping as DWR dumps more water out of the main spillway than is flowing in. Damage to the main spillway appears to have stabilized, according to DWR’s acting director Bill Croyle. His agency inspected the emergency spillway this morning, and repairs are underway to both spillways.

CA DWR spillway repairs

What’s happening to all the water DWR is releasing? Is it being wasted?

As engineers frantically try to lower the water level of the lake, they are releasing a lot of water from the dam’s main spillway. That water is flowing out down the Feather River to the Sacramento-San Joaquin River Delta and eventually to the sea . But it’s not technically being wasted because there’s nowhere we could actually store it, said Jay Lund, a professor of civil and environmental engineering at UC Davis. There simply isn’t enough water storage in California to hold this year’s historic snow and rainfall.

How much of Southern California's water comes from Lake Oroville in particular?

Lake Oroville is the largest reservoir in the State Water Project, the system of reservoirs and canals that supplies nearly a third of Southern California’s drinking water. Built in 1960, the SWP is the critical piece of infrastructure that has allowed Southern California to flourish.It moves water from the Lake Oroville 444 miles to Lake Perris in Riverside County, irrigating farms and providing drinking water to cities along the way.

In the winter, hardly any of our water comes from Lake Oroville. That’s because the SWP is flush with rain and snowmelt flowing into it from a myriad of rivers, lakes and streams. Oroville’s storage isn’t needed.

But come summertime, when those natural flows have dried up, Lake Oroville becomes a principal source of water for the SWP. Oroville will have, ideally, been filling up all winter. Then, engineers will slowly release that water so Southern Californians can drink and water their lawns, and farmers can irrigate all summer.

What’s going on with the dams in Southern California? Are they safe?

The conditions that led to the Oroville crisis are too much rain and snow melt at the same. We don't have that problem here, but we do have a number of dams in Southern California. And our main problem isn't too much water all at once, it's really sediment washed down from mountainsides that were burned during catastrophic wildfires, like the Station Fire in 2009. The sediment means there’s less space in the reservoirs to hold storm water.

 In Los Angeles County alone, five of the 14 flood control dams are filling up. Devil’s Gate Dam in Pasadena is in the worst shape. County officials have been trying for years to remove 2.4 million cubic yards of sediment from behind the dam, but environmentalists have sued to block them. They want less sediment removed.

They point to this winter in particular and argue that we've gotten above average rain and the dam hasn't overflowed, so we don't need to take that much sediment out. But officials with the LA County Flood Control District say the rain we've gotten has been parceled out in steady chunks, so we haven't had the kind of intense rains they've had up north.  

On Monday afternoon, LA County Supervisor Kathryn Barger introduced a motion to inspect all the county's dams.