Nantahala River Gorge:
The Nantahala River Gorge is part of a much larger geologic formation known as the Appalachian Plateau. The Appalachian Plateau stretches from southeastern New York to northern Alabama. This formation was formed due to the collision between the North American and African continents 270 million years ago. This collision led to orogenic uplift: the creation of the Appalachian Mountains. At the Nantahala Gorge, the most common type of bedrock is quartzite. Quartzite is a metamorphic rock created when folding occurs in the uplift process. Although this is the most-erosion-resistant rock in southeastern North Carolina, rivers such as the Nantahala can erode the surrounding landscape due to the presence of softer rock layers like sandstone. Downcutting is the process of river water eroding bedrock and deepening a gorge or valley. Rivers in mountainous areas tend to create narrow and deep valleys and take many years due to erosion-resistant rock, while streams in low-lying areas can downcut wide and shallow valleys.
Types of Downcutting:
Downcutting can produce three main types of valleys.
- Slot canyons form when a gorge is formed from different rock layers. Canyons with more erosion-resistant layers located closer to the surface and soft layers underneath typically form slot canyons. Rivers erode a narrow portion of resistant rock and then erode a wide soft layer of rock.
- V-shaped valleys occur in landscapes where mass wasting and downcutting coincide. Downcutting can lead to the instability of river banks. Mass wasting occurs when an unstable riverbank collapses due to gravity, giving the gorge a distinctive v-shape.
- Stair-step canyons form due to the combination of factors present in slot and v-shaped valleys. Landscapes prone to stair-step canyon formations consist of alternating layers of hard and soft rock. As a river cuts through hard rock, it cuts a narrow passage and widens as it cuts through soft rock. The mass wasting of erosion-prone soft rock gives the canyon a stair-like appearance.
What are rapids?
Rapids are shallow and fast-moving parts of a river or stream. Rapids slowly erode the streambed and can even accelerate the process of downcutting. They are often the site of differential erosion or the process of quickly wearing away erosion-prone rock (softer rock) than erosion-resistant rock (harder rock). Differential erosion can best be seen by the obstacles such as rocks or larger boulders in a river. These rocks break up the stream's flow and lead to the creation of patches of whitewater and small waterfalls, accelerating the erosional process.
Four main components affect the intensity and appearance of rapids. A large amount of water flowing quickly has a better chance of forming rapids. The gradient is the slope of the riverbed where the water is flowing. Constriction refers to where the river is forced into a smaller or narrower channel. Constricted river channels allow water to speed up to fit the necessary volume through a decreased area. As a result, a higher gradient and more constriction correlate to faster-flowing waters. The final requirement for the formation of rapids is obstacles. Many erosion-resistant rocks and boulders are enormous obstacles in a river full of rapids. If you already have water moving quickly through a narrow channel, large boulders create more turbulence in the water, making whitewater appear.
Logging Tasks:
- Estimate the size of the boulders (obstacles) in the river. How has differential erosion affected the appearance of rapids at the GZ?
- Take a look at the area around the gorge. Based on your observations and what you’ve learned in the description, how has the downcutting of the Nantahala River affected the landscape?
- The GZ is located at the bottom of the Nantahala Gorge. From where you stand, what is the valley's shape (slot, v-shaped, or stair-stepped)?
- As of June 2019, earthcaches now contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://www.whitewaterguidebook.com/how-do-rapids-form-in-a-river/
https://www.nps.gov/articles/appalachiannplateausprovince.htm#:~:text=As%20the%20mountains%20rose%2C%20streams,make%20up%20the%20Appalachian%20Plateaus
https://geomodelsvt.wordpress.com/2019/10/10/searching-for-the-deepest-gorge-in-the-appalachian-mountains/
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/downcutting
https://education.nationalgeographic.org/resource/rapids/
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