ALMOST 40 YEARS LATER, I'm standing knee-deep in the cool water of Sespe Creek. It's a transcendent, peaceful stream, and Steve Larson, now 41 years old and a public-school teacher from Sacramento, and I have been camping alongside it for a couple of days in June, with temperatures nudging 100 degrees. We've dumped our packs to relax near one of the many places where the trail crosses the creek, as we work our way back toward civilization.

This isn't just any crossing, though. This is the place where, in January 1969, the Sespe killed Deputy Sheriff Chester Larson, Steve's father. It also killed three other men and the six young boys they were trying to evacuate from the area. "It probably happened right about here," I call to Steve from the middle of the river, where it deepens ever so slightly.

Steve doesn't answer. He's still on the bank, subdued by the weight of the past. He was just two years old when his father was killed, and after the tragedy he began to speak with a stutter. He was never told the details of the accident, only over and over that his father had died a hero. As he grew up, he was determined to escape the ghosts in his family. At 19, he went off to Chico State. There, he met Marcie, and they married in 1994. Now they live together with their two young children, Sarah and Owen, in Sacramento, where he's been teaching for ten years. But a few years ago, just past the age his father died, Steve realized he needed to find out exactly what happened on the Sespe and what kind of man Chester Larson really had been—and how the rescue his father attempted had ended in so much death.

After a while, Steve wades in and starts to wander alone among the dark boulders. The malign potential of the creek is hard to grasp. In fact, the only hint that the Sespe is capable of violence is the flood debris that necklaces the trees lining its banks. This is the first time he's ever been here.

Every year, people fall off rock faces, drown in rivers and lakes, succumb to exposure, and are broken or suffocated by avalanches. You almost never hear about these tragedies, unless they happen on Mount Everest or catch the attention of the cable-news beast. But these invisible disasters are searing nonetheless. They almost always involve moments of nobility and courage and heartbreaking miscalculation. They snuff out promising lives and heap sadness, guilt, and unanswered questions on the families and friends left behind. Nature unleashed—a howling storm, a flash flood, a blizzard—has elements of divinity and drama that expose perfectly the frailty and flaws of human nature.

What's the half-life of tragedy in the wild? In the case of the Sespe, I discover, the pain still runs hot through the lives of everyone involved. Steve is struggling over his feelings about the father he never knew. The families who lost children still experience a corrosive legacy of sorrow and anger. And the lone survivor of the accident continues to battle the agonizing sense that he somehow could have done more.

"It's like cancer. We have cancer," says Debra Cassol, 57, who was 18 when the Sespe killed her two younger brothers, Bobby and Ronny. "Sometimes we get a little remission and we're not so sick. But then it always comes back. It always comes back."