51����

When most people are asked to name a river, they often reach for the most famous: the Amazon, Nile, Ganges, Mississippi, Thames. But these aren’t the only remarkable rivers out there. From a river that experiences a thundering wall of water that hurtles upstream to an underground river that is home to bird-catching spiders, here are 10 more from around the world – and solar system.

The Itchen, England

A kingfisher dives between the white flowers of common water-crowfoot as clouds of mayflies hover overhead and an endangered white-clawed crayfish forages on the gravel riverbed. Salmon and trout head upstream, avoiding the otters scrambling amongst the yellow irises.

This is the river Itchen, a chalk stream that flows for 40 kilometres through Hampshire in southern England. It is “a globally important river”, says Martin de Retuerto at the . The variety of life and abundance here “truly is quite astounding”, he says.

Chalk streams are globally rare habitats. . The water comes from the chalk aquifers below, making it mineral-rich, clear and a stable temperature throughout the year. Plant growth is encouraged by the large amounts of light that can penetrate this clear water, alongside the high availability of minerals, especially calcium. “With a chalk river, the type and quality of water gives rise to very complex and diverse plant communities that are up there in a global context,” says de Retuerto.

Like all rivers in the UK, the Itchen has been modified by humans. Recent restoration work has focused on rejoining the river to its flood plain, opening passages for salmon to travel upstream and reintroducing lost species, such as the Eurasian otter. One success story is the . “They nearly went extinct in the early nineties when the crayfish plague from North American signal crayfish got in the river,” says de Retuerto.

The biggest threats to the Itchen are abstraction of water from the aquifer and , says de Retuerto. “We’re not treating [the Itchen] like the finely tuned, high-performance thing that it is.”

The Diamantina, Australia

Winding through arid west and , has . Large sections are dry for months at a time until water floods downstream following summer monsoon rains in its upper sections. “It’s really spectacular,” says at Flinders University in Adelaide. “It’s this gushing torrent of water that wakes up the outback.”

As the water travels downstream, it bursts out of its main channel and fills hundreds of intertwined channels. In the wettest years, this . Only , on average, does enough rain fall for the water to reach all the way to its terminus: Kati Thanda-Lake Eyre in South Australia.

Birds quickly arrive at the flowing river to feed and breed, including . “It’s beautiful being out in the centre of Australia when it’s rained,” says Shanafield. “It’s just amazing to see all the wildlife that comes out.”

The plants and animals living in the area are adapted to this “boom and bust” cycle, such as . “These ecosystems can wait years if they need to and then, as soon as it rains, they come to life,” says Shanafield. “[They] then disappear again and wait until there’s water in the stream.” During dry periods, provide .

Around 70 per cent of Australia’s rivers only flow for some of the time. “What’s interesting to me is that, if you look at a map of Australia, it looks like there’s no rivers here, or just the Murray-Darling river snaking down the east coast,” says Shanafield. “Actually, there’s rivers all over the place.”

The Qiantang, China

For a spectacular natural phenomenon, look no further than the Qiantang river in China’s Zhejiang province, which is home to the . “It is an almost vertical wall of water rushing upstream along the river with its foaming front and thundering noise,” says Dong-Zi Pan at the Zhejiang Institute of Hydraulics and Estuary, who has for 15 years.

The bore forms as an incoming tide is funnelled into the narrow entry to the Qiantang from the wider Hangzhou Bay and passes over a sand bar at the river’s mouth that reduces the water’s depth. Together, these concentrate the water into a wave that sweeps upstream at between 4 and 7 metres per second. Normally, the bore is about 1 to 2 metres in height, says Pan, but it – and it can get even larger if it hits an obstacle. “It’s different every time and makes me feel exhilarated,” he says.

The largest bore of the year is seen in August, when the sun, moon and Earth are in their closest alignment – creating the largest incoming tide – and the prevailing winds are in the right direction, says Pan. The bore can travel up to 100 kilometres upriver and hundreds of thousands of people line the banks when the largest bores are predicted to appear.

Caño Cristales, Colombia

This beautiful river is the Caño Cristales in Colombia. It is a riot of colour between June and November: reds, greens, blues, yellows and purples all glisten under the crystalline water.

These colours are partly due to the plant , also known as Macarenia clavigera. Found only in this region of Colombia, it has green hues in shady regions and red ones in sunny spots. “The other colours of Caño Cristales are given by its surroundings: the sky, the sand, the stone,” says at Cormacarena, a local government department with responsibility for the river.

To survive the dry season, the plant nestles between the stones in order to get moisture from the humidity trapped there, says Leyva Quijano. Once rain refills the river, the plant is “reborn”, she says. This rainwater flowing off the land is also responsible for the circular pits seen in the riverbed. Over millions of years, sand and small stones carried into the river have been whirled around in eddies at the same spots, carving out the pits, says Leyva Quijano. This creates different depths of water, adding to the range of colours.

The river is increasingly popular with tourists, and one challenge is to prevent damage, says Leyva Quijano. Climate change is also having an effect. “The rainy and dry seasons are changing, which affects whether the plant has developed,” she says. “If it has not reached an appropriate height, it can be uprooted by the force of the waters [following rain].”

The Elwha, Washington state

In 2014, something remarkable happened to the Elwha river in Washington state: it could flow from source to sea unimpeded once more. Two hydroelectric dams built in the early 1900s had been removed in .

The dams had made a huge impact on the river. Built without passages for fish to pass through, , including salmon central to the . They also flooded cultural sites and disrupted sediment flow, which caused to deteriorate. Fish populations crashed.

The , alongside environmental groups, . In 1992, the US Congress passed the , with work beginning on deconstruction in 2011.

It took just .

As the dams were breached, decades’ worth of sediment trapped behind them was released, with , which grew by 60 hectares in five years. This sediment pulse was , although they are expected to bounce back as the Elwha returns to its natural state.

The Vjosa, Albania

Known as the last wild river in Europe, the Vjosa is free flowing for almost its entire 270-kilometre length, with only the first 10 kilometres at its source altered by humans. “That is very, very rare in Europe, especially in central Europe close to the global tourist hotspot, the Mediterranean Sea,” says at environmental group RiverWatch.

Rising in the Pindus mountains in Greece, passes through narrow gorges and canyons into a wide, braided floodplain filled with sandbanks, willows and poplar trees, before meandering towards its estuary and entering the Adriatic Sea.

This , alongside the clear, clean water, means it is home to , including . “For wildlife it is important, because it is one of the last intact rivers at all in Europe,” says Eichelmann. “Lots of species, especially insects, have their last ‘home’ in the Vjosa.” Some of the rarest are the and the .

Plans to build 40 dams along the Vjosa and its tributaries sparked in Albania. “It is only now that locals and Albanians became aware how unique their river is,” says Eichelmann. On 15 March, the river was designated Europe’s first wild river national park, which should protect the river and its tributaries within Albania.

Puerto-Princesa underground river, the Philippines

Flowing through lush rainforest and past pinnacles of limestone rock, you would be forgiven for thinking the Cabayugan river on the island of Palawan in the Philippines is just another tropical river – until it abruptly disappears into the rock to spend its final 8 kilometres underground.

Here, it becomes the Puerto-Princesa underground river and flows through a series of spectacular caverns until it. It is home to an “extraordinary biodiversity for an underground river”, says Paulo Agnelli, who explored the caves as part of an expedition by La Venta, an organisation that explores hard-to-reach geographical features. “We found 17 species of vertebrates and at least 84 species of invertebrates.”

roost above the river, “which animate the tunnels along the river at dawn and dusk [as they] go out in search of insects”, he says. These are , while their guano supports ecosystems both in the caves and within the river. Also seen are freshwater and marine fish, molluscs, shrimp, scorpions and “huge and beautiful spiders” that can catch bats and swiftlets, says Agnelli.

“The co-existence of as many as five different ecosystems in a single cave is absolutely uncommon, if not unique,” says Antonio De Vivo, also part of La Venta. The river has the longest known underground estuary, with seawater reaching up to 5 kilometres upstream during the dry season. The contrast between the seawater and the colder freshwater “leads to the development of very rare phenomena, such as the formation of true clouds inside the huge chambers of the cave, even with short ‘rainstorms’”, he says.

Whanganui river, New Zealand

Found on New Zealand’s North Island, the Whanganui river rises on the slopes of the active volcano Mount Tongariro before flowing downstream for 290 kilometres to the Tasman Sea. It was the first river in the world to be granted legal personhood, following the longest legal battle in New Zealand’s history, .

The river is spiritually important to the Whanganui Iwi, who recognise it as their ancestor and who, for 150 years, from threats such as the removal of gravel, destruction of eel weirs and proposed hydroelectric dams. In response, government legislation and tribunals were used to cut away those rights and ownership of sections, such as the riverbed, was vested to the Crown.

As part of settling these disputes, the was passed in 2017 by New Zealand’s parliament, giving the river the same rights and responsibilities as a person. It includes – a representative from the government and from the Whanganui Iwi – to defend the river’s legal rights and maintain its health. , including improving spawning grounds for fish and habitats for the declining freshwater mussel.

Unnamed river, Antarctica

Antarctica may be a world of ice, but there is liquid water there if you know where to look. flowing into the Weddell Sea. “Now we know they exist, there will be others discovered,” says at Imperial College London, who was part of the team that found the river.

. However, it turns out enough heat is generated from the friction created as an ice sheet moves over rock, as well as heat from Earth itself, to melt its underside. “This river is special because it’s not surface melting that causes it, but basal melting – completely beneath the ice,” says Siegert.

This under-ice river helps lubricate the ice sheet as it flows towards the sea, allowing it to move more quickly than it would otherwise, says Siegert. This process could be affected as the world warms because of climate change. “As the ice sheet does change due to fossil fuel burning, so, too, will the flow of these rivers,” he says.

Vid Flumina, Titan

Earth isn’t alone in having rivers. Mars once had rivers larger than Earth’s, although all that is left of them now is dried-up valleys. But there is: Saturn’s icy moon Titan. The 400-kilometre-long Vid Flumina in Titan’s north polar region was spotted by the Cassini probe in 2012. This remarkably straight river is and flows into a sea called Ligeia Mare.

Analysis of the radar data sent back by Cassini suggests it . But, at -179˚C, Titan is too cold for liquid water. Instead, .

“We think it’s raining in the highlands and flowing down into these river valleys,” Thomas Farr, then at NASA’s Jet Propulsion Laboratory in Pasadena, California, told 51���� at the time of Vid Flumina’s discovery.