Hot summer temperatures reaching into the triple digits and cool, swift-running waters conspired last weekend to claim the lives of two young girls who looked to the Middle Fork of the Kaweah River in Sequoia National Park for momentary relief from the heat, according to park officials.

Dangers abound in the National Park System. At Yosemite National Park there's Half Dome. At Zion National Park there's Angel's Landing. At Cape Cod National Seashore you've got riptides, and at Yellowstone National Park there are wild animals and incredibly hot geothermal features. At Sequoia, it's the park's rivers that pose perhaps the greatest threat come summer.

Park officials know well the dangers. Each spring they issue warnings about the snow-melt-swollen streams that can be deceptively dangerous to folks looking to cool off when temperatures grow warm and turn to hot. Signs also are posted near popular river access points, and warnings are printed in the park's newspaper. Four years ago Sequoia initiated its "River Rover" program, in which volunteers patrol stretches of rivers most likely to prove dangerous to park visitors. The program was built around a half-century of data that allowed park officials to narrow down not only the time of year most drownings occurred, but where they occurred, even the time of day.

According to Sequoia dispatch records, high temperatures in the park's gateway town of Three Rivers, California, ranged between 110-116 degrees last weekend when a 13-year-old girl drowned not far from Hospital Rock on Saturday afternoon and when a 14-year-old girl drowned in the same stretch on Sunday around 8 p.m. Both days saw River Rovers pass along the popular swimming hole to warn folks about the potential danger, according to park officials.

This summer the park has a dozen volunteers patrolling the rivers. They generally work two-four hours one day a week. Their schedules are staggered to maximize locations reached and time spent in the area. The focus is on coverage during the hotter periods of the day. Last weekend, park officials say a River Rover was in the Hospital Rock area on Saturday from approximately 11 a.m. - 1 p.m., and on Sunday from approximately 10 a.m. to 4 p.m. Additionally, on Saturday a park ranger was along that stretch of river from about 1 p.m. until shortly before 3 p.m., according to park officials. The call for help went out right about 3 p.m., they said. On Sunday evening, there were an estimated 12-20 people along the stretch of river where the 14-year-old disappeared, though park officials don't know how many were in the water at the time she went under.

The drownings, park spokesman Malinee Crapsey says, were unfortunate coincidences.

"We have a long, rocky fast flowing river and it could as easily as happened in a number of other places. We did have River Rovers out and they were at that location earlier in the day," Ms. Crapsey said Tuesday. "We also have signs at every trail that leads to the river warning people regarding the dangers in the river.”

Until last weekend the River Rover program had been an overwhelming success, according to National Park Service officials. "Since 2005 when they implemented this, they were having two deaths a year, and since they implemented the program they’ve had none," Dr. Sara Newman, the agency's risk management program director in Washington, D.C., said earlier this year for a story about staying safe in the parks. "What they do feel confident about is that they’re educating people. They’re finding that people were very surprised to learn how dangerous that river was. It doesn’t look it.”

While records can point to no drownings, or two drownings, they can't point to prevented drownings, said Ms. Crapsey. "We will never know how many drownings we’ve prevented. We know the number of drownings has gone down (over the years)," she said.

Whether there will be more drownings at Sequoia this summer is impossible to say. But park visitors will be looking for places to cool off.

"We’ve had a tremendous heat wave here and there have been drownings and near drownings all over the state," said the park spokeswoman. "Virtually every turnout along the highway that has a trail down to the river had two or three cars crammed into it (last weekend), people trying to get down to the river. It’s such a tragedy, I can’t stretch my mind for it to be symbolic of anything more than it was hot."