In April 1846 the Donner and Reed families left Springfield IL for California. They split off from a larger wagon train to follow a so-called shortcut, but that and other difficulties cost them valuable time. When they finally reached the Sierra Nevada Mountains at the end of October they were trapped on the eastern side by heavy snows and food supplies were quickly depleted. A small group set out to get help, but along the way some died and were cannibalized. Seven of this group reached safety on the western side of the mountains in mid-January 1847. Californian relief parties sent to rescue the rest of the Donner Party found that some of the survivors had begun to eat their dead too. The last survivor of the Donner Party arrived at Sutter’s Fort on April 29, 1847. Of the original 87 pioneers, 39 died.
Interested in this bit of history? Search the Library Catalog to find titles such as Ordeal by hunger; the story of the Donner Party (3rd-fl. Main Collection, F868 .N5S7x 1960), Unfortunate emigrants: narratives of the Donner Party (NetLibrary ebook), and Wagons west: the epic story of America’s overland trails (3rd-floor Main Collection, F593 .M475 2002).
Online resources include the brochure of the Donner Memorial State Park and a couple of digitized diaries of Donner Party members: