You can watch videos on the web, but this is a great experiment to try at home, if you do it carefully and don’t burn yourself! On a day with very cold, dry air, throw a cup of boiling water into the air. Use a sweeping motion that aims the water away from you. The water will break into tiny liquid water droplets, most of which will evaporate before they hit the ground.

Evaporation plays a key role in this fun phenomenon. Evaporation is the transition of water from the liquid phase to the gas phase. The rate of evaporation depends on the temperature difference between the water and the air, and evaporation is more rapid in low humidity. Hot water is closer than cold water to evaporating, so your cup of near-boiling water is already close to becoming a vapor.

By dispersing the liquid water into a collection of liquid water droplets, you’ve increased its surface area, which also speeds evaporation. The combination of hot water, cold, dry air, and high surface area causes most of the tossed near-boiling water to evaporate before it hits the ground, and the tiny droplets that don’t evaporate will freeze into ice crystals while still in the air.

If the water is not hot enough, evaporation will be slower. Instead of evaporating, the liquid will hit the ground first, and then freeze.

Photo: Oct. 7, 2010, friedrich glorian

With red sludge still visible, women survey the damage in Hungary. Eight died when a dike burst at a factory that processed ore into alumina, a raw material for aluminum.

Have you seen the photos of aluminum sludge surging through villages in Hungary, heading for the Danube River? With eight people dead, and new cracks appearing in the wall containing a pond-ful of alkaline sludge, we’re left to hope that the toxic crud is defanged by dilution before it does too much damage to the mighty Danube.

Still, the spill got us to thinking about the plight of the world’s rivers. Rivers are our foremost source of freshwater, used for drinking, industry and agriculture. Their wetlands and floodplains clarify water, temper floods, and provide an irreplaceable habitat for countless plants and critters, many of which are the major source of protein for hundreds of millions of people.

But a new study in the journal Nature shows that the globe’s rivers are being lambasted by pollution and invasive species. Heavy burdens of artificial fertilizer have created dead zones at the mouth of hundreds of rivers. Rivers are being over-fished, channeled into barge canals, and drained for irrigation, industry and drinking water.

Graphic: Barry Carlsen, copyright University of Wisconsin Board of Regents

A new analysis of 23 threats to global water security and biodiversity shows many regions with a high cumulative level of threats.

When the study¹ assessed river health in terms of pollution, biological change, watershed disturbance and water resource development, rivers carrying 65 percent of the total amount of water that rivers bring to the ocean “is moderately to severely threatened on a global basis,” says study co-author Peter McIntyre, a professor of zoology and freshwater expert at the University of Wisconsin-Madison.

Dam difficult

Both human water supplies and the natural world are endangered, McIntyre says. “One-quarter of the world’s vertebrate species live in fresh waters, and hundreds of thousands of plants and animals are at risk because they live in places where threats are high.” In total, biodiversity is more threatened in freshwater than it is in saltwater or on land, McIntyre says; ominous declines are being seen in fish, turtles, mussels and plants.

Lest “biodiversity” sound frivolous, estimates suggest that the value of “ecosystem services” like clean air and clean water exceeds the global economic output. The necessity of clean water is obvious, but we are also utterly reliant on plants, above and below water, to convert carbon dioxide into oxygen.

And these ecosystem services are best served by stable ecosystems.

Managing freshwater is a delicate balancing act, and some experts anticipate that tightening supplies will lead to disputes or even water wars later in the century. The U.S. government says if current trends continue, “by 2025, one-third of all humans will face severe and chronic water shortages,” with the first and worst problems appearing in Africa and the Middle East.

Already, the Colorado River in the United States, and the Yellow River in China, are so thoroughly exploited that they scarcely reach the ocean. Low flows and massive pollution plague rivers in China, India, the Middle East and Africa.

Nile denial

Photo: Bionet

The Nile River supplies virtually all water in Egypt (notice how fields cluster along the river?) and major portions in Uganda, Sudan and Ethiopia. The Nile is polluted by sewage and agricultural chemicals, and is failing to supply growing populations along its dry lower stretches with enough water for a good standard of living. With a watershed that includes parts of 11 nations, disputes over the Nile’s water could devolve into war.

Water security vs. environment: Inevitable tension?

Although pollution, invasive species and overfishing play major roles in declining freshwater biodiversity, dams and associated water diversions are a fundamental part of the tension between environment and river development.

Dams are built to store and divert water, supply hydroelectric power and prevent floods. Dams, and the locks that allow ships to traverse them, remain a keystone of river management in Western Europe and the United States, which is home to an estimated 75,000 dams.

While dam construction is largely over in Europe and North America (where some dams are even being removed), the 20th century was epic for dam building, says Bradley Udall, director of the Western Water Assessment at the University of Colorado. Udall notes that the volume of water stored behind dams has risen 350 times since 1900, to 5,000 cubic kilometers.

At the same time, Udall notes, due to such alterations as damming, draining, levees and development, “We have destroyed one-half of wetlands worldwide, which are very important for all kinds of ecological services, including water purification.” (Watch 23,000-plus large dams spread across the world.)

Chinese (river) checkers

Dam building is booming in developing countries, as an answer to floods and shortages of water and electricity. China’s Three Gorges Dam was essentially completed in 2008, after more than 1 million people were moved away from a new lake that is expected to cover 400 square miles. With a planned electrical output equal to more than 20 large nuclear plants (about 10 times greater than Niagara Falls), Three Gorges was also intended to halt disastrous flooding on the Yangtze River.

The series of dams that China is building or planning along the Yangtze and its tributaries will generate even more electricity than Three Gorges.

Yangtze River in When, China

Photo: Doris Antony

The river, shows intense pollution and human habitation in a city of about 9 million.

Dams can raise issues in any location. As Three Gorges proved, they displace riverside villages and cities and drown archeological sites. As is happening at the Glen Canyon dam in the United States, reservoirs can fill with silt, losing storage capacity and causing erosion as downstream areas are deprived of their normal silt supplies.

Dams also divert money that could be used for other purposes.

Granted, dams are a critical source of usable water, but they can also be a scourge of native plants and animals. “There is definitely a tension between human infrastructure and biodiversity conservation,” says Laurence Smith, a professor of geography at the University of California at Los Angeles, and author of a new book on environmental trends².

China is embarked on the largest water project in history, a 50-year program to move water from the Yangtze toward population centers in the drier north. Designed to move 50 cubic kilometers per year, the project aims to reduce sandstorms and water shortages while bolstering sinking aquifers in North China.

U.S. Fish and Wildlife Service

Rivers in the North, like Alaska’s Koyukuk, are far less impacted by pollution, diversion and damming.

Failing fish

Courtesy Peter B. McIntyre

A man fishes at Igamba Falls, on the Malagarasi River, Tanzania, site of a proposed hydroelectric dam. Fish are a major source of protein — and dams are a major cause of fish declines.

Altering rivers with dams enacts fundamental changes in ecosystems, says Smith. “A lot of the most biologically diverse riverine environments are seasonally flooded wetlands and flood plains. Biodiversity is not found in a big reservoir behind a dam… It is more the episodic flooding [of natural rivers] that gives this diverse habitat.”

Dams block the migration of important fish species, including the salmon, which is vanishing along the Atlantic and Pacific coasts of the United States, where dams block the upstream spawning journey.

That problem is widespread, says McIntyre. “In the tropics, species like big catfish, and the family known as the tetras, are very intensively fished. You have regions where people depend on these migratory fish, and if you put in a dam to stop the migration — rivers are aquatic highways — you profoundly change the system. There’s a real concern that if fisheries collapse, hundreds of millions of people worldwide who get a majority of their protein from freshwater fish could go hungry.”

In the central United States, massive dams and engineering projects on rivers like the Illinois, Missouri, Mississippi, Tennessee, Wisconsin and Ohio have also been blamed for ecosystem destruction.

For example, locks and dams north of St. Louis on the Mississippi stabilize the water level so large barges can traverse the river. But that stability, combined with extensive levees on the banks, has eliminated vast wetlands that once bordered the river. When the river no longer surges in the spring and subsides in the fall, remaining flat land along the river turns to muck that can no longer support native plants and animals.

Photo: Jason Sturner

Massive engineering projects along the upper Mississippi River have essentially changed the river into a barge canal.

Biodiversity black hole

One reason to foster biodiversity in rivers and watersheds is this: Biological systems with many interacting species tend to be more stable, and people, like other animals, have adapted to a fairly stable environment. “In experiments with bacteria, if you strip away species, you eventually hit a point where the basic properties change,” says zoologist Peter McIntyre. “It can be on a plateau of high function for a while, but there is a threshold, and we can’t predict where it occurs, things start to fall apart.”

The classic analogy, McIntyre says, “is popping rivets on the wing of an airplane; you pop one too many, and boom! down you go. In the global river context, we are rolling the dice, we know we are losing species. The rates of extirpation and extinction are highest in freshwater; and that is where we are seeing the worst human impacts.”

Scientists who are looking more broadly at the health of river ecosystems are hampered by a lack of information. “There are no global data sets” that would support an exact measurement on the biological health of rivers around the world, says Carmen Revenga, a freshwater scientist at the Nature Conservancy.

Still, new evaluations of biodiversity are delineating the difficulties. Revenga says a recent assessment listed 253 endemic species of freshwater fish in the Mediterranean — meaning they are found nowhere else — and 56 percent of them are threatened with extinction. Another survey found severe declines among 40 percent of the 300 species of freshwater turtles, she adds. “Nobody would have guessed it was that bad.”

The inevitable tension between environment and human water use is growing more intense in dry places such as Africa and Australia, with heavy population pressure and intense land usage.

When farms, people or industry get thirsty, “Freshwater biodiversity has not tended to play role in discussions about water security,” Revenga says. “Usually there is a lot of focus on providing water that is secure and safe. Irrigation took precedence at first, and now cities take precedence, but the ecosystem hardly gets included.”

The sad, dry Aral Sea

Photo: Gilad Rom

The Aral Sea in Central Asia dried up after the rivers that fed it were diverted to irrigate vast cotton farms.

And that leaves less water for ecological purposes, Revenga adds. “When we calculate the amount of runoff in a basin, we assume we can tap all the water that’s available” for human uses. “The conservation and environmental community has not interacted with the water supply community, and the environment is almost forgotten.”

Mono Lake in California, whose water was diverted to Los Angeles in the 1940s, is one example showing that cities and farms have come first in American water management. According to the Mono Lake Committee:

In recent decades, California has been pressured to allot some water to environmental purposes, part of a gradual rebalancing of water use in the dry, densely populated American Southwest.

We’ll explore evidence of progress in water management in the next Why Files, but note that cities like New York rely on watershed protection to ensure a clean, adequate water supply. “It’s a very good strategy to protect upland forest, and reduce siltation and runoff into streams, but a lot of projects don’t look at biodiversity in the river,” Revenga says. Watershed protection is rare, and in any case the ecological benefits are secondary to the need to provide clean water to cities, she adds.

Climate change

As more people look to rivers to supply more water, there’s one final factor to consider: the climate. Brad Udall of the University of Colorado, an expert on the waters of the West, told us that climate change is not just about temperature. “You could make a compelling argument that it’s about changes to water cycles; changes in the quantity, quality and timing, almost all of which are detrimental” to freshwater supplies.

In general, Udall says, studies of ancient climate show that “wet areas get wetter and dry areas get drier.” In the Colorado River basin, where climate change has been intensely studied, “we can expect a 10 percent to 20 percent reduction in runoff by 2050.”

Photo: Laslovarga

Hoover Dam and its reservoir Lake Mead are major factors in Western water management, but at what environmental cost?

Because so many rivers in the American West are fed by melting snow, warmer winters already have a major impact, Udall says, with the earlier spring causing earlier runoff in the rivers. Studies project that the floods could happen as much as 60 days earlier in the spring, “and we are already seeing 20-day advances, especially in lower-level snow-dominated systems, like in the Pacific Northwest.”

At the same time, river flow is likely to diminish earlier in the late summer, and the water will also be warmer, Udall says, which poses problems for life. “Many critters in the water are stressed in high temperature, which also carries less dissolved oxygen.”

Dry conditions in late summer also contribute to a longer wildfire season in the West.

Climate change may be even more catastrophic where drinking water comes from rivers sourced in melting glaciers, Udall warns. Large cities like Bogotá and Lima in South America, “could go from having a glacier upstream one day to not having it the next. The United States does not have that problem, but in the Andes, there is potential for very harsh consequences.”

The Ganges: A river or a sewer?

Photo: Daniel Bachhuber

Worshippers leave heaps of offerings alongside the Ganges river in Varanasi. It’s easy enough to hose away the dregs into the river, but that just adds more pollution to the “Mother Ganges.” Says Science Daily, the Ganges “contains untreated sewage, cremated remains, chemicals and disease-causing microbes. … Cows wade in the river. People wash their laundry in it and drink from it. … The Ganges River is a major source of disease.”
If this can happen to a sacred river…

Summary judgment

Rivers collect runoff from their watersheds, and therefore carry messages about conditions from most of the land on our planet. As the authors of the recent Nature study found, trying to assess the health of rivers around the world is not for the data-shy. Differences in economy, history, geography and culture all affect how we view rivers, and how we decide whether to use, alter or preserve them.

Photo: Steve Dewey, Utah State University, Bugwood.org

Purple loosestrife is striking, but it invades wetlands near rivers, reducing biodiversity, destroying habitat for native species, and reducing the ability to filter water.

But most “decisions” that affect rivers, like allowing them to be polluted with chemicals, topsoil or fertilizer, or building a dam to store water for the dry season, are made not with rivers in mind, but with another goal, like growing more food or getting people something to drink.

“You don’t miss your water ’til your well runs dry,” pertains to rivers as well as groundwater, says Peter McIntyre, one author of the recent global river survey. “In the industrialized world, we go home at night, turn on the faucet and get beautiful, clear water, it’s safe to drink and bathe; it poses no risk to us and our kids. Mass investments in engineering and infrastructure have granted us this water security.”

As developing countries, where people struggle to find water for faucets, farms and factories, embark on the dam-building that was so crucial to European and American water supplies, saying “don’t do what we did” seems hypocritical at best and repugnant at worst.

And yet experience shows that dams can damage or destroy plants and animals in rivers and their floodplains. We’ll concede that questions about biodiversity, the environment and the long term seldom interest people who are hungry or thirsty. But they may still be worth asking. Will a proposed dam harm an essential fishery? Will it produce benefits over the long term, or will it silt up in a decade because trees have been stripped from its watershed?

There are lessons to be learned from the water-management experience in Europe and North America, and one of the most significant one is to expect a continual tension between human water use and biodiversity. “I am not pretending there is an easy answer,” McIntyre says, “or that I should have the right to dictate to that person whether they build a dam or not.”

Coming Oct. 28: Part II : The Why Files will discuss some river-management ideas for balancing human and environmental needs.

Historians tell us the Spanish introduced pressurized water systems to the New World. But a new study indicates that the Maya were building pressurized pipes between about 450 and 750 AD, in Palenque, a major Mayan city in modern-day Mexico.

The Maya built a large number of cities in the Yucatan, Guatemala and Belize, before their cities were suddenly and mysteriously abandoned around 800. The Maya, whose descendants still live in the region, wrote with hieroglyphs, had extensive knowledge of astronomy, and their economy was strong enough to support cities such as Palenque, Chichen Itza and Cobal.

Until now, nobody had found evidence for pre-Spanish pressurized water in the New World, say the two authors of the new study.

The evidence takes the form of a narrow constriction in the underground Piedras Bolas aqueduct that routed water from a spring into Palenque. Unlike many Mayan cities, Palenque was built in low mountains, with only about 2,200 hectares of reasonably flat land. Untamed streams would gobble valuable real estate, so the Maya built limestone conduits to rout water through the city.

In some cases, the Maya plastered the inside of conduits with stucco to prevent leaks. And like modern builders, they Maya covered the conduits with stones that paved city streets and plazas.

Streaming, but not video

The suggestive constriction was six meters below the spring that supplied the stone pipe, and that height differential put the water under pressure, says co-author Christopher Duffy, a professor of civil and environmental engineering at Penn State University. The system is “analogous to a modern water distribution system. The water tower produces a ‘hydraulic head,’ or water pressure. The pipes go underground, and back up into the home, where water flows under pressure.”

The small opening at the bottom allowed the Maya to close off the conduit, so it would stay full of water. Air in the system will neutralize the hydraulic head, Duffy says.

Unfortunately, the Palenque site has been disturbed, and tantalizing questions remain, Duffy says. “We don’t know how they distributed the water from this point, but we can’t see any other purpose, other than as a control point in the buried conduit.”

Paving paradise to put up a … fountain — or a toilet?

Archaeologists already know that the Maya had an extensive irrigation system, fed by nine streams that ran through Palenque to the fields below.

The constricted conduit, one of nine, had a capacity of about 68,000 liters, and it alone could have stored enough water to supply scanty rations for several thousand people for a week during the dry season.

The pressurized pipe could have supplied a fountain where people could dip jars to collect drinking water. But the putative fountain was “probably beautiful,” says co-author Kirk French, a lecturer in anthropology at Penn State. “Everything the Maya did at Palenque was over the top, grandiose, in art and architecture.”

Fountains also serve a social purpose, says French. “They are in a central part of the city, where people can fill jugs and socialize. It’s funny, we refer to ‘water-cooler conversations,’ but it seems this has been going on for a very long time.”

Did the Maya’s pressurized plumbing have a more, er, “sanitary” function? “We don’t know the exact application,” admits Duffy, who specializes in hydrology, “although we were recently told, after the paper came out, that there are sweat baths, and perhaps toilets, in the palace at Palenque.”

In fact, the palace has “four toilet-like features,” French says, “They are in a line, at the right height, and share the same drain, but it’s hard to prove that they are toilets.”

The sanity of sanitation

Toilets or not, the newly discovered plumbing shows that the Maya “are better engineers than they ever got credit for,” Duffy says. Although the Maya may have never seen pressurized water flow in nature, people are inventive, especially when it comes to something as important as water.

“We think this is the first example in the New World, but a lot more will probably be discovered,” says Duffy. “The Maya built like the Romans. They were practical. They would build, if it failed, they would build again. It’s a standard engineering strategy. Do something, fail, learn, and do it again.”

– David J. Tenenbaum

Water is an exceptionally interesting chemical with many important implications for life on Earth and the circulation of the atmosphere. It is the only chemical that naturally exists in all three phases (solid, liquid, and gas) in our atmosphere.

At any moment, the atmosphere contains an astounding 37.5 million billion gallons of water, in the invisible vapor phase. This is enough water to cover the entire surface of the Earth (land and ocean) with one inch of rain.

What’s more, this amount of water is recycled 40 times each year in what is known as the hydrological cycle. That means a water vapor molecule has an average residence time in the atmosphere of only nine days: the raindrop that fell on your head last Tuesday on average had evaporated into the atmosphere nine days before.

This huge amount of water is processed through an endless cycle of evaporation, condensation, and precipitation all over the globe. Evaporating water requires energy – in fact, it takes 600 calories of energy to evaporate each gram. When that gram of water condenses back into liquid, that same amount of energy is released back into the atmosphere.

The amount of energy released as 37.5 million billion gallons of water condenses would be enough to power the Madison, Wis., metro area for 144.5 million years! Think about that the next time you get caught in a spring rain shower!

There are plenty of cats that love water, according to Sandi Sawchuk, a clinical instructor at the School of Veterinary Medicine.

Big, wild cats, especially those that live in hot, arid areas, often love to swim. An Asian species known as the fishing cat uses webbed paws to dive for fish, frogs and crayfish.

Among domestic breeds, the Turkish Van is known for swimming. But most house cats shy away from water.

“Because cats groom themselves, we as owners tend not to introduce them to bathing like we do our dogs,” Sawchuk says. “Ask somebody who has show cats, which have to be bathed regularly and have been in the water since they were young. Those cats will tolerate it. There’s no fight at all.”

The spray-bottle method of controlling cat behavior — giving them a squirt between the eyes when they jump up on the counter — would make just about anyone wary.

“What’s the cat going to think about water then, when it’s only used for punishment?” says Sawchuk, who has three cats at home.

It’s not uncommon for the owners of Sawchuk’s feline patients to describe their cats sitting on the edge of the bathtub, batting at the stream of water from the faucet or showerhead.

“When they have the ability to control how much they can get near the water and how much exposure they have, they don’t seem to mind at all,” Sawchuk says.

Both lungs and gills have a bed of very small blood vessels with thin walls that the gases can easily travel across. But gills have a much harder job than lungs, Malison says. “It’s a big challenge for a fish. The air we breath is 20 percent oxygen, or 200,000 parts per million.”

Water holds 4 to 8 parts per million of oxygen, he adds. “It takes an awful lot of work for the fish to exchange gases, particularly oxygen. It just takes a lot of energy.”

Because fish must open a “terrific” amount of blood vessels to the water, they may have problems controlling salt flow, Malison adds. A freshwater fish will constantly lose salt through its gills, while a salt-water fish may have to spend a lot of energy keeping excess salt out.

Despite these challenges, gills are much older than lungs, Malison says. Complex organisms with spinal columns arose in the sea hundreds of millions of years before they moved to land.