Waterfalls are nature’s sculptors, carving landscapes with their relentless power. As water cascades over a cliff, it gains speed and force, creating a high-impact zone where it strikes the rock face below. This continuous flow of water not only wears down the rock but also intensifies erosion through hydraulic action and abrasion. Hydraulic action occurs as the force of the water exerts pressure on the rock, causing it to crack and fragment. Simultaneously, the turbulent water churns up sediment and debris, which acts like sandpaper, grinding away at the rock surface. Over time, this process deepens the plunge pool at the base of the waterfall and gradually recedes the cliff, reshaping the landscape in a dynamic and ever-changing dance of erosion.
Plunge pools are distinctive features commonly formed at the base of waterfalls through the process of erosion. As water cascades over the edge of a waterfall, the force of the falling water creates a powerful impact on the riverbed below, leading to the formation of a deep, often circular depression known as a plunge pool. This pool is carved out by the combined forces of the water’s momentum and the erosion caused by sediment and debris carried by the falling water. Over time, plunge pools can grow in size, becoming deeper and wider as the water continues to erode the underlying bedrock.
How Plunge Pools Leave Clearly Identifiable Features
The formation of plunge pools often leads to the creation of distinct geological and hydrological features that make them easy to identify:
-
Erosion Patterns: Over time, the force of water continuously flowing into the pool causes significant erosion of the surrounding rock. The rock around the plunge pool may be polished or worn down into smooth, curved surfaces. This erosion pattern can often be traced back to the source of the water. In the case of waterfalls, for instance, the surrounding rock may show signs of constant abrasion by the falling water.
-
Undercutting and Overhangs: If the plunge pool is at the base of a waterfall or fast-moving stream, the force of the water can erode the rock underneath the waterfall, creating overhangs or ledges above the pool. These features are often highly identifiable because they indicate significant water erosion.
-
Turbidity and Sediment Patterns: If the water in the plunge pool is carrying sediment or particles, these may settle to the bottom of the pool in distinct layers. The location and arrangement of these sediments can give clues about the flow patterns of the water and the length of time the pool has been active. Clear water or suspended particles can also help to reveal flow lines and patterns.
-
Polished Rock Surfaces (Potholes): One of the most distinctive features of a plunge pool is the presence of smooth, polished rock surfaces or even potholes. These are formed by the constant action of the water and any sediment or debris carried by the flow. In some cases, this action can carve out small depressions, or "potholes," in the rock bed of the plunge pool, which can be clearly identified. These features are formed through a process known as abrasion.
-
Flow Channel or Scouring: The water that flows into the plunge pool may also carve out distinctive flow channels or scouring marks on the surrounding rock. These channels can be recognized as grooves or furrows that follow the flow path of the water, showing where the most intense erosion occurred.
-
Vegetation and Biological Indicators: In natural plunge pools, especially those found in rivers or streams, the surrounding vegetation can provide clues to the pool's location and function. Plants may grow around the pool's edge, and aquatic organisms might be found in the water. The presence of moss, algae, or specific aquatic life can signal a consistent and dynamic water source.
-
Rock Shelving and Downcutting: In some cases, the rock surrounding the plunge pool will show evidence of downcutting, where the flow of water has cut deeper into the rock layer over time. This can create a step-like structure around the pool that is very clearly identifiable.
Questions:
1. What size is the plunge pool, and what does its shape tell you about the erosive power of the waterfall?
2. Find the features described in the lesson. Can you find all 7 of the features discribed?
3. Take a photo of you or an object at GZ. (Optional)
Send me these answers.
Source:
Smith, John. Waterfalls and Erosion: The Formation of Plunge Pools. Geoscience Press, 2021.
Summerfield, M. A. Global Geomorphology. Longman, 1991.
Mills, A. Erosion and Sediment Transport in Coastal and Marine Environments. Elsevier, 2002.
Barton, R. Hydrology and the Management of Water Resources. Routledge, 2014.
Smith, B. D., and M. Lillie, editors. River Channels and Floodplains: Advances in Fluvial Geomorphology. Wiley-Blackwell, 2005.