The state shut down releases from the spillway for a time but then restarted them to counter inflows to Oroville Lake from the week's storms.
The water level at California's second largest reservoir crept to within half a foot of the top early Saturday, making it increasingly likely that Lake Oroville would start lapping over its emergency spillway.
Earlier this week, chunks of concrete flew off the almost mile-long spillway, creating a 200-foot-long, 30-foot-deep hole. Throughout the week, they have attempted to stem the rising reservoir levels by releasing test flows of 20,000 to 65,000 cubic feet per second.
A mass of white-water cascaded down the damaged spillway on Friday after officials upped the flow in an attempt to avoid disastrous overflowing of the reservoir, SFGate reports.
This is the first time the uncontrolled emergency spillway has been used in the dam's 48-year history.
For fear of mass fish kills amid the debris- and mud-filled water flowing into the hatchery, state Fish and Wildlife crews have rushed to transport about four million baby salmon to a downstream hatchery that is safe from rapid inflows.
Water flows through a break in the wall of the Oroville Dam spillway on Thursday. Oroville and other lakes are brimming and have begun releasing water to make room for more runoff.
The cost could approach $100 million, though department spokesman Doug Carlson said the estimate by a department engineer is an early, ballpark figure.
A rainbow appears over Feather River as water cascades down the damaged spillway at Lake Oroville Dam on February 10, 2017.
California's Oroville Dam is the tallest in the United States.
It was 90 percent full Thursday and more water than anticipated was flowing into it thanks to a combination of warmer than typical temperatures and rain falling on snow, said Bill Croyle, acting director of the Department of Water Resources.
Officials said they expected the cavity to widen as a result - and it did.
A never-before-used emergency spillway, located down an adjacent hillside, is one possible solution if the reservoir overflows, but Carlson said this option isn't ideal. The emergency spillway, a lip on the north end of the dam, is at 901 feet.
On Thursday, a fish-rescue effort involving about 40 people and tanker trucks took 4 million chinook salmon to safety.
Associated Press writers Kristin J. Bender and Ellen Knickmeyer contributed to this story from San Francisco.