In 1869 John Wesley Powell decided to set off down the Green River and follow it to the Colorado and then down the Colorado through the Grand Canyon. All of this was territory that had been unexplored by Europeans. Edward Dolnick recounts the passage in Down the Great Unknown. It's a fascinating story told masterfully of a courageous -- or foolish -- adventure.
His companions had no experience running rapids and their equipment was sturdy but not designed for shooting rapids. Fortunately, by starting high on the Green, they were able to learn some of the basics without killing themselves. Water, because it cannot be compressed and is fluid, does some strange things when running through narrow canyons and over rocks. Speed is not the greatest hazard: "Waves ricochet off rocks and cliffs and collide with one another; water rushes over rocks and dives down into holes and moves upstream to fill in 'empty' spaces behind obstacles." Water is moving in so many directions at once and at so many different speeds that obstacles such as rocks, dangerous in and of themselves, become even more hazardous.
Many of the canyons were very deep making portages around bad rapids impossible. Their first hint of difficulty came after Brown's Park, a lush hidden valley favored by cattle rustlers, called Lodore Canyon. The entrance was described as a "dark portal to a region of gloom." The walls of the canyon extended upwards some 2,000 feet. "The Gates of Lodore hinge inward, cruelly joined, hard rock, ominous, and when the mists skulk low between the cliffs, they become an engraving by Gustave Dore for one of Dante's lower levels of hell." This a description by a modern writer who extols the river.
And this was before they got to the tough parts.