Water started flowing over an emergency spillway at the nation's tallest dam, on Lake Oroville, for the first time Saturday after erosion damaged the Northern California dam's main spillway.

Officials hoped to avoid using Oroville Dam's emergency spillway, fearing it could cause trees to fall and leave debris cascading into water that rushes through the Feather River, into the Sacramento River and on to the San Francisco Bay. Crews prepared for several days, clearing trees and brush.

Water began running over the emergency spillway around 8 a.m., said Ted Thomas, a spokesman for California's Department of Water Resources. It was the first time the emergency spillway has been used in the reservoir's nearly 50-year history.

Unexpected erosion chewed through the main spillway earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don't know what caused the cave-in that is expected to keep getting bigger until it reaches bedrock.

A spokesman for the Water Resources Department has said officials estimate the cost of repairing the dam could approach $100 million, but noted the estimate was an early, ballpark figure.

Officials have emphasized Oroville Dam is sound and there is no imminent threat to the public.

State officials also had been attempting to rescue millions of hatchery-raised fish imperiled by muddy water flowing downstream alongside the damaged spillway after sections of its concrete walls collapsed earlier this week

About 150 miles northeast of San Francisco, Lake Oroville is one of California's largest man-made lakes, and the 770-foot-tall Oroville Dam is the nation's tallest. The lake is a central piece of California's government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in Southern California.