|Looking at the Grand Canyon from one of the many viewpoints is amazing enough but walking a little way down below the rim is much more rewarding and gives a better appreciation of the scale and variety of the great gorge. However, this may be harder than it seems. Going down is easy; coming back up is not - descending a canyon is of course the opposite of climbing a hill which most people will be familiar with, where the return journey is always easier. The paths are very steep and weather is hot - over 90 °F at the south rim in summer (see Grand Canyon weather) - and the temperature increases steadily down the canyon so that it is usually 20 degrees higher at the river. The trails take at least twice as long to ascend as to descend. It is most essential to carry adequate water - only the Bright Angel and North Kaibab trails have some along the route. The trailheads have warning signs but still many people hike down for an hour or so with tiny water bottles and unsuitable footwear.
The National Park Service have taken to issuing advisory notices 'closing' some of the trails in especially hot weather to try and reduce the number of casualties - apparently up to 20 people per day during the summer require treatment for heat exhaustion at the Indian Garden (on the Bright Angel Trail) and Phantom Ranch (by the river) ranger stations. Besides lack of water, the biggest problems are hikers overestimating their ability and going too far down, and people straying too close to steep cliff edges. It is particularly recommended not to hike to the river and back in one day.
On the other hand, some people do overestimate the difficulties and come equipped with ski poles and all manner of mountain climbing equipment for a walk down some of the maintained trails; they also tend to travel very slowly. Some of the Park Service warnings too are a bit excessive; I walked down the South Kaibab Trail to the Colorado and back (13 miles during the day in summer), which took 6.5 hours; on the return journey I met two US 16-year olds also returning who had previously descended down the Bright Angel Trail, making a 15 mile round trip for them. So if you are fit and well-prepared there should be no problems.
But for an in depth look at what can go wrong, the definitive account is 'Over the Edge: Death in Grand Canyon' by Ghiglieri and Myers, published by Puma Press in 2001. This 408 page book describes all known deaths in the canyon from the explorations of John Wesley Powell in 1869 to the present day; 550 cases in all, ranging from accidents specific to the park - falls, drownings, heat stroke and flash floods, to coincidental mishaps like murder, suicide and plane crashes. The book is more than a simple narrative, as it includes reasons for specific accidents, and much advice on safe travel in the canyon.
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