Because its geologically difficult to divide Idaho into different regions that evenly distribute the waterfalls in the state, the divider between the two regions for Idaho has been set at the Salmon River.
The southern half of the state has two primary geologic features. The Snake River plain dominates the southern half of the region, stretching up to a hundred miles wide and 350 miles long, the flat landscape is surprisingly heavy with waterfalls thanks to the massive Snake Plain Aquafir emptying into the Snake River Canyon. To the north, the Sawtooth Mountains rise from the plain to elevations over 10,000 feet, creating some of the most rugged mountains in the country. Many of the mountains are accompanied by dozens of lakes on their flanks, often with streams flowing from them, falling over the lips of hanging valleys formed by glaciers long since melted. Though the area receives a fairly heavy snowpack, many of the waterfalls in the Sawtooths run very low in the late summer because much of the range is solid rock and the water runs off very quickly. The largest waterfalls in Idaho occur in both areas, accompanied by dozens, if not hundreds of others.