The Fresno County Sheriff's Office says a few children were playing in the shallow water when they fell off a sandbar.
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Adults rushed into the river, including a father attempting to rescue his daughter.
But neither knew how to swim well, and they both drowned.
"Quite frankly we're lucky we didn't have more drownings with all the people that were in the water that did not know how to swim," said Lt. Joe Smith.
Last week, a five-year-old boy died after being pulled from Lake Kaweah. He was not wearing a lifejacket when he fell off a houseboat.
Millerton Lake has not had any drownings so far this year.
California State Park rangers hope weak swimmers rent one of their free life jackets or swim in an area where a lifeguard is watching.
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"It doesn't matter which body of water you happen to be at, whether it's Millerton Lake, or backyard swimming pool, whatever it is...if you don't know how to swim, take necessary precautions to keep yourself safe," said California State Park Supervising Ranger Steve Barber.
Rangers are enforcing standard boating laws, looking for people driving unsafely or under the influence.
There is also a new one which requires operators of motorized vessels to carry a "California Boater Card."
The deadline to get one depends on your age and you'll have to pass an approved boater safety exam first.
"My hope for this new law is that it's going to reduce accidents and vessel collisions, water-related injuries, and fatalities out on the water, because the expectation for all the boaters is the same. You put yourself through some type of standardized training so that you know how to operate a vessel."