Geology of Rapids:
Rapids are areas of shallow, fast-flowing water in a stream or river. Rapids tend to form in younger waterways, with a straighter and faster water flow than in older rivers. Softer rocks in the riverbed erode or wear away faster than harder rocks. This process is known as differential erosion. The result of differential erosion is that the more substantial rocks remain as the riverbed wears away. The effects of differential erosion are shown by the presence of whitewater in a series of rapids. Whitewater occurs when the river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and white.
More About Differential Erosion:
Rapids are one of the many examples of differential erosion. The process can form some larger hills and mountains too. Harder rocks are more resistant to weathering, whereas softer rocks are rapidly worn away, producing a landscape that is formed by erosion at irregular rates.
Classes of Rapids:
Rapids are classified based on their turbulence and presence/absence of obstacles that lead to the creation of whitewater.
Class A - no water movement
Class I - the water is smooth, and there is little presence of whitewater. There are small obstacles and rocks present in the water.
Class II - there are large rocks present in the water (>3 feet) that create ledges and passages between rapids. There is a lot of whitewater present, and the water turbulence is quick.
Class III - enormous rocks and eddies combined with fast river flow create waves. The entire series of rapids is covered in whitewater.
Class IV, V, VI - the series of rapids could contain cascades (waterfalls). The currents are violent, and there are massive waves present.
Logging Tasks:
- Walk down the dirt path to get a good view of the river and examine the appearance of the rapids (they should be about 50-75 feet downstream from the bridge). Do they take up the entire width of the stream? Is there any evidence of whitewater or obstacles (such as rocks, etc.) in the rapids?
- What class of rapids are you observing at the GZ? As differential erosion continues to occur on this stretch of stream, do you think the class of rapids will increase in severity in the future?
- How do you think differential erosion has contributed to the formation of the rapids?
- 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://en.wikipedia.org/wiki/Whitewater
https://paddling.com/learn/classification-of-rapids-water-level-and-canoeists#:~:text=Class%20II%20%2D%20Moderate.,maneuver%20canoes%20and%20read%20water
https://education.nationalgeographic.org/resource/rapids
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