The Middle Fork country is profoundly rugged, the product of a 100 million year collision between oceanic and continental tectonic plates. The resulting pressure eventually uplifted the central Idaho mountains.
The river itself has only been at work for two million years, carving into the mountains and creating the deep canyon.
The Middle Fork drops 3,000 feet in approximately 100 miles. The result is a fairly dramatic change in terrain over a seven day river trip. You launch in high alpine forest and depart in semi-desert. Along the way, boaters marvel at the massive granite of the Idaho batholith, the twisted metamorphic rock that speaks of intense pressure and heat, and the white sandy beaches.
Even those who have rafted through Impassable Canyon a dozen times can still find something to astonish them. In part, that's because the river literally changes from year to year, as high water and occasional blow-outs create new rapids and re-fashion old ones.
Some of the state's most famous rapids -- Velvet, Powerhouse, Tappen, Weber, Rubber -- are the result of the tumultuous geology of the Middle Fork.