Photo: The Associated Press

Swift water rescue teams conduct an exercise on the American River in Sacramento.

California rivers fed by this winter's massive Sierra Nevada snowpack have been turned into deadly torrents, drawing warnings from public safety officials ahead of the Memorial Day weekend's traditional start of outdoor summer recreation.

At least seven people, including two children, have died or gone missing this spring in the grasp of powerful rivers plunging down from California's towering mountain range, and there have been numerous rescues.

“This year we’re seeing higher water, faster water and colder water,” said Capt. Justin Sylvia, a fire spokesperson in Sacramento, which is crossed by the American River.

Sacramento has already had 20 water rescues this year, nearly as many as all of 2022, Sylvia said Tuesday as crews practiced swift-water rescues on the lower American River near its confluence with the Sacramento River.

Memorial Day weekend is typically one of the busiest, if not the busiest, times of the year, and “floating down the American River is like a quintessential Sacramento activity,” said Ken Casparis, spokesperson for Sacramento County regional parks.

“Probably thousands of people use the river for floating or swimming or rafting, what have you, and this weekend conditions are shaping up to be pretty dangerous, so we have been urging people to stay off the river,” he said.

Even just wading along the shore is being discouraged, said Casparis, who was hoping for chilly weather to discourage river use. Forecasters predicted mild weather in the interior of Northern California except for chances of thunderstorms in the mountains.

With Californians expected to flock to the outdoors, the Governor's Office of Emergency Services on Thursday issued a broad caution about conditions they might encounter, including fast-moving water, following months of severe weather.

An extraordinary series of storms this past winter buried the Sierra range in deep snow that is now melting, swelling Central Valley rivers that just months ago were running low because of years of extreme drought.

Reservoirs that store water and provide flood control must release high flows into rivers to maintain room for incoming runoff. That, in turn, changes rivers. Sandbars and ledges may become steep drop-offs and lead to an unexpected plunge into cold water.

“It can really give a shock to the body,” said Daniel Bowers, Sacramento city's director of emergency management. Experts say muscle control can be lost in minutes.

The recent tragedies include an 8-year-old girl and her 4-year-old brother, who were swept away by the Kings River on Sunday. The girl's body was found that afternoon and the boy's body was found nearly 2 miles (3.2 kilometers) downstream on Monday, the Fresno County Sheriff's Office said.

The fatal accident occurred even though both the Kings and San Joaquin Rivers have been ordered closed to recreational users since March 14.

In the Sierra northeast of Sacramento, a man was swept away by the American River on April 29, two days after Placer County authorities first issued warnings. His body was found Friday in a lake miles away. Another man who vanished in the river on Mother's Day remains missing.

Placer County's messaging about the risk is blunt. “If the public doesn't listen to our warnings this year, people are going to die, more people than we've seen over the last few years,” sheriff's Sgt. Kevin Griffiths says in a public service announcement video.