100-year-old Shasta County dam creating conditions of 'extreme peril'

Damon Arthur | Redding

With winter rains on their way, officials worry a dam that creates a small lake 17 miles west of Redding could collapse, inundating downstream homes with up to 20 feet of water if sediment and debris clogging two outlet pipes is not cleared.

Two 30-inch outlet pipes at Misselbeck Dam have been clogged with silt and debris since last summer, forcing water from Rainbow Lake to flow over the top of a deteriorated 100-year-old spillway, said Charles Tucker, president of the Igo-Ono Community Services District, which owns the dam.

Currently there is no imminent danger from water flowing over the spillway, he said. But with rains due next week and winter approaching, the situation could deteriorate, and possibly lead to the collapse of the dam, district officials said.

"We're waiting for help from folks who should be helping us and we're rather disappointed so far," Tucker said. "We're in a pretty ticklish situation. We need some big boy help."

The district has sought help from the Federal Emergency Management Agency and the California Office of Emergency Services, but they have been slow to react, he said.

Noting "conditions of extreme peril to the safety of persons and property," the Shasta County Board of Supervisors declared a local emergency in July and renewed that emergency Nov. 5.

"The damage from a dam breach when Cottonwood Creek is already at flood stage is exponentially greater, likely reaching many structures in the Cottonwood area," according to a letter the district sent to the supervisors.

The outlet pipes became plugged after tons of sediment from hillsides surrounding the 113-acre lake were washed into the reservoir last winter and spring during storms.

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The 2018 Carr Fire left a burn scar covering 90% of the 12-square-mile watershed above the lake, according to a letter the district sent to the supervisors last summer.

Without vegetation to hold soil in place above the lake, the rains washed tons of sediment, and other debris washed into the lake, burying the pipes in about 20 feet of soil, much of it decomposed granite, Tucker said.

The district cannot afford the $1.7 million to $2.1 million to clear sediment from the lake so water can again flow through the outlet pipes. District officials have asked for help from FEMA and Cal OES.

But funding from FEMA is in question because the agency told the district the problem is not related to the Carr Fire, he said.

With the outlets plugged, the lake level can't be lowered, forcing water over the spillway, according to the district letter. The California Division of Safety of Dams has determined the spillway is unsafe to use.

There are holes completely through the spillway floor and water is scouring away the decomposed bedrock supporting the spillway. Winter rains could completely destroy the spillway, the district said.

Floating debris is also building up along the spillway, raising the level of the lake. If that continues, water could begin to flow over the top of the dam, which could lead to "total failure of the structure," the district said.

The letter says the Office of Water Safety at Sacramento State University estimates that if water overtops the dam, the structure "would be destroyed for its entire height and nearly its entire width in a period of 6 minutes."

"Failure of the dam or spillway would endanger lives and property between the dam and Cottonwood," the district said.

Several structures on Sunny Hill Road downstream of the dam would be inundated with 20 feet of water within 15 minutes of dam breach and multiple structures on Gas Point Road would be under 10 feet of water, the district said.

Structures near the main stem of Cottonwood Creek and structures on Ponder Way and Denise Way would also see water up to 5 feet deep, the district said.

"The worst-case scenario is the dam breach occurring when Cottonwood Creek is already at flood stage, which is the more likely scenario given the current outlet works condition," the district said.

County Public Works Director Pat Minturn said his office was aware of the problems at the dam and his office had determined that bridges downstream of the dam are tall enough that they could withstand the water flow if the dam broke.

It wasn't clear if residents and property owners living downstream of the dam had been notified about the issues at the dam. Officials at the sheriff's office could not be reached for comment Thursday afternoon.

Damon Arthur is the Record Searchlight’s resources and environment reporter. He is among the first on the scene at breaking news incidents, reporting real time on Twitter at @damonarthur_RS. Damon is part of a dedicated team of journalists who investigate wrongdoing and find the unheard voices to tell the stories of the North State. He welcomes story tips at 530-225-8226 and damon.arthur@redding.com. Help local journalism thrive by subscribing today!