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Geo-Joint: Remarkable Rivers

Posted on April 12 2016

Water flows downhill, and takes the easiest path that gravity pulls it along. Depending upon the kind of geography and geology in a drainage basin, that stream course will develop its own individual qualities, from sculpted rocks, to unique water chemistries, to particular plant assemblages.

The Cristales River, or Caño Cristales, in Colombia is one such “special circumstances” river. Its waters support the growth of a water plant with the name Macarenia clavigera, which resembles algae or moss, but is neither. It grows with a rich green color in shade, but in sunlight, under the right conditions, it changes to colors ranging from pink, to red, to fuchsia in a most striking display.

The show only occurs from around June to December, during the wet season. Even then water levels have to be just right – too much, and the clarity diminishes, slowing growth; too little, and the plants die off, or go into dormancy. At the “Goldilocks level” however, the riotous display of reds and pinks, along with the blue of the water, the yellow of the sand, and the greens of shaded Macarenia have given people to call it the “River of Five Colors.” The river is quite remote and only reasonably accessible by plane, then boat, and then on foot, and was until recently considered too dangerous a place to visit because of insurgent activity. The area has since been secured, but tourism has had to be regulated as it became a major draw, and the fragility of the ecosystem put it at risk of destruction from overuse.

Another brightly colored river is the Rio Tinto in the region of Andalusia in Spain. This time the shocking red color comes from an abundance of iron in the surrounding landscape, and the highly acidic water itself carries the crimson hue. Iron lends the color, but copper, gold and silver mining operations have fouled the waters with heavy metals for thousands of years. So much so, in fact, that a suite of unusual bacteria which can digest iron and sulfide minerals have colonized the river bottom. The oxides they produce have added to the river’s toxicity. Their ability to withstand these extreme conditions has made them a topic for scientific study. Indeed, the whole river environment is under study as a facsimile of the Martian subsurface environment which NASA plans to probe further in years to come.

A river on the Philippine island of Palawan, the Puerto-Princesa, defines a national park of the same name. The river itself is a subterranean flow in this karst limestone region, which is nothing unusual in and of itself. Puerto-Princesa’s claim to uniqueness is the fact that at its terminus, it flows directly out of a cave into the sea. Roughly five miles long, the river is affected by the ocean tides and is brackish along its lower half. The park runs from mountain peaks to the sea and the entire drainage of the Puerto-Princesa is protected, a key feature in preserving the diverse wildlife both on the surface and in the underground river. The caves at the mouth of the river are wide and tall, allowing easy access for small tour boats to ferry visitors past the always-fantastical shapes of the eroded limestone walls and the elaborately deposited dripstone within. The landscape outside the caves is similarly bizarre, with high sculpted limestone cliffs and tall blocky islets jutting out of the blue-green water. It is an other-worldly location, like the Cristales and the Rio Tinto, special places made extraordinary by flowing water.

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