A river that flows uphill
The Onyx River is Antarctica's longest, measuring approximately 32 kilometers (20 miles) in length. It flows through the Wright Valley, one of the McMurdo Dry Valleys, which form the largest relatively ice-free area on the continent, covering around 4,800 square kilometers. The river's existence is unusual in one of the world's driest and coldest deserts. Its source is the meltwater from the Wright Lower Glacier and the nearby Lake Brownworth.
Unusually, the river flows west, away from the Ross Sea. This inland, or endorheic, drainage pattern occurs because the mouth of the Wright Valley is blocked by a glacier, forcing the water to flow toward the valley's center. The river's destination is Lake Vanda, an ice-covered, closed-basin lake. The river only flows during the brief austral summer, typically for a few months between November and February, when temperatures rise enough to melt the glacial ice. In some years, the flow is so minimal it never reaches Lake Vanda, while in others, flood conditions can cause significant erosion, with discharge rates once reaching 20 cubic meters per second.
Life in suspended animation
The Onyx River contains no fish. Its ecosystem is entirely microbial. The riverbed is lined with extensive photosynthetic mats primarily composed of cyanobacteria, such as species of Nostoc, Phormidium, and Oscillatoria. These dark-colored mats, which give the river its name, are biological activity in an otherwise barren area. They are responsible for most of the primary production in the ecosystem, fixing atmospheric nitrogen and carbon.
This simple ecosystem also supports microscopic animals, including nematodes, rotifers, and tardigrades. Tardigrades, also known as water bears, are famously resilient invertebrates, often less than 1 millimeter in size. They can survive the extreme desiccation when the river dries up by entering a state of suspended animation called cryptobiosis. In this state, they lose most of their body's water, their metabolism nearly stops, and they can endure for years until liquid water returns. When the glacial meltwaters begin to flow again each summer, these microbial and micro-animal communities rehydrate and become active, repeating a cycle of dormancy and life in one of Earth's most extreme environments.