Which posed the greatest challenge to pioneers crossing the western mountains?

Although many neophyte pioneers believed Indians were their greatest threat, they quickly learned that they were more likely to be injured or killed by a host of more mundane causes. Obstacles included accidental discharge of firearms, falling off mules or horses, drowning in river crossings, and disease. After entering the mountains, the trail also became much more difficult, with steep ascents and descents over rocky terrain. The pioneers risked injury from overturned and runaway wagons.