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.
The report does not address an alternative explanation for the green spot -- that it's fed by a natural spring uncovered during construction of the dam -- that has been raised in past inspections and safety reviews of the dam.
As part of the dam's five-year federal safety review in 2014, for instance, a panel of independent consultants wrote that "the green spot is believed to be associated with pre-existing natural springs in the downstream left abutment area of the dam foundation."
The concentration of moisture, the panel's report said, could be due to the composition of the rock and earth used to build the dam’s downstream embankment. The fill, which may contain excessive volumes of very fine, dense material, “may prevent free drainage of flows from those underlying springs.”
“This issue has a high historical profile that needs to be conclusively addressed,” the consultants said in their report to the Federal Energy Regulatory Commission. In a 2016 letter to FERC, DWR said it was exploring the issue and intended to deliver and answer by late 2018.
Les Harder, a former deputy director of the Department of Water Resources who currently serves as an outside consultant for DWR, said in an interview Wednesday that a spring could account for some moisture observed on the dam's downstream face.
But, Harder said, "it's not reasonable to assume it's contributing in any way to the wet area" on the dam.
Harder, a 30-year department official who moved to frequent DWR contractor HDR 10 years ago, insisted that the source of the green spot has been well understood since at least 1967.
"People saw this before there was any reservoir," he said. "This behavior, the wet spot and vegetation, developed during construction."