The state Department of Water Resources has released a report on the Oroville Dam's "green spot," declaring the extensive area of persistent moisture on the face of the dam is due to seasonally trapped rainfall and poses no threat to the dam's integrity.
The department produced the report amid intense public speculation -- and a UC Berkeley-affiliated study -- about the source of the green spot and whether it signaled a serious structural flaw in the 770-foot-high dam that allows water to flow through the structure from the reservoir behind it.
The report cites historical evidence -- observations made during the dam's construction in the mid- and later 1960s, before the giant Lake Oroville rose behind the structure -- that the lush vegetation that marks the green spot during winter and spring months is the result of rainwater that becomes trapped in the area and slowly seeps out.
The report, which avoids using the term "green spot," quotes a 1967 observation by DWR's Division of Safety of Dams that the seepage area "should not be a problem except for public relations and maintenance" if other parts of the dam performed as designed.
Chief among the design features the report notes are the dam's dense "clayey" core and an internal drainage system that's intended to capture seepage through the structure. The report echoes earlier DWR statements that the level of seepage observed in the dam is low for a structure of its size.