There is a plaque here commemorating the event.  On Aug. 28, 1869 Seneca and Oramel Howland and Willima Dunn left the Powell party and climbed out of the canyon to be killed by Indians or Mormons. They were bickering with Powell and thought that they could not survive the rapids in front of them. There is speculation that Mormons killed them (see Dolnick).  

Powell Geographic Expedition of 1869

The Powell Geographic Expedition was a groundbreaking 19th century U.S. exploratory expedition of the American West, led by John Wesley Powell in 1869, that provided the first-ever thorough investigation of the Green and Colorado rivers, including the first known passage through the Grand Canyon. The expedition, which lasted approximately three months during the summer of 1869, endured hardships and dangers down the river but resulted in the discoveries of many new natural features in the Colorado Basin. In 1875, Powell published a classic account of the expedition (combined with elements from later expeditions in 1871-72) called Report on the Exploration of the Colorado River of the West and Its Tributaries, re-issued and revised in 1895 as The Exploration of the Colorado River and Its Canyons.

• http://en.wikipedia.org/wiki/Powell Geographic Expedition of 1869
• Pages 272, 284 - Dolnick, E., 2001, Down the Great Unknown: John Wesley Powell’s 1869 Journey of Discovery and Tragedy Through the Grand Canyon: HarperCollins Publishers, New York, 367p.