Geology

Idaho, along with neighboring states was once ocean front property on the Pacific. Idaho was most likely a flat coastal plain. The mountain range to the east eroded and the resulting sediments were laid down. These layers are would have been over 8,000ft thick and are now referred to as the ‘Belt Sediments.’  Sometime later the Pacific Plate began to slide under the North American plate creating a subduction zone. As the plate slid underneath, it melted and the large masses of magma rose up and solidified creating the Idaho Batholith. As the masses of magma rose they pushed rocks aside and bulged the crust enough to create volcanoes such as Mt. St. Helen. It was the belt sediment layers that were pushed to the east and west of the bulge. Where the magma masses encountered other rocks they created zones of contact metamorphism, changing the surrounding rocks with pressure and heat.

Things calmed down for a bit and then the subduction zone encountered a crustal plate that would not slide underneath the North American plate. This would have been a set of Island Mountains, and the crustal collision created the Rocky Mountains. (This same type of collision, much more recent, is what gave birth to the Himalayas.) When this occurred it triggered more activity from the magma under the surface of what is now Idaho and gave rise to the Casto Plutons. It also further displaced the Belt Sediments and jumbled them in with other rocks. Evidence of these granite masses and their contact metamorphism is evident along the Middle Fork.

Middle Fork Formation:
The Middle Fork was has been formed by flowing water, and was not carved by glaciers. Some of the side creeks show signs of glacial influence in their U shape, and these supplied sediments that have helped form the Middle Fork. River channels are formed by a fluctuation in the balance between sediment load and water energy. When the energy is less than the sediment load then ground is laid down as the sediments settle out. When the water energy is greater than the sediment load then a channel is cut. The bedrock exposed by this process started its formation over 100 million years ago. The other rocks exposed include granites from the Idaho Batholith, which created metamorphic rocks upon contact, and pieces of the broken Belt Sediments.

Recent Changes:
Free flowing rivers create very dynamic environments. Every spring the snow melts high in the mountains and causes the river level to rise. If the weather warms quickly, or if there is warm rain on snow, then the high country snow will melt very quickly which causes creeks with small banks to overflow and blow out. When creeks blow out, the debris of rocks, sediment and wood is washed down into the river and sometimes creates a new rapid. When new rapids are created they can also act as natural dams and create reservoirs upstream of the new constriction perhaps flooding out older rapids.

After big forest fires in 2007 the loss of trees and their stabilizing roots meant that a lot of soil was loosened.  The following year there were massive rainstorms during the summer, which caused numerous creek blowouts and the formation of many new rapids. These changes keep the river exciting and also inspire respect for the power of water.

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