1
Introduction
Every culture recognizes that water is life, that rivers are the veins and arteries of the world. Rivers figure prominently in a Haida creation story, handed down by word of mouth among the First Nations of coastal British Columbia for innumerable generations, captured here by Martine Reid:
As the Raven flew away, gasping to restore his breath and recover from his fright, drops of fresh water trickled down from his bill. These drops, falling on the mainland, became lakes and ponds of fresh water. From these, streams and rivers flowed in all directions.
Norman Maclean wrote semiautobiographically of his childhood home in Montana:
Eventually, all things merge into one, and a river runs through it. The river was cut by the worldâs great flood and runs over rocks from the basement of time. On some of the rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs. I am haunted by waters.
These examples come from western North America. But one could point to any nation, any language, any creed, and find other stories similarly demonstrating a clear understanding of how powerfully important rivers are to us at every level.
Much of that significance is very practical. Rivers provide our drinking water. They give us food by irrigating our crops and watering our livestock and serving as the homes or birthing places for fish and wildlife. Rivers keep the lights on and commerce in motion by driving hydroelectric turbines. Their water is key to virtually any industrial process, from building cars to manufacturing computers. Rivers have served as transportation pathways for much of human history, and continue to do so to this day: the Danube still carries freight across central and eastern Europe, much as the Romans used it two millennia ago. It is no coincidence that a nationâs GDP is linked to its water supply availability. Virtually every major city worldwide is built on a river, and rivers often delineate our national borders. Their valleys define where we are from, our homes, our nicknames, our industries, our animals, our culture, our booze, our sports teams, our place of mind. California alone provides a host of examples: the Valley Girl pop culture of the 1980s refers to the San Fernando Valley, which carries the Los Angeles River; Silicon Valley technology derives its name from the Santa Clara Valley, through which Coyote Creek flows; and the wines of Napa and Sonoma come from their respectively named river valleys. And what rivers bring, rivers can also take away; water is life, but it can also be death. We choose to build on floodplains, and then riversâ waters sweep away our homes and even our lives. Floods are the most expensive type of natural disaster in the United States. Drought is subtler but even more dangerous. It is reported to cause more human suffering globally than cyclones, earthquakes, and floods combined. And according to a United Nations report, deprivation in adequate clean water claimsâÂthrough the associated disease aloneâÂmore lives than any war claims through guns. Developing a broad understanding of these issues has never been more important. Population growth and economic expansion are taking their toll on our watersheds, by increasing the demand we place on them for simultaneously providing both clean water and a dumping ground for waste, by likely affecting the climate that drives river flows, and by interrupting the natural landscape processes that regulate river levels.
1.1. Aftermath of the 2015 Memorial Day flood in Texas. Photo: US National Weather Service.
How do we approach this? Where do we start? Stand on the bank of a river, and youâll notice one thing immediately: the water isnât at a standstill, but flowing by. It came from somewhere, and itâs on its way somewhere else. At this moment, you are watching one part of the planetary-Âscale water cycle, the fundamental conceptual framework on which all of hydrology is based. Figure 1.2 illustrates some of these ideas. Ours is essentially an ocean planet: roughly three-Âquarters of Earth is covered by water, almost all of which is sea (salt) water. Freshwater constitutes the remaining 3.5% of the total planetary water store. And of that, more than half is locked up in glacial iceâÂmainly the gigantic Greenland and Antarctic ice sheets, but also alpine glaciers and ice fields atop various mountain ranges across the world. Groundwater is the second-Âlargest component of the worldâs freshwater reserves. This is water stored underground in aquifers, which consist of the innumerable empty spaces between sand grains, for example. Rivers, on the other hand, hold only about 0.006% of Earthâs freshwater resourcesâÂa negligible proportion of the total amount of water on the planet. How is it, then, that rivers are front and foremost in our minds? Is it just an error of perception? Not quite. The importance of rivers lies in the fact that everything is dynamic, linked together in a big loop, and rivers are the interstate highways in that global cycle. Rivers donât store much water at any given moment in time, but they move a great deal of it, making it available for drinking water and to drive turbines for hydroelectric power generation and the myriad other functions it serves.
Letâs start our initial exploration of this cycle with the oceans. Evaporation under the sunâs warmth moves water from the ocean surface upward into the sky, leaving behind the salt, a little like how a still concentrates alcohol to make whiskey. The atmosphere then moves that water around the world, through gigantic circulation cells and hemispheric-Âscale wind patternsâÂbasically, the weather. Like rivers, the atmosphere acts mainly as a highway for water. For all the mighty thunderstorms and hurricanes we see on the nightly news, relative to other planetary stores the atmosphere actually contains very little water at any given time. Atmospheric water eventually falls to the land surface as rain and snow. It then undergoes all sorts of fascinating and important interactions, and weâll talk in detail about many of these. For now, just note that much of the rain and snowmelt eventually makes its way to rivers, which transport the water back to the oceans againâÂcompleting the cycle.
While this planetary water cycle forms the core concept behind all hydrologic studies, there are many aspects to hydrology and different takes on the subject, reflecting the diverse array of implications that rivers have. For instance, civil engineers will often be concerned with understanding extreme river flow rates for designing bridges or delineating flood hazard zones. Forestry scientists are interested in the ways that forests, and different forestry practices, affect river flows and water quality. Agricultural and soil scientists study irrigation engineering and the impacts of agriculture on runoff. Atmospheric scientists often consider the potential flood impacts of their weather forecasts. Water supply managers are interested in overall river flow volumes and how these vary from year to year, especially periods of droughts. Oceanographers and life scientists occasionally make significant forays into the field due to its profound impacts on coastal oceans, and aquatic habitat quantity and quality. Geoscientists provide insight on topics like groundwater resources, water pollution, and the way that rivers change their paths, sometimes destructively.
1.2. Many processes drive the movement and storage of freshwater. This intricately interlocked assemblage of processes is known as the hydrologic, or water, cycle. Image: U.S. Dept. of the Interior; U.S. Geological Survey; John Evans, Howard Perlman, USGS. http://ga.water.usgs.gov/edu/watercycle.html.
This book, though, looks at rivers through the lens of physics. We wonât be strict and exclusive in that emphasisâÂkey topics from geology, biology, and chemistry, among others, will also be brought to bear. But weâll always return to our physics-Âinspired view of rivers. Our main goal in doing so is to shed a little light on the rivers that sustain our world. I aim to do that in a way that makes sense to you, regardless of how little, or for that matter how much, you might know right now about either rivers or physics. But Iâm also aiming to communicate a view thatâs something more than just an assemblage of facts and explanations about the mathematical tools we use to forecast floods, or how greenhouse gases and climate change might affect water supplies, or how satellites monitor the glaciers and snowpack that power large rivers on every continent, or how urbanization impacts the ecological value of watersheds. Weâll broach all those subjects, and many more. Yet across these individual topics, two larger threads will be interwoven: how looking at something from a fresh perspective can engender new types of understanding, and the interconnectedness of the environment and, indeed, of ideas.
We can consider this first notionâÂthe value of looking at something familiar in a new wayâÂat a couple of levels. One could be called, for lack of a better word, professional. The study of how water moves underneath, across, and above Earth is called hydrology, and the scientists who study these things are called hydrologists. Although basic physics has always had a strong bearing on hydrology, it probably wouldnât occur to most folks to regard hydrology as a branch of physics. Yet we take exactly that philosophy here. We can explore this idea by juxtaposing the geophysical study of rivers alongside the astrophysical study of the night sky. Of course, there are some obvious differences: while one looks up to the stars, the other looks to the creek in your backyard; one searches billions of light-Âyears away, over expanses so vast that time and space merge, while the other offers a refreshing dip on a hot summerâs day and then abruptly changes its mind and wipes out your house as its waters rise. But deep likenesses are also evident. Both apply similar methods to the analysis of uncontrolled natural experiments. Both can appeal convincingly to sophisticated quantitative tools like nonlinear differential equations and statistical mechanics and information theory, all of which weâll touch on later. Both seek to better understand the complex dynamics of systems we can control little if at all, not some carefully designed and tightly managed laboratory experiment but the very world around us, challenging us to tease the signal from the noise. Both peer into the heart of nature, in search of a flash of revelation. We posit here that reimagining hydrology as a sister science to, say, astrophysics, adds value by suggesting new ways of tackling some very difficult and very important questions about our natural environment, the ways we affect it, and the ways it affects us.
But thereâs also a much wider component to this idea of examining with a fresh eye something we think we know well. Itâs occasionally suggested that by attempting to reduce the universe to a rational, compact, fact-Âbased explanation, the natural and social sciences can in a sense drain the joy out of our experience of the world, amounting to a sort of psychic vandalism. And at its most narrow-Âminded, science may do exactly that. Love, for instance, is a lot less intriguing if you choose to believe itâs just a specific set of chemical reactions in your brain, and dismissing spiritual experience as a socially organized delusion is a condescending insult to cultures worldwide. At its best, though, science is just the opposite of this: an act of creativity and imagination, which adds new layers of meaning and significance to our familiar, everyday experience. I find that the day-Âto-Âday change in the water level of a river I drive past on the way to work every morning, which might otherwise be so mundane I wouldnât even notice it, transforms into something magical when I think of the fractal dimensionality of this variability, how the tiniest hour-Âto-Âhour changes in water flow after a rain shower are linked across a vast continuum of time to the greatest variations in planetary climate and water cycles as Earthâs orbit across the solar system slowly wobbles. A dramatic view of a river meandering across a desert landscape of red sand and sagebrush at twilight is made even richer by being able to look even deeper into this scene and recognize some of the levels of causality and complexity that contributed to it, from the rise of the mountains in its headwaters as a continental plate split apart over millions of years, to the way that the river shifts its channel when a thunderstorm descends from the skies to deliver a flash flood. And being able to contextualize the relationship among weather, rivers, and spawning salmon in terms of the same mathematical communications theory that drives the modern information age adds new depth to our intuitive understanding of an intricately intertwined natural world.
That brings us to the second thread that runs throughout this book: the notion of interconnectedness. Again, we can look at this at a couple of levels. The most basic and literal, though by no means simple, level is in terms of how rivers actually work. A river isnât just a single thing, but a complicated and interlinked collection of things, from the changing climate overhead, to the forests and snowfields upstream, to the thirsty farms and factories and cities downstream, not to mention the ecosystems of the watershedâÂthat is, the area draining to any particular point on a river. And all of this can vary radically from one place to the next: in another part of the world, the aforementioned forests might be replaced by tropical jungles, or dry grasslands, or arctic tundra. So too for the hydrological sciences, which span all these topics and more, and need to accommodate tremendous regional differences between river basins. To understand rivers, then, we need to understand a lot of system components and how they all interact with one another.
At another level, though, weâll also spend a good deal of time in this book examining the broader interconnectedness of ideas. The tools of physics and mathematics that we can use to learn something about how El Niño affects river flows are also used to study deep-Âspace phenomena like pulsars. The numerical methods that illuminate and quantify how wetlands impart a kind of inertia to stream water level variations are the same ideas and techniques used in stock market analysis. Looking at the mathematical and computational wherewithal thatâs brought to bear on flood forecasting also gives us a window into the world of artificial intelligence. And that mathematical theory of communication we mentioned earlier as a way of studying how weather, rivers, and ecosystems interact? Physicists are using these concepts to study black holes and, indeed, the information processing capacity of the universe itself. This universality of ideas, and especially mathematical ideas, is truly amazing, and weâll return to it again and again.
In summary, looking at hydrology in this new light, as not just a pragmatic exercise in applied science and engineering but instead a terrestrial equivalent to more ethereal disciplines like astrophysics, brings to mind exciting and even exotic ways of imagining questions about rivers that weâll explore here. Can rivers remember? Does a river choose its own course, or is it told where to flow? How do clouds talk to fish, and how much do they have to say? What does artificial life have to do with devastating floods and landslides? How do variations in Earthâs orbit affect the alpine âwater towersâ that sustain much of the worldâs population? What is a digital rainbow, and how does it help us understand the way that rivers respond to climate variations? And what might be the greatest buried treasure of all? Drawing on examples from all over the world, and Âwritten to all audiences, this collection of short, light, illustrated chapters shows how the methods of physics, mathematics, and statistics can be used to answer these questions and more. Along the way, weâll explore some surprising connections: between a good-Âluck charm and El Niño, black holes and rain gauges, the history of life and the geology that helps determine where rivers flow, Wall Street derivative pricing models and river forecast systemsâÂeven between James Bond and groundwater wells. And in using this exploration of connections and explanation by analogy to develop a full-Âblooded, intuitive understanding of these topics, some of which (like feedback loops) are fundamental and pervasive throughout both the natural and artificial worlds, weâll see not only how the cosmopolitan intellectual realm of physics informs our view of rivers, but also how rivers can inform our view of physics and of the world as a whole.
2
Why Rivers Are Where They Are
Perhaps the most basic question we could ask about a river regards its origin and placement: why is there a river here? This also happens to be one of the more challenging questions about a river to answer, because each watershed has its own unique history and personality. On the other hand, the laws of nature donât change from one watershed to the next, so itâs a manageable task to trace out some broad generalities about ...