http://byustudies.byu.edu/showTitle.aspx?title=7194 Chad M. Orton, BYU Studies 45:3, 2006
One of the most prominent stories in Mormon folklore is the story of the Martin Handcart Company, and three [four] of the rescuers carrying all of the company members across the Sweetwater on a cold winter day. This story was popularized by Soloman Kimball. The popular story goes that when the Martin Company reached the Sweetwater, they were overcome with the thought of having to cross another river, especially since the day was very cold, it was a cold part of the day, and there was ice floating in the river. The rescuers entered the water, and carried the entire company across. The effects of being in the water for several hours caused these young men, all 18 to suffer the effects of this the rest of their lives, and to eventually die early. Brigham Young when told of their sacrifice promised them places in the Celestial Kingdom of God.
If not for Solomon Kimball this story probably would not have been known at all. It is correct in generalities, but not correct in particulars. All of the rescuers did their duty, however they were not all at the river that day. Several of the rescuers entered the water to carry pioneers across. Five rescuers are named in different histories, none of them were 18, two were older and three younger. In truth they did not carry all of the handcart members. They had several wagons with them, and many of the invalids and children where taken to Martin's Cove in the wagons. Of those who had to ford the river, there was a hierarchy of who received help as the rescuers could not help everyone. Woman, children and elderly received the first choice. It is true some of the men where also carried across, but most forded the stream. Although most of the handcarts were abandoned at Devils Gate, they still took several to the Cove to carry the cooking utensils etc. These the pioneers had to pull across the River. They did receive help from the rescuers as the River was muddy and the opposite bank was steep. Some of the rescuers did have health effects, but it cannot be verified that these effects actually caused premature death. In fact some of the Valley Boys who helped at the Sweetwater lived long lives. A couple died young, but the first death was 16 years after the rescue. That these boys were guaranteed Eternal Life in the Celestial Kingdom by Brigham Young may also not be accurate. Although Brigham Young said that the rescuers were portraying true religion by serving in this manner, that is not quite the same as being guaranteed Celestial Glory.
For anyone studying the handcart story, and particularly this rescue, I would recommend this article. It is interesting how things different sometimes between what we grow up believing, and what may have really happened. Mr. Orton does provide extensive footnotes and sources for his conclusions.