What made early navigation on the Columbia River difficult?

Early navigation on the Columbia River was primarily hindered by the presence of numerous rapids, which made travel difficult for the canoes and barges used by early fur traders and immigrants. Specifically, there were 21 rapids between the river's mouth and the confluence with the Snake River.1

These navigational challenges were significant enough that the most difficult sections had to be circumvented by land, leading to the construction of the first railroad in Washington—a mule-drawn tram—to bypass the worst points.1