A man and his son were riding a jet ski nearby and threw a flotation device to Kalem, Bonner said. But Kalem could not reach it. He slipped under the water and never resurfaced.
Kalem's friends and classmates assembled at Wasatch High School around 10 p.m. Wednesday for an impromptu memorial service.
Leno Franco, Kalem's father, thanked the couple of hundred students and parents who turned up late Wednesday.
He told them that Kalem was a "true Wasp," remarking on the school's mascot. Franco said his son loved Heber and all his fellow students.
"You guys set a great example for him," Franco said.
Kalem was one of 13 children in the family; he was the third oldest boy, Franco said.
Forrest Cone, a junior at Wasatch, called Franco a "foul weather friend."
"It seemed like whenever things got harder, he was there," Cone said.
The mourners closed with a school cheer: "All for one. One for all. All for Wasatch!"
With mountain runoff from late spring snowfall, the reservoir is unseasonably cold just over 55 degrees, Bonner said.
"That's a very cold temperature," Bonner said. "Your body will start to cramp up."
None of the boys was wearing a personal flotation device, Bonner said.
"I understand when you're swimming, it probably gets in the way," Bonner said. "But it could save a life."
Utah's chilly, swollen waterways have claimed 10 lives since April. Near the scene of Kalem's drowning, waders and swimmers of all ages buckled into life vests. Pleasant Grove's 13-year-old Ashlynn Miner said she swims without a vest "only in pools and when people are watching."
The danger of swimming in cold, mountain water is that the chill doesn't set in "until you're out there," said Mindy Holm, of Eagle Mountain. Holm said her children, Makenna, 12, and Dyllen, 11, are required to use vests in the reservoir no matter how old they are or how well they swim.
"You jump in, and you don't think it's too cold ... or too far," she said. "You wouldn't know until it's too late."
Divers used an underwater camera to find Kalem's body about two hours after he went below the surface, Bonner said.
Of the 10 drownings in Utah's lakes and rivers, eight of the victims have been children. The most recent death was that of a 2-year-old boy who fell into the Virgin River during a family camping trip.
Utah's water deaths
Ten people have drowned in Utah's lakes and rivers since spring began.
1 • Richard Jay Valenzuela, 4, of Santaquin, drowned April 15 after he tripped, hit his head on a rock and fell into Utah Lake
2 • Ryan Bishop, 21, of New Hampshire, died April 28 after his canoe capsized during a nighttime excursion in Glen Canyon National Recreation Area.
3 • Vincent Larsen, 2, of Woodland Hills, fell into Starvation Creek on May 28; he died the next day.
4 • Shirley Palmer, 73, of Tuscon, Ariz., died May 31 after her inflatable kayak flipped on rapids in the Green River.
5 • Danny Ellison, 13, of Provo, died June 7 after he fell into the Provo River.
6 • Jayson Robert Corporon, 8, of Tooele, died June 8 after he fell into the American Fork River just below Tibble Creek Reservoir.
7 • Donaven Lopez, 2, died June 8 after he went through a hole in a park fence in Riverton and fell into a canal.
8 • Uri Grois, 15, died June 11 after he and a friend were swimming and fell over a waterfall that spills into the Virgin River.
9 • A 2-year-old fell into the Virgin River on June 22; he died Sunday at a Salt Lake City hospital.
10 • Kalem Franco, 17, of Heber, drowned June 29 while swimming from the shore of Deer Creek Reservoir to an island.
*Two children also drowned in swimming pools.