Antarctica's longest river flows back out of the sea

Onyx, the longest river in Antarctica, does not flow into the sea but flows in the opposite direction due to the unique geological features of this place .

Antarctica has a lot of water, but most of it is frozen. Because of this, many people think that rivers don't really exist on the continent. But that's not true. Antarctica does have some rivers, which flow for at least a few months each year. The longest of these is the Onyx River, and it's quite a strange river.

The 32km long Onyx semi-permanent river. (Photo: Mozgova).

Technically, the Onyx River is a glacier-fed river . At 32km long, the Onyx is short compared to the 6,650km Nile, but considering the icy landscape it flows through, its length is impressive.

The Onyx River is located in the McMurdo Dry Valleys, flowing west from the terminus of the Lower Wright Glacier into Lake Vanda. This means it flows away from the sea.

This is an example of an endorheic stream—a water system that does not drain into the ocean. The river exists because of a unique geological feature: Glaciers block the entrance to Wright Valley, forcing the Onyx to flow only further inland , toward the ice-covered Lake Vanda.

Antarctica has nine such glacier-fed rivers , but they are different from most. While rivers like the Amazon and Congo originate in the mountains and are made up of many small streams, the Onyx began flowing when the Wright Lower Glacier melted. As a result, the Onyx is a semi-permanent river, forming only during the Southern Hemisphere summer, when temperatures are high enough to melt the ice.

Rain does not contribute to the Onyx's flow because the dry valley rarely rains, and what falls sublimates before reaching the river. However, melting glaciers usually provide enough water to sustain the flow. Over the past few decades, there have been some flooding events during particularly warm summers. In 1984, researchers from New Zealand were even able to row the river. As temperatures rise due to climate change, the Onyx's flow season is starting earlier and lasting longer, although the amount of water flowing appears to be decreasing.

Not only is the Onyx River the largest ice-flowing river in Antarctica, it is also its most ecologically important. It has its own microscopic ecosystem, supporting a wide variety of microscopic organisms, from tardigrades and nematodes to cyanobacteria and phytoplankton. Algae also flourish in the Onyx's waters, despite the lack of fish.