The Yellowstone National Park is one of the country's most popular national parks, catering to more than 3 million visitors per year, but it appears that tourism has had an unwanted impact on the natural appearance of the place, particularly its hot springs.

A group of researchers revealed how Yellowstone hot springs looked like years before tourists started to contaminate them with make-a-wish coins and other debris that changed the pool's chemistry.

Joseph Shaw from the Electrical and Computer Engineering Department of Montana State University, together with colleagues, simulated the vivid colors and optical characteristics of the park's hot springs using a simple mathematical model that takes into account several factors, including the spectral reflection of each of the pools caused by microbial mats, the optical absorption and scattering of waters, as well as the solar and skylight conditions that were present at the time the measurements were taken.

Two years ago, the group took measurements and images at several pools in Yellowstone, including Sapphire Pool, Grand Prismatic Spring and Morning Glory using handheld spectrometers, DSLR cameras and long wave infrared cameras.

By using the collected data and previously available information regarding the physical dimensions of the pools, the researchers were able to come up with a simple model whose renderings of the hot springs looked strikingly similar to the actual photographs.

The researchers even managed to simulate how the Morning Glory Pool looked like between the 1880s and 1940s when its temperatures were much higher than those of today. During this period, the Morning Glory's waters were a uniform deep blue and the shift in its appearance can be blamed on tourists.

The pool's orange-yellow-green appearance today has something to do with thrown coins, trash and rocks that over the years partially obscured its underwater vent, lowering the temperatures and resulting in changes in the composition of the microbial mats.

The researchers confirmed in their study a general association between shallow water temperature and the composition of the microbial mat with the observed colors, albeit the color patters in the deeper parts of the pool are attributed more on the absorption and scattering of the light in the water.

"The measurements and simulations confirm that colors observed from shallow-water features arise primarily from the spectral properties of the microbial mat, which is related to the water temperature, while colors observed from deeper water arise primarily from the wavelength-dependent absorption and scattering in the water," the researchers wrote in their study published in the journal Applied Optics on Dec. 19.

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