Here's an excellent LA Times article on the hazards of hiking without water:
Here, he says, is the spot where they found Margaret Bradley, a 24-year-old University of Chicago medical student and marathoner.
Just three months before, the 115-pound Bradley had finished the Boston Marathon in a few ticks over three hours, a solid performance in temperatures well over 80.
"I focused on keeping myself hydrated," she told the magazine Chicago Athlete afterward, "and not letting the adrenaline from the crowd make me do something stupid."
But last month, when she and a companion decided to try a 27-mile trail run in a single day, that caution was missing. A cascading series of miscalculations, say rangers, turned this scholar-athlete into the Grand Canyon's first dehydration fatality in four years.
In a single hour, a hiker in desert heat can easily lose a liter of moisture through sweat — maybe, some experts say, as many as three liters (a liter is slightly more than a quart).
Without water, write authors Michael P. Ghiglieri and Thomas M. Myers in their book "Over the Edge: Death in Grand Canyon," dehydration, hyperthermia and exertion in the canyon can "turn people, inch by inch, into heat zombies…. Kids and young adults seem to run at full function in the heat, sweating appropriately and seemingly going strong, but abruptly, when dehydration kicks in, they crash quickly and often unexpectedly. And die."
At least part of the tale, however, can be gleaned from rangers who were there. By Park Service accounts, the runners began their day about 9 a.m. at Grandview Point, the highest spot along the canyon's South Rim, where the trail head is 7,400 feet above sea level, nearly 5,000 feet above the river.
Here is where the two runners made their first mistakes. They set off nearly four hours after sunrise, several hours later than rangers advise distance hikers to begin on summer days, and they were traveling dangerously light. Bradley's companion had four liters of water. She carried fruit, three protein bars and just two bottles of water (about 1.5 liters). They carried no maps, and Bradley apparently had no flashlight or headlamp.
From Grandview Point, the two headed down an unmaintained, waterless path built by a prospector about 100 years ago, descending 2,600 vertical feet in just three miles. From there they planned to descend farther, then follow the Tonto Trail across the notoriously hot and shadeless Tonto Plateau, about 1,000 feet above the river. Then they'd climb back out on the busier South Kaibab Trail, which tops off at 7,000 feet.
It's unclear why they thought they could do this route in a day, or where they expected to get water. "Not recommended during summer," says the Park Service's free trail guide. "No water."
"This would be a two- or three-day backpack trip with a lot of planning," says Yeston, who served as incident commander. "And the optimum time to do it would be fall," adds Ken Phillips, the park's search and rescue coordinator.
In the first seven months of this year, park personnel have carried nearly 200 hikers out of the canyon by helicopter, most of them suffering from "environmental causes" — exhaustion, dehydration, sometimes water intoxication, which happens when hikers drink plenty but fail to take in salt to help keep their electrolytes balanced.
By 3 p.m. they were in trouble. Bradley's companion couldn't run anymore. He stopped, overheated and exhausted, and curled up in the shade of a bush to rest. In six hours they'd covered about 12 miles, with 15 still to go. Now the temperature was over 100, and their water was gone.
As a hiker heats up, says Yeston, "the body is going to start to divert blood to the parts of the brain that are more basic. The parts of the brain that you might have used to make nuanced decisions about your situation — they're compromised. Long before a person seems drunk or delirious, they're already going to have a subtle loss of fine motor coordination and critical thinking … and even difficulty referencing past experiences."