The California Department of Water Resources, (DWR) and the incident command team managing Lake Oroville, cited concern that erosion at the head of the auxiliary spillway threatened to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. Those potential flows could exceed the capacity of downstream channels.
To avert more erosion at the top of the auxiliary spillway, DWR doubled the flow down its main spillway from 55,000 cubic feet per second (cfs) to 100,000 cfs.
The concern was whether the concrete structure at the head of the auxiliary spillway remains intact and prevents larger, uncontrolled flows. Current flows are contained with downstream channels. Flow over the auxiliary spillway weir began Saturday morning and has slowed considerably. DWR officials expect that flow to stop entirely soon, which will reduce the erosion on the downstream side of the structure. Oroville Dam itself is sound and is a separate structure from the auxiliary spillway
Water levels in the reservoir have risen following heavy rain and snow after years of severe drought. However, it is the first time that Lake Oroville, which lies 65 miles (105km) north of Sacramento, has experienced such an emergency in the dams near 50-year history.
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