To understand rivers, let physics be your guide

Where the River Flows
Sean W. Fleming
Princeton Univ.
$26.95

Spend an hour wandering along a river and you may wonder why the water rushing by chose this particular path over any other. While many nature writers might offer philosophical musings on the subject, Where the River Flows author Sean Fleming has physics on his side.

Physics isn’t the lens through which most people think about rivers. Fleming, a hydrologist, aims to change that. Only about 0.006 percent of the world’s freshwater is in a river at any given moment. But these hydrological highways transport a massive amount of water across the planet. Physics can explain where that water moves and help predict the ecological impact of its travels.

The physical force that water exerts on its surroundings (whether it carves a canyon, for instance) is just the beginning. Equations that quantify the rate at which particles disperse through water can help scientists predict whether a farm dumping manure into a river will make a swimming hole downstream unsafe. And hunting for patterns in streamflow measurements over time can be something like using a prism to spread white light into a rainbow. Both are a type of spectral analysis, in which a complex system is separated into its individual components.

Fleming makes some more unusual connections, too. Information theory — a framework that can be used to quantify the amount of information conveyed by a measurement — is most commonly associated with computer science. But Fleming explains how the concept can help scientists find value in variable rainfall data, which is important for making accurate predictions about river flow. He also delves into fractal geometry. Geologists find similarities to fractals (series of the same pattern that repeats at different scales) in aerial views of river basins, where tiny rivulets flow into larger and larger streams, as well as in streamflow data collected over different time intervals.

Fleming’s decades of experience shine through in this book. Abstract physics concepts feel more relevant when applied to concrete phenomena that readers can visualize. But Where the River Flows isn’t a light read. Occasionally, it wanders into textbook territory, walking through equations in great detail when conceptual examples might have conveyed the same information. Readers who don’t mind wading through such diversions will find that the math does, in the end, illustrate a point. Those who choose to skip the equations will still find nuggets of wisdom elsewhere in this book.

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