Physics Photo of the Week

February 22, 2013

Dendritic erosion
It's intriguing to compare physical appearances between small objects and much larger objects.  This photo emphasizes the brief erosion channels that appear in the sand on a beach formed by the receding wave.  The dendritic appearance of the small beach drainage channels resembles the dendritic appearance of rivers draining a continent on Earth.  Once a small channel becomes eroded, it channels more and more water, with other channels joining forming the dendritic appearance.   The resulting miniature "gorges"  eventually eventually empty into the water's edge and soon become erased by the next wave.

Compare the sand channels in the beach with Colorado plateau erosion just north of the Black Canyon of the Gunnison in western Colorado in the right hand photo.  The steep walled Black Canyon traverses the lower part of the aerial photo made by myself on a commercial flight.  The canyon's rim is at the top of the steep scarps about 1/4 from the bottom of the photo.  The broad plateau area from the scarps toward the center of the photo slopes gently to the north, away from the Black Canyon, toward the next valley and irrigated cropland in the photo's center near Crawford, CO.  The morphological similarity with the beach sand erosion is striking! 

The erosion channels that appear in the beach slope are spaced about 10 cm apart.  The erosion channels in this Colorado mesa are about 700 m apart.  This self-similarity spans space by a factor of 7000.  Similar erosion channels can be seen in a map of the rivers and tributaries on a continental scale (about 1000 times greater than this plateau, and 10 million times the scale of the beach sand channels).  The time for the beach channels to appear and disappear are about 10 seconds - the period between waves.  However, the time scale for the Colorado plateau erosion channels is millions of years compared to a few seconds for the beach sand.  The time scales for these similar formations span a factor of a factor of billions! 

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

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