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Oroville – The state Department of Water Resources halted flows down Lake Oroville’s spillway Monday afternoon in an effort to get the Hyatt Powerplant up and running and to assess damage.

In order to use the powerplant under the dam, the department needs to remove debris in the Diversion Pool at the base of the spillway that started crumbling Feb. 7.

DWR needs to remove the debris, which has in effect created a dam across the river below, and lower the water level in the Diversion Pool to a point at which it is deemed safe to operate the plant.

The debris build-up began when a major break opened in the concrete chute this month. With no other way to get enough water out of the lake, the department had to let the spillway flow with the realization the bottom part of the ramp would be torn up.

With the absence of flows, the department is able to survey the extensive damage to the spillway. The flow down the broken spillway carved a new path and a huge ravine south of the spillway, toward the dam. That debris, rocks, trees and dirt, mostly ended up in the Diversion Pool.

DWR spokesperson Lauren Bisnett said the department estimated at 3 p.m. Monday that there was 500,000 to 1 million cubic yards of debris in the Diversion Pool. The higher figure is the equivalent of a football field stacked 470 feet high with debris, she said.

Figuring that mass in terms of dump truck loads that must be hauled away, the average commercial dump truck can carry up to 18 cubic yards of material. That’s 55,556 truck loads that will be hauled away and dumped near the dam.

Bisnett said erosion measurements in and around the spillway were still being taken.

“We knew there was going to be significant damage,” she said.

At a press conference Monday morning, DWR acting Director Bill Croyle said he hoped to have the plant operating this week but couldn’t give an exact date. Use of the spillway is anticipated again in March, he said.

“This is a big day. There’s going to be a lot of changes,” Croyle said.

He said the department may not go “full blast,” or 15,000 cubic-feet per second, right away through the powerplant. Previously a department spokesperson said the maximum flow from the plant was 13,000 cfs.

When asked how many turbines were operational, he said one of the six was not. To compensate for the stopped spillway flows into the river, the department is ramping up releases from the Thermalito Afterbay, Croyle said.

Flows began decreasing starting at 6:45 a.m. Monday morning, down from 55,000 cfs. As of 2 p.m., inflows to Lake Oroville were coming in at 24,089 cfs, a department spokesperson said.

The lake’s surface was at 838 feet of elevation. That’s 63 feet below its 901-foot capacity, which when exceeded causes flows to go down the problematic emergency spillway as happened a week and a half ago, triggering evacuation orders for more than 180,000 people.

With the spillway turned off, there’s still enough space in the lake to handle more than 40 days at the current rate of inflow without filling, the DWR said.

Lowering flows will allow the department to see if it’s possible to operate the spillway with flows lower than 50,000 cfs without causing further damage to the structure, Croyle said.

“Our goal is to better understand the dynamics of that spillway,” he said.

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Though the department expects to have to use the spillway again soon, Croyle said it doesn’t want to get into a pattern of turning the spillway on and off. He mentioned that inflows for June were expected to go down to 12,000 cfs, which the powerplant could handle on its own, without adding to storage levels. Inflow levels in February were 570 percent above average, Croyle said.

“We don’t want to get into this yo-yo thing, where were loading up, ramping down,” he said.

Bisnett said crews were monitoring pools in the river from above and would rescue any trapped fish on Tuesday.

Butte County Sheriff Kory Honea said an evacuation plan for residents downstream of the dam is now complete and he was meeting with entities on Monday to finalize it. He said later in the week, he would meet with local media to discuss those plans. Preparing a social media campaign and community meetings in areas affected are also in the works, Honea said.

“Everybody (will have) an opportunity to look at the plan and understand the key early warning triggers,” he said.

He said he was optimistic about not having to use the emergency spillway again because of the work by the DWR, the U.S. Army Corps of Engineers and private contractors. Croyle said the four primary work areas of the emergency spillway were complete.

When asked if the emergency spillway repairs had been tested, he said it isn’t possible to test the area without using it and assured the department is doing everything it can to mitigate concern.

Honea and Croyle emphasized the no-fly zone in the spillway area is still in place, as the Department of Water Resources is operating helicopter and drone missions. Honea said that would be in effect until May 13 unless he lifted it sooner. Exact coordinates and more information about the no-fly zone is available on the FAA website.

“I understand people are interested to see what’s going on but there will be lots of photographs, videos taken so everybody will be able to see what’s going on,” he said.