State orders dam spillway inspections, including Don Pedro’s, after Oroville crisis

California officials have ordered owners of 93 dams to reinspect their flood-control spillways after the Oroville Dam crisis, saying the spillways need a closer look after a preliminary review.

The list released by the Department of Water Resources includes some of the largest dams in California, such as the New Exchequer Dam on the Merced River, New Bullards Bar on the Yuba River, and Lake Almanor Dam on the Feather River in Plumas County.

Also on the list is Don Pedro Dam, on the Tuolumne River, which contains the sixth largest reservoir in California. The Turlock Irrigation District owns the dam.

“Specifically, what they are looking at is whether or not conditions exist at Don Pedro that are similar to conditions that exist at Oroville, and we believe they are not similar,” TID spokesman Calvin Curtin said. He said the focus primarily will be on the geological conditions underneath the spillway.

Curtin added there are differences between the spillways. He said Oroville’s is used more often (Don Pedro’s spillway was opened in February for the second time in its history) and Don Pedro’s spillway is about a quarter mile from the dam and the water from the spillway flows down a separate channel before entering the river about a mile downstream.

Curtin said the spillway has done well in recent inspections. He added TID is completing a contract with its consultant to do the spillway inspection. He expects work will start in August and be completed by the DWR’s November deadline. An estimate for the cost of the inspection was not available.

Two dams operated by the Merced Irrigation District, New Exchequer and McSwain, also were included in the order.

The 93 dams represent less than 1 percent of the 1,250 dams overseen by the Department of Water Resources’ dam safety division. DWR said the probes were ordered in recognition of the emergency at Oroville, which prompted a mass evacuation, and that California’s dams are 70 years old on average.

Preliminary assessments showed each of the spillways on the list “may have potential geologic, structural or performance issues that could jeopardize its ability to safely pass a flood event,” according to letters the dam-safety division sent to the dam owners. “Therefore, we are requesting that you perform a comprehensive condition assessment of the spillway as soon as possible.”

DWR officials have been notifying the owners of the dams since June. The complete list was released Thursday by the agency.

“It will not be known which spillways, if any, will need repairs until the comprehensive assessments are completed and reviewed by (the dam-safety division),” DWR said in a note accompanying the list. “Dam owners of these spillways have been directed to perform any needed maintenance repairs prior to the next flood season. (The division) has already received immediate responses from many dam owners in compliance with the notice.”

Oroville’s crisis began when a giant crater opened in the main spillway Feb. 7. DWR officials throttled back water releases to limit the damage, but a heavy rainstorm filled Lake Oroville and water began spillway over the adjacent emergency spillway for the first time in the lake’s 48-year history.

When it appeared the emergency spillway might fail, officials ordered the immediate evacaution of 188,000 downstream residents. DWR quickly ramped up releases over the main spillway, arresting the flow of water over the emergency structure, and lake levels fell. The evacuation order was lifted two days later. The cause of the crater has yet to be determined. An independent forensic team has cited two-dozen possible factors but won’t finish its investigation until this fall.

Modesto Bee staff writer Kevin Valine contributed to this report.

This story was originally published July 27, 2017 at 3:50 PM with the headline "State orders dam spillway inspections, including Don Pedro’s, after Oroville crisis."