The Shasta Sheriff's Office says the kids slipped down a riverbank in part of a small landslide.
They were hurt by the rocks and debris. Both were pronounced dead at the scene, authorities said.
Numerous first responders including the sheriff's dive team rushed to the scene around 9 a.m. about 200 miles (322 kilometers) north of San Francisco, the statement said.
They had been at a local campground when it happened.
"This is a camping area and the people were at the camping area, so it has nothing to do with any field trip groups or that type of thing," said sheriff's spokesperson Tim Mapes.
"It just is an absolutely tragic incident," he said. "We offer our condolences."
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The names of the juveniles were not immediately released.
The Sheriff's office says the area where this happened is downstream from the dam, and the Bureau of Reclamation has now temporarily reduced water releases as they complete their investigation.
The Associated Press contributed to this report
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