Trinity Lake, the state’s third-largest reservoir, was filled to 97 percent of its storage capacity Tuesday, and a snowpack estimated at 150 percent of normal still looms over the watershed. If the reservoir were to overtop the dam, the results would be catastrophic, said Keith Groves, a Trinity County supervisor representing the district that includes Trinity Dam.
“It would take out bridges … and a big section of (Trinity County) would be wiped off the face of the planet,” Groves said. He said 3,500 people live in the immediate pathway of potential flooding.
Completed in 1962 as part of the Central Valley Project, Trinity Dam was the largest earthen dam in the world until it was eclipsed by Oroville. It sits on the Trinity River, 45 miles northwest of Redding, and just above Lewiston Lake, which was formed by a second dam 8 miles downstream. A 3-mile tunnel diverts water from the Trinity River Basin to the Sacramento River Basin, providing water for hydroelectric production at four power plants and irrigation to the agricultural industry in the San Joaquin Valley.
Concerns about the dam’s safety date to 1974, when an unseasonably warm storm, known as a “pineapple express,” dropped heavy rain and snow in the Trinity Alps. The water level in the lake rose so high it nearly overwhelmed the dam, said Groves.
At that time, the Reclamation Bureau, which operates the dam, reduced the reservoir level it considers safe to 1.85 million acre feet, around 75 percent of the lake’s total capacity. This week, the reservoir was holding more than 2.3 million acre feet of water.

We take projections and predominantly we see the inflows in the months of April, May and June. It's typically 90 percent of the inflows from Trinity is from snowpack only 10 percent is rain," said Bader.