A Lake Oroville Community Update from the Department of Water Resources

August 30, 2017

The California Department of Water Resources (DWR) today published the Assessment of the Vegetation Area on the face of the Oroville Dam.

This past Friday, DWR shared these findings and conclusions with the independent Board of Consultants convened for the Lake Oroville Spillways Emergency Recovery Project, who provided feedback to the Department last week. The Board of Consultants agreed with DWR’s findings that the vegetation is caused by rain and there is no dam safety concern. The BOC will provide official comments in their next memo which will be posted here.

Highlighted conclusions in the assessment:

– The vegetation area poses no threat to the integrity of the dam. The area is caused by temporarily trapped rainwater.

– The area was observed in 1966 and 1967 during construction, before the reservoir was filled. Construction operations during the 1966-67 rainy season resulted in ponding of water on the fill surface and the creation of stratified lenses or layers within the fill in the area where vegetation is now present.  In addition, rainfall during the 1966-67 wet season also resulted in small erosion channels, or rills and gullies in the area where vegetation is now present. These rills and gullies were repaired with cobbles, not regular dam fill, which allows rain water to percolate into the area rather than drain quickly down the face of the dam.  Numerous inspection reports and aerial photographs taken during construction and prior to reservoir filling document the presence of wet areas seeping rain water that had percolated into the dam.

– Ongoing seepage measurements at the base of the dam have remained consistently low and virtually unchanged since construction of the dam. Any increase in seepage through the dam would be noticed almost immediately.

– The vegetation area dries out during the hot summer months, which it would not do if it had a constant source of seepage through the dam. The vegetation growth cycle begins again during the next wet season, indicating that it is rain-fed growth.

– The vegetation area is currently brown and dry despite the Lake Oroville lake level sitting almost 100 feet higher than the vegetation area. If any water did make its way through the dam’s core, it would be intercepted by a vertical drain, preventing it from flowing to the downstream area of the dam where the vegetation is located.

We hope this assessment clears up any misunderstandings about the integrity of Oroville Dam. As always, please feel free to reach out if you have any additional questions.

— Oroville Outreach Team