|This article does not cite any references or sources. (May 2009)|
Plunge pools are formed under the force of a natural source, such as a waterfall or rapids, but also as the result of scour from man-made objects such as spillways and bridge abutments. The swirling water, sometimes carrying rocks within it, erodes the riverbed into a basin, often featuring irregular and rough sides. Plunge pools can remain after the waterfall has ceased to exist or the stream has been diverted.
Plunge pools are erosional features which occur in the youthful stage of a river. When soft rock has been eroded back to a knickpoint, water constantly bombards its base. Because this rock is often less dense than surrounding strata, the water from the higher grade continues eroding downward.
Garden feature 
A plunge pool, also known as a cold plunge, can be a recreational water feature in landscape design, and constructed in gardens. They can be small-diameter swimming pools used for wading in warmer climates or adjacent to saunas.
These plunge pools are built to the same codes and specifications as swimming pools in regards to structural engineering, sanitation and filtration requirements, safety and liability precautions, and building permits. They can be unheated in climates without seasonal freezing, since their purpose is not for active recreation or exercise. Their smaller surface and ease of covering reduce evaporative water losses, and need for potable water refilling. Their popularity has increased with concerns and needs for domestic water conservation, more sustainable landscaping design, and reduction of resource consumption with energy-efficient landscaping.
See also 
- Stream gradient
- List of waterfalls by flow rate
- List of waterfalls by type
- Sustainable landscape architecture
- Marshak, Stephen, 2009, Essentials of Geology, W. W. Norton & Company, 3rd ed. ISBN 978-0393196566