Ask the R-S: While Lake Shasta is brimming, Trinity Dam is not even half full. Why?
While Lake Shasta is brimming with water, Trinity Lake is less than half full.
There are several reasons why that happened this year, according to the Bureau of Reclamation, and why the situation at Trinity Dam may not be as dire as it sounds.
Winter rainstorms filled Lake Shasta to 98% of its capacity, 116% of its historic average for this time of year, according to the California Department of Water Resources.
The water level at the dam is lapping a little more than three feet from the top, Bureau of Reclamation Area Manager Donald Bader said.
“We cranked the releases up,” letting more water out through the dam because of the storms this week and last weekend, Bader said on Tuesday. “We don’t want to get totally full,” and there’s still water coming into the lake from this week's rains. “We want to leave a little bit of room. We’re going to creep up as close as we can to the top, but we’re not going to go over,” he said.
But 40 miles northwest of Redding, Trinity Lake is only 39% full: 49% of its historic average. That means its water level sits 120 feet below the top of the dam, Bader said.
There are three reasons why Trinity didn’t fill the way Shasta did this winter, he said.
Trinity Lake had the worst consecutive three years of drought on record for that reservoir. Its last “good year” ― one when the water level reached its historic average ― was 2019. Those years of drought taxed the reservoir’s storage, so it started the year low, Bader said.
The good news is Trinity Lake “got more water and snowmelt this past year than we did in the previous three years combined,” he said.
Another reason water levels differ is the way the two reservoirs fill up, Bader said.
Lake Shasta has a big drainage basin. While Trinity Lake is a large reservoir, it has a very small basin. That means Trinity fills more slowly throughout the year as mountain snow melts, he said.
“It’s harder to refill Trinity in one season. You go over Buckhorn Mountain, the big divider between both dams. Trinity has a really small area above the dam for drainage.” Bader said.
But Trinity Lake won’t get as much melt water this June as some other California reservoirs that rely on snowmelt, according to Bader.
Overall, California saw record amounts of snow last winter, including mountainous parts of Siskiyou County that got more than 200% of historic snowfall norms, according to the National Weather Service.
But most of the wet storms that dumped rain and snow on much of Shasta and Siskiyou counties bypassed Trinity Lake, Bader said, and the area only got about 120% of its historic snowfall.
The good news is there’s more water on the way for Trinity Lake as snow as far north as southwestern Siskiyou County melts and flows into the reservoir, Bader said. The lake could reach 50% of its capacity this summer.
North State waters directly and indirectly fed by Trinity Lake are getting a little help from Shasta Dam, the largest reservoir in California. That's because the areas the two reservoirs cover overlap, Bader said.
Water from Trinity Lake is often used to help fill Whiskeytown Lake, but that won’t be necessary this year because there’s enough water being released through Shasta Dam to compensate, he said. Having so much water in Lake Shasta it's necessary to release it “is a good problem to have.” It means the water supply outlook is sunny going into next winter, Bader said.
Meanwhile, the bureau is saving every drop it can in the Trinity Reservoir, he said.
“Shasta is going to end the summer very high in storage. That means the Bureau of Reclamation can conserve water in the Trinity Dam this year," Bader said. "We’re not scheduling any Trinity water to be released this year” into the Sacramento River, and “hopefully the area will have a good wet winter next year.”
This story resulted from a question to the reader-driven Ask the Record Searchlight, our newsroom effort to answer the community's questions about curious happenings across the North State. Please submit questions you'd like us to tackle to RRSEdit@redding.com using Ask the Record Searchlight in the subject line.
Jessica Skropanic is a features reporter for the Record Searchlight/USA Today Network. She covers science, arts, social issues and news stories. Follow her on Twitter@RS_JSkropanic and onFacebook. Join Jessica in the Get Out! Nor Cal recreation Facebook group. To support and sustain this work,please subscribe today. Thank you.