OROVILLE, Calif. _ Rainwater erosion alongside the Oroville Dam's main spillway appears to have contributed to the heavy damage that prompted a crisis, forcing more than 100,000 to be evacuated from their homes, a report reviewed by the Los Angeles Times showed.
A summary of the incident, prepared by state water officials four days after the crater in the concrete chute appeared, said flowing water from heavy rains hit the hillsides where the massive concrete spillway runs.
Flowing water during heavy rains was "diverted ... effectively eroding and undermining the spillway, causing a section to collapse," said the incident summary.
It's unclear whether the "flowing water" was a primary cause of the spillway damage or one of many.
The spillway follows the slope of a dirt- and tree-covered hill that helps secure the towering dam, which is America's tallest. The spillway fracture began as a 200-foot-wide hole that was 35 feet deep but over the last week has gotten significantly worse. Photographs of the first damage show soil washed away beneath the sidewall of the concrete spillway and along its outer edge.
The damage to the spillway began a chain of events that led to the mass evacuations. Officials turned off the spillway to investigate, and that caused water levels at Lake Oroville to rise. Then a storm dumped more rain than expected, causing the lake to exceed its limits and push water down an emergency spillway.
On Sunday, officials detected encroaching gullies that threatened that emergency spillway, and fearing that it could collapse, ordered numerous communities downstream to evacuate. Since then, officials have been using the damaged main spillway to lower the level of the reservoir, easing the crisis.
The state Department of Water Resources has provided no public statements on the cause of the collapse. Agency spokesman Chris Orrock, at the incident command post in Oroville on Thursday morning, said the cause remained under investigation. Orrock confirmed that the incident summary refers to the initial damage and not to subsequent damage to the concrete spillway when it was put back into use.
Bill Croyle, acting director of the California Department of Water Resources, said determining the cause of the spillway collapse could take months.
Large swaths of Northern California are on track to experience their wettest winter on record, with many areas having already surpassed their average precipitation for an entire year.
