Westward migration in the early/mid 19th century depended on a strict timetable. If you left too early, there would be no grass to feed your oxen while you crossed the Great Plains. If you left too late, you'd get trapped in the mountains on the west coast. Any delays contributed to the latter situation - and that's where the Donner Party ran into trouble.

The Donner Party left Springfield, MO late. They took a longer than usual break to mourn the death of the elderly mother of one of the members. And finally, they took the Hastings Cutoff, which was virtually unexplored, untested, and unguided since Hastings (the man who created it) urged the party to take the path and then abandoned them to their own way near the beginning.

After the Hastings Cutoff, the Donner Party just hit failure after failure. They had to clear-cut the paths of forest and underbrush, which is backbreaking work (imagine cutting down virgin forest with a hand-axe). They had to cross the salt flats in Utah on foot, during which they lost a substantial number of their oxen and some members were forced to abandon their wagons.

By the time they reached the Truckee River (their winter camp), they were physically and mentally exhausted. They had no supplies, very few live animals, and already knew they were dangerously behind schedule.

Another contributing factor to their failure to prepare properly is the amateur nature of the people in the party. The Donners and the other major families were Midwestern farmers, but that doesn't help much in a wilderness survivalist situation. Further, all the families involved were just that - families. The proportion of young children in the party meant there was a lot of mouths to feed that couldn't contribute to survival work. There were 35 children in the party of around 65 (variable, because some party members left during the trip). Many of those 35 children were small - 26 were under the age of 10. Add to this the fact that women were not expected to work as hard as the men were. This isn't to say the women didn't contribute, but it was the men who were running themselves exhausted in the above-mentioned crises on the trail (Cutting trees, clearing paths, hunting).

So, when the party gets to the Truckee River, they are a) physically exhausted, b) dangerously low in supplies, c) extremely low morale knowing the Hastings Cutoff was a massive failure, d) short-staffed for workers with the ability and knowledge to attempt to survive (i.e. men) and e) trying to survive in unfamiliar and extreme wilderness conditions (see u/CedarWolf's post for details for Patrick Breen's account of snow in the Sierra Nevada). They tried their best to survive, but by the time the Donner Party was in *serious* trouble, they were already backed pretty far into a corner.

If you're interested in this topic, I recommend checking out the book Desperate Passage, by Ethan Rarick. It covers the whole crisis beginning to end, and is a fun read (considering the topic). The Donner Party is an excellent study in failure of crisis management, but also in human endurance in increasingly bleak odds. Strongly recommend.

Hey, something I actually know something about and can contribute a bit. The Donner party was led by two brothers, Jacob and George Donner, but they were following the path set down by another man, Lansford Hastings. It was supposed to be a shortcut to California, so they left the established route and followed the shortcut that had been laid out for them.

However, as they approached the shortcut, they found a note from their guide saying the trail ahead was more dangerous than he had anticipated, so they should wait for him to return. When Hastings didn't show up after a week, they sent a scout ahead, who came back with a note telling them to take a different path, which turned out to be even rougher than the first.

This new path put them up towards the Donner Pass, and much later in the season than they had intended. There were a series of unusually heavy snows, starting in late October and continuing every few weeks through February.

You can read a more in-depth description of the snow here. Patrick Breen kept a journal with the weather and conditions logged in it, and you can read the original text in a few places. (Though I don't have access to the library with it right now, I found a copy of the text online, and you can see where he mentions the heavy snows.) It's kind of haunting, considering what happened.

Anyway, since the Donner party was higher on the mountain, they were subjected to heavier snows than they would have been in the valley, and they became quite buried in. Some of Patrick's accounts mention 13 foot snow drifts with an extra 6 or 7 feet being piled on top of that with successive snows.

The Donner party did try to send a scouting group out to seek rescue, and they were aided by a pair of Native Americans who gave them some supplies and advice on winter survival. However, when the scouting party ran out of supplies, they shot and ate both of them, considering them to be less human. It's one of the tragedies of the whole story.

Despite all of the above, roughly half of the folks involved survived. It took four separate rescue attempts to get the rest of the survivors out. They had been trapped there from late October until March the following year.

One thing you have to remember is that this was in the 1800's and they were traveling by wagon, in a large group, with families. It's one thing to look at the pass in a modern context and wonder why they didn't just come down from the mountains, but once their wagon wheels were frozen and buried in the snow, they were stuck.

tl;dr: They were delayed and crossing late in the season on a rough, unfamiliar trail, during unusually heavy snows, which left them stuck and buried in, and they were trapped there.

Edit: Found text of Patrick Breen's diary. Will update with more sources as I can.

The Donner Party made a couple of fatal choices that led them into a deadly situation. These were not savvy frontiersmen who knew how to survive in the wilderness. They were emigrants who wanted to go from the settled East to the promise of opportunity in California. They were naive and ignorant in many ways.

Their first mistake was to take the infamous "Hasting's Cutoff" an alternative route in the eastern Great Basin that cost them a lot of time. They had left Missouri late (they were the last group of wagons to head West that spring), and the Hasting's Cutoff cost the group additional time. The result was that they reached the foot of the Sierra Nevada late in the season. Although a storm was threatening, they decided to risk the ascent; to do otherwise would have meant that they would have needed to winter in the valley on the east side of the Sierra (around the current location of Reno), with little to supply their existence until spring.

There is no going around the Sierra Nevada - it is a formidable wall of mountains many hundreds of miles long. There are a few high-elevation passes which provide the only means across the mountain range. Donner Pass, the current location of Highway 80, is one of the best passes and was and is the most traveled. Despite this, if there are chains or snow tires required in one spot in the US, it is likely to be over Donner. Even today, it is a tough piece of real estate to traverse in the winter.

The winter of 1846/47 was particularly bad. The first snow storm that caught the Donner party arrived early and dumped a lot of snow. The group could not move their wagons and they could not walk out, so the decided to camp where they were and wait for the snow to melt. Unfortunately, it was just "one of those winters" when the snow kept coming. Forage was completely covered, so the animals were certain to die, and they provided the last good meals of the refugees. The men tended to die first: studies indicate that they likely had burned more body fat through heavy labor and they were exposed to more of the elements because they were doing the outdoor work, burning calories while attempting to hunt and erecting primitive shelters. For whatever reason, many men died, leaving mostly women and children. Fishing was a possibility, which they tried, but the Donner Lake fish are best caught from a boat rather than on shore, and the group lacked boats. Hunting was nearly impossible because the group was not prepared to traverse across the snow. So they remained at camp (with camp fires - they did, at least, know how to build a fire!).

There were several attempts to reach the party. A group of Native Americans tried to approach the camp with supplies, but they were seen as threatening and the Donner Party attacked them. People from California made several attempts to get supplies to the group, but they were met with the same challenges of deep snow.

It was, simply, a really bad situation.

There are several sources on this. George R. Stewart, (1936). Ordeal by Hunger: The Story of the Donner Party, remains the classic. In addition Don Hardesty and others offer an excellent modern analysis: The Archaeology of the Donner Party, University of Nevada Press (1997).

edit thanks to /u/fluffkopf

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