Mullan suffered a great deal in his efforts. Trying to find a pass over the mountains began before he started constructing the "road." He was on the Montana side a year or 2 before the $ was authorized. Then, when he worked his way back to build the road from the west, Montana wasn't any easier for him and the guys with him. I believe that was when one of them lost his legs to frostbite. Not just a toe or 2 . . !
Now let see how things went on his building the road from Walla Walla: They progressed at breakneck speed! What was it - something like 600+ miles of "road building" in 2 years? He had something like 100 men at first and a lot of these guys were along for protection from the Indians. Yeah, the INDIAN trails were already there. They'd been herding 1,000's of horses along those trails.
Then Mullan gets into the mountains of Idaho and has this MAJOR problem with trees, trees, and more trees! Fallen trees, big trees, little trees! He complains to the Indians on the Coeur d'Alene River, when he finally reaches them, "Why the heck haven't you cleared some of those dang trees?!" Or, words to that effect.
Didn't it occur to him that they didn't want anyone coming up from the lake thatta way? He realized latter, and said as much if'n I remember correctly, that he should have gone by way of the Clark's Fork River. But, more than that, the Coeur d'Alene got around just fine in their world - in canoes! They didn't want half the people in this part of the world herding horses, etc. thru their river valley.
Horses to Montana, they could run them up the valley here to the Clark's Fork - where they'd been driving horses over the mountains so that the Blackfoot, etc. could buy them and hunt their buffalo. It was a "planned obsolesce" deal. They'd take a horse over this year, the Montana Indians would hunt off of him thru the season. Winter would come and the horse would die in the severe weather. Meanwhile, the Indians here would have all these lovely young horses on the Palouse. They'd take those over the next year and sell them. It was a BIG deal!
Steve