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 Times showed. A summary of the incident, prepared by state water officials four days after the crater in the concrete chute appeared, said 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.