The effort to protect Oroville Dam entered a critical phase Monday when engineers shut off water flowing out of the damaged main spillway, giving officials their first unobstructed view of the eroded concrete chute since a crisis prompted mass evacuations earlier this month.
For the next five to seven days, geologists and engineers will have an unhindered view of the concrete spillway, which on Monday was revealed to have severely deteriorated on its lower half during the last two weeks of use.
Repairs for the spillway are estimated in the hundreds of millions of dollars, and no timeline has been set for when they would be completed.
With the water flow shut off — and with it, the frothing rapids in a plunge pool at the bottom — the top priority is to clean out tons of sediment, rock and debris that have prevented the Hyatt Powerplant at the dam's base from operating, said Bill Croyle, acting director for the state Department of Water Resources.
The plant helps provide power and drinking water for the surrounding community and, in dry times, allows engineers to control the reservoir's water inflow and outflow without having to use the eroded spillway, Croyle said.
Engineers need to get the plant up and running again before a weekend storm pushes the reservoir's level up and before the Feather River drops too low to support Chinook salmon downstream, Croyle said.
Monday's shutoff has been more than a week in the making and gives geologists, engineers and work crews a chance to assess the geology of the earth beneath the spillway and how it's eroding under different conditions.
Experts hope that studying the damaged area while it's dry may help them find the balance they need to control the reservoir's level with the spillway while also clearing debris at the bottom.
To this point, when water flowed too slowly off the damaged spillway, it sped up erosion and moved it closer to the dam — a dangerous process engineers cannot allow, Croyle said.
When water was released in large amounts — as it has since the reservoir briefly reached capacity weeks ago — there's no way to clear debris near the bottom.
"Our goal is to aggressively attack the debris pile," Croyle said Monday. "Once this sort of gets into a zero-flow environment, it'll be a little less stressful."
But water flow down the spillway isn't the only variable engineers have to consider, Croyle said. With the spillway closed, the reservoir will continue to fill. If it gets too close to a level officials deem unsafe, they will have to begin releasing water again.
The National Weather Service said rain isn't expected until the weekend, and it's too early to say how much could fall. Melting snow may carry lots of water into the reservoir this spring or summer, but it should not be anywhere near the pace that recently pushed the reservoir to capacity, said DWR chief hydrologist Maury Roos.
"At the moment, things look pretty safe," he said.
The main concrete spillway was damaged earlier this month after a week of powerful storms and after an earthen emergency spillway that was used when the reservoir reached capacity also rapidly eroded.
Officials need to keep the dam water levels in check because of damage to the emergency spillway that carries water when the reservoir goes above capacity. Damage to that spillway earlier this month prompted the evacuation of 100,000 people. Officials were able to use the damaged main spillway to reduce water levels, easing the crisis.
Interviews and records suggest that the near-catastrophe grew out of fundamental problems with the original design of the emergency spillway that were never corrected despite questions about its adequacy.
The "solid" bedrock that officials thought would stand up to the force of the spill was soft and easily eroded. The long concrete lip of the spillway was not anchored into the rock. Critical power lines were strung across the spillway, which consists of nothing more than an earthen hillside covered with trees and brush.
For breaking California news, follow @JosephSerna on Twitter.