The image above is from a holiday snap in 1958. The one below of more recent times.
The colour change from pristine blue to murky yellow-green is thanks to tourists who, over the years have thrown coins, rocks and rubbish into the spring.
According a study by researchers from Montana State University and Germany’s Brandenburg University of Applied Sciences the objects clogged up an underwater vent which then caused the water temperature to cool, allowing pigmented bacteria to thrive.
A general relationship between shallow water temperature (hence microbial mat composition) and observed colours was confirmed in this study. However, colour patterns observed in deeper segments of the pool are caused more by absorption and scattering of light in the water.
These characteristics — mats having greater effect on colour in shallow water, and absorption and scattering winning out in the deeper areas — are consistent across all the measured pools.
The spring has always left an impression on those who have seen it. The researchers quote a member of the 1871 Hayden Expedition report:
“…nothing ever conceived by human art could equal the peculiar vividness and delicacy of color of these remarkable prismatic springs”
The research was carried out through measurements using handheld spectrometers, digital SLR cameras for visible images and long wave infra-red thermal imaging cameras for non-contact measurement of the water temperatures.