They found the Columbia. They missed the South Pass (used by the later Oregon Trail), which would have made for a heck of lot easier trip over the Rockies. But, by going as far as possible up the Missouri to the Bitterroots, they left themselves much too far north to find it.
Your main point is that they had no real way of knowing the best way up front. That's why being the pioneer is a big deal.
Thought they went to far south...missed the Snake river that goes into the Columbia!