Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves (20 page)

The thing is, water stays frozen between minus 460 degrees Fahrenheit and plus thirty-two degrees Fahrenheit—and the temperature of most of the universe lies within this range. Water’s gaseous state, steam, is maintained within an even larger temperature range, from 212 degrees to 2,700 degrees, when its molecules start to break apart. Thus ice and steam carry water’s banner throughout the cosmos. The liquid form prevails in just a very narrow span, from thirty-two to 212 degrees.2

And even the right amount of warmth is not enough for water to maintain its liquid state. Though we mostly live in temperatures between thirty-two and 212 degrees, given that Earth’s mean temperature is fifty-nine degrees, we still wouldn’t see liquid water unless we were under pressure. In parts of the cosmos—such as Mars, during that planet’s summer—enough heat exists to turn water into a liquid, but there is virtually no pressure, so H2O remains exclusively vapor and ice on the red planet.

The weight of Earth’s atmosphere provides the needed pressure. Reduce it and water readily boils off into a gas. All you need to do is drive up a hill to prove this for yourself. The boiling point plunges by roughly one degree for each five hundred feet you climb. So the hottest coffee in Denver is ten degrees cooler than the hottest coffee in Boston. Atop Mount Everest, water boils into vapor so readily that its maximum liquid temperature is about 160 degrees, unless one uses a pressure device. If astronauts hauled a bucket of water from their spacecraft to fill a decorative birdbath on the moon’s desolate surface, it would boil furiously while simultaneously freezing solid.3 Result: a grotesque modern art sculpture.

So H2O is common, but its liquid form is rare. Yet this is what makes up most of our planet’s surface as well as the human eyes that observe the whole pageant. We’ve got miracle stuff all around us.

A generous 326 million cubic miles of water cover the globe. Of this, 97.2 percent is ocean. Some 0.65 percent is in the form of freshwater lakes, streams, underground aquifers, and vapor and mist in the atmosphere. About 2 percent is locked in the form of ice. And all this liquid is poised to move the moment it finds a way to wriggle closer to the planet’s center. It starts out by falling from the sky, making the water-flow scenario inevitable.

A truly immense amount of water is continually cycled through the air as vapor turns into plunging droplets. In a given year, 91,000 cubic miles of water fall as rain. If it all could collect on the surface, it would form a global layer nearly four feet deep. This is our world’s annual rainfall, and it has to go somewhere.

Rain runoff begins as broad sheets that find narrow crevices or urban sewer drains in which to flow. From this point on, the water either seeps into underground pools or follows a channel whose width can vary from that of a narrow brook to that of the Amazon.

Streams and rivers can lope along at just half a mile an hour or race at twenty-five miles per hour, the fastest measured anywhere. Usually rivers flow at about walking speed; their average is three miles per hour. Even the Nile, during its famous yearly inundation, only rushes north at five miles per hour.

Rivers overwhelmingly do their damage in flood periods, when they’re apt to overflow. Water is eight hundred times denser than air, so it decisively pushes anything it hits. It can move two-hundred-pound objects when it’s just a foot deep. Still, much of a stream’s erosion comes from the embedded particles—which in fast-moving scenarios can be entire boulders—that scrape away the sides.

After heavy rains the reliable runoff sequence starts with narrow rivulets that flow in channels and scour out steep-banked V-shaped streambeds. Over time, the sides erode and the channel becomes a wide, flat-bottom waterway. During nonflood periods the stream flows in the middle of this newly created valley.

Einstein seems to have been the first to point out that rivers tend to obey pi, the number 3.14159 and so on, with its never-terminating digits. Meaning that a river’s straight-line distance from source to sea, divided into its actual meandering mileage over the ground, is pi. Rivers tend to build themselves a loopy path because the slightest curve will lead to faster currents on the outer side. This creates extra erosion and a sharper bend, which in turn produces a further increase of flow speed, accelerated erosion, and an even sharper twist to the river. (The removed sediment is typically deposited at the very next inner bend, building up the subsequent curve.) But a natural process limits water’s desire for circuitousness: too much of a curve makes the river double back on itself, effectively short-circuiting the process by creating oxbow lakes. We’re left with half circles and an overall value of 3.14. You can see this beautifully from the air or on a map.

The ratio of curved to straight sections varies from river to river. A ratio closest to pi is seen most commonly in rivers flowing across gently sloping terrain. In a river cascading down steep topography, the waters are too fast for the pi effect to work.

We’d expect to see this on other worlds as well, except no other place has rivers. Mars had flowing water millions of years ago, but no one’s yet sure whether it stuck around for a long time or appeared only in sudden, temporary, white-water events. The fluvial channels along the Martian surface are ghosts from a long-ago era, and some appear curvy indeed. The only other liquid water within four light-years of the Mississippi is not the flowing variety but huge subsurface reservoirs. Jupiter’s moon Europa and the Saturnian moon Enceladus boast warm, alluring saltwater oceans made possible by the weight and pressure of a mile of floating ice above them.

Sediments eroded and carried away by rivers add up to a staggering mass. Some consist of dissolved solids, such as salts, but the bulk is its suspended load, which is what makes a river turbid. It also transports a so-called bed load—material that moves along the channel floor by sliding and rolling.

The Mississippi alone carries 750 million tons of material to the sea each year. Two-thirds of this is in suspension (no surprise, given that river’s famous chocolate color), two hundred million tons are in solution, and fifty million tons are in its bed load.

The flow rate is all-important. Water’s kinetic energy (impact force) increases by the square of its speed. So when water doubles its speed, its talent for creating damage rises by a factor of four. During a flood, when river speed can easily increase threefold—say, from two to six miles per hour—its ability to scour its banks is multiplied ninefold. Very powerful. That’s why such overwhelming damage and reshaping happen during floods.

Groundwater is usually on the move, too, but only by a few feet a day as it creeps through porous rocks or inches through cracks. Experts estimate that the hidden water within two thousand feet of the surface equals twenty times the volume of all the world’s rivers and lakes combined. When it comes to fresh water, we only glimpse the iceberg’s tip.

It is water, too, that most endangers us, even if only 5 percent of the 2,420,000 people who die annually in the United States succumb to any kind of a motion-based mishap.4

Most of these unintentional fatalities stem from car accidents or falls and such. Nature, by and large, can plead not guilty. All combined nature-induced fatalities account for just one death in a thousand. Nonetheless the pure drama of storm violence and the fact that some of us are indeed carried off by winds and earthquakes ensure headlines disproportionate to the actual peril. In a typical recent year it was water—floods—that killed one hundred Americans, while lightning killed sixty-five and tornadoes and other windstorms seventy-five. By contrast, car accidents killed thirty-six thousand people.

In the matter of aquatic motion, classical writers saved their reverence for the oceans. The ancients spoke of the seven seas, a term that gained household familiarity thanks to its use by Rudyard Kipling. Naturally, because all seas are interconnected, there’s really only one global ocean, though salinities, currents, and other attributes vary from place to place.

Beyond the science is the sea’s magic. Its size seems designed to induce humility. Watching it as it tirelessly moved and roared, Neanderthals stared at its swells, and so will the final humans, and even then its heartbeat won’t falter. We cannot see air move or the galaxy rotate or the sun pulsate, but here at the shore Aristotle’s “eternal motion” seems self-evident. The sea requires no intellectualizing.

What paradoxical sadism that this water should have slaughtered so many via the weapon of thirst. Desperation-induced seawater drinking first produces severe diarrhea, then confusion, brain damage, and finally death by renal failure. Drinking water with more than a 1 percent salt content rapidly increases blood sodium and pressure levels. The body reacts not to the water but to the salt, and the kidneys can remove it only by utilizing fresh water. Seawater is not even close to potable, being 3.5 percent salt by weight. (Typical city tap water has less than one hundred parts per million of sodium. The legal allowable maximum salt content is one thousand parts per million, or 0.001 percent.)

In bodies of water where evaporation is high but replenishment by rivers is low (e.g., the Persian Gulf and the Red Sea), the salinity can easily reach 4.2 percent. Conversely, the Baltic, into which a large amount of fresh water discharges, is just 2 percent salt. Ocean salinity is thus another action-based consequence of river activity. But enough beating around the bush: let’s turn to the “big three” movers of the seas.

Waves. Tides. Currents. Each is epic. Each delivers untold tons of force.

Of all places to explore the tides, none competes with the Bay of Fundy in the Canadian Maritimes. I came here to see it firsthand, to stand atop a riverbank in Nova Scotia outside the town of Truro. The muddy bed of the Salmon River lay sixty feet below me. In its middle, an unimpressive stream a foot deep flowed leftward, toward a bend in the distance, and then presumably to the sea, unseen from this spot. An enterprising Canadian had built a restaurant on this promontory. Picture windows looked down toward the sandy abyss below. Nothing much, yet this is one of the most amazing places in the world.

Here is the location of a legendary tidal bore. At least, it’s legendary among ocean lovers and people in the Canadian Maritime Provinces. Bores exist in just a few places on our planet, which explains why most folks have never even heard the sentence “It’s a bore!” spoken with any kind of excitement.

The world-famous Bay of Fundy has a narrowing shape and, unseen beneath the surface, a precisely sloping seabed that together channel and amplify incoming tides. The Atlantic waters enter the fifty-mile-wide bay, and the constriction forces them to rise as they travel the 150-mile length. In the Minas Basin, near Wolfville in Nova Scotia, and here in Truro and a few other nearby places, truly bizarre consequences follow.

It all happens because, although the average coastal tidal range is five feet, here the sea frequently rises and falls sixty feet. Six stories. A vertical six stories. At high tide one sees floating ships, well behaved, tied to piers. A mere six hours later the boats are way below, sixty feet down in the mud, and the pier’s odd full height, equal to a large apartment building, stands awkwardly revealed. As in a tidal wave’s early, sickly, perilous stage, during which the unwary are lured into its grip, the ocean has retreated far off in the distance, separated from onlookers at the coastal road by a half mile of kelp and pools and happy gulls.

The world’s tides fluctuate more dramatically when the moon and sun align, either together, at new moon, or on opposite sides of the heavens, at full moon. Twice a month, then, coastal communities experience these spring tides, whose name is confusingly awful, since they have nothing to do with spring or any other season. The term’s origin is unknown. Perhaps people thought they were like a spring, a fountain of water. At these times, the high waters come nearly to the boardwalk, and the low tide exposes normally hidden stretches of muddy sand. This is when clammers, checking their tide tables, grab their buckets and shovels and head out. It’s when the ocean typically rises and falls an additional foot or two beyond the average tidal range.

But here alone this is not the case. The strange, complex, watery wonders of the Bay of Fundy virtually ignore the monthly moon-and-sun alignments. Here the seas scarcely change during the time of spring tides. Instead, Fundy’s tides increase when the moon comes closest to Earth—its monthly perigee. This changing-lunar-distance effect is small everywhere else. Here it matters mightily. That’s why it’s wise to check the lunar perigee tables before arranging any trip to Fundy if you want to witness the best tidal spectacles.

As I sat there I was a mile inland, with no sign of the ocean at all. Then, as if choreographed, people started filing out of the restaurant and standing on the raised embankment. All heads were turned to the left, toward that river bend a half mile away. People looked at their watches, their smartphones. There were hushed, expectant conversations.

And suddenly, there it was. The tidal bore. Rounding the bend like a living creature, and extending from bank to bank, a two-foot-high wall of water materialized and marched toward our position. When it arrived below us, roaring as its wave collided head-on with the river’s flow in the opposite direction, its momentum carried their combined waters rightward. The ocean easily won the aquatic tug-of-war. The bore continued rightward until it was out of sight.

The show was not over. During the next hour, the river channel kept filling, higher and higher. It was the sea, marching farther inland, exploiting the low riverbed for its own advancement. By the time I left, the water was perhaps thirty feet deep, exhibiting a rapid march in the opposite direction from the way it was flowing when I’d first arrived.

Every coastal community has its own peculiar tidal sagas—albeit not as exceptional as they are here—for tides are often intricate and not always fully understood. Their origin is mainly lunar, though the sun exerts its own tides, a bit less than half the strength of the moon’s.