IN 1704, a mill dam was built in Billerica, Massachusetts, on the banks of the Concord River. It was a small dam, but the social ripples it sent intensified over the course of 150 years, especially after 1798, when the mill and its dam were sold to a group of early capitalists, who raised the dam’s height because a higher fall meant more power and thus better financial returns. This was the dawn of the Industrial Revolution in the United States, and the Concord was at its epicenter; indeed, just a few miles downstream, near the confluence of the Concord and Merrimack Rivers, stood the famous Lowell Mill complex, the United States’s first factory system.

But there was a problem. The town of Concord, upstream from the dam, like many of its neighboring villages, was a farming town whose pastoral economy also revolved around water power. The Concord River is nearly flat, and when spring came it inevitably flooded, which was good for the farmers because when those yearly floods receded they left behind a nutrient-rich layer of silt. From this silt grew rich meadow hay, which fatted cows and livestock, which were sold to nearby towns, all of which left Concord’s farmers well off. But after the Billerica dam’s height was increased in 1798, farmers in Concord noticed that the floodwaters were slow to drain, and so spoiled their meadows’ hay. Thus began six decades of legal wrangling to determine whether the Concord River’s flow was best used to power a machine or to grow a blade of grass.

In 1859, an association of Concord’s farmers hired Henry David Thoreau to measure the abutments of all the bridges that crossed the river upstream from Billerica. (Bridge abutments act as mini-dams, impeding a river’s flow.) The association’s plan was to have Thoreau figure out just how much the bridges contributed to the flooding, with the hope that the answer would be not very much. This evidence could then be taken to the State of Massachusetts, which was in the midst of conducting an investigation into the source of Concord’s flooding, and ultimately used to justify tearing the Billerica dam down.

Though it has been generally forgotten, Thoreau was regarded in his own day as Concord’s preeminent land surveyor. He drew up scores of maps between 1849 and his death in 1862. (They’ve been scattered to archives throughout the Northeast, but the vast majority of them are held by the public library in his hometown, which has made them available, free to all, online.) As the literary critic Patrick Chura showed in his 2010 book Thoreau the Land Surveyor, surveying “was an essential […] component of the author’s life and character” that left its indelible mark on nearly everything Thoreau wrote, from his first book, A Week on the Concord and Merrimack Rivers — a literary map of the river running through his hometown — to the posthumous essays published in The Maine Woods. One of the last projects that Thoreau ever worked on — he fell ill with the tuberculosis that would kill him just a few months later — was an enormously detailed seven-and-a-half-foot map of the Concord River. This map has always been a mystery to Thoreauvians. It’s not clear why he made it, since Thoreau’s employers had no use for a map: they were after statistics, not cartography. Nor does it appear to be connected to any literary project.

This mysterious map is at the center of Robert M. Thorson’s newest book, The Boatman: Henry David Thoreau’s River Years. Thorson made his first Thoreauvian splash in 2014 with Walden’s Shore: Henry David Thoreau and Nineteenth-Century Science, an insightful book that reads Walden; Or, Life in the Woods not as a literary or philosophical text but as a series of geological experiments. For Thorson, himself a professor of geology, the most impressive thing about Thoreau is what a gifted scientist he was, and it was to “counterbalance […] a recent trend in ecocriticism that refracts science through literature without being scientific” that Thorson wrote Walden’s Shore. The Boatman is, in essence, a 200-page coda to Thorson’s earlier book, but rather than geology, Thorson turns his attention to potamology, the study of rivers. In it, Thorson argues that Thoreau “properly interpreted most of the key ideas of fluvial geomorphology a half century before the subject was invented.” He was, in Thorson’s words, “a lone genius” whose contributions to science we’ve too long ignored.

Thorson anchors that claim with an astonishing reading of Thoreau’s river map, supplemented by dozens of relevant entries from the author’s 47-volume journal as well as his handwritten notes preserved in the Concord Free Public Library. These are rich troves for the potamologically literate: the map itself is covered in a riot of detailed jottings on river depth, width, and the composition of the bottom, and one can find among Thoreau’s ephemera dozens of pages of minute figures and calculations. Part of what makes Thorson’s work on Thoreau so unusual is that he hardly bothers with literary, political, or intellectual approaches to his subject at all — he’s after data, and when he finds it, he checks it, weighing it against today’s best practices. (Thorson has generously posted all of this research online.) He comes away from his historical data-crunching deeply impressed with Thoreau’s skill: “[W]orking on his own, Thoreau inaugurated a truly scientific investigation of the largest, most powerful and wildest thing in his life, the Concord River.”

But it’s what Thoreau did next that confirms the extraordinary character of his scientific achievement, according to Thorson. Once Thoreau had his measurements in hand, he “generated a half dozen thought experiments that he tested” against his findings. He then combined these with his understanding of the deep geological history of Concord, supplemented by hours of painstaking research in Harvard’s library into cutting-edge French hydraulics, to come up with a general theory of river dynamics. Thoreau had figured out exactly how his river worked, from its subsurface currents to its eddies to the way it carves the bed in which it lies. As Thorson puts it, he “may have known more about rivers […] than anyone else in America.”

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The Boatman is an impressive feat of empirical research, and Thorson’s conclusions are an important contribution to the scholarship on Thoreau as natural scientist. Had he stopped here, it would be one of those books frequently and admiringly cited by a small circle of academic specialists. The Boatman, however, has grander ambitions: Thorson hopes that his Thoreau can lead us, slide rule in hand, through the hot times of the Anthropocene. Yet this is where he starts to run into trouble.

There are any number of definitions of “the Anthropocene,” a term popularized in 2000 by the ecologist Eugene Stoermer and the atmospheric chemist Paul Crutzen, but most revolve around the notion that human-generated global climate change is a calamitous break in world history. That’s not quite Thorson’s take. Instead, he’s curiously, cautiously celebratory: for him, the Anthropocene is an age in which humans have at last become aware that our agency “is completely interwoven with nature.” Though global climate change, ocean acidification, rampant pollution, and deforestation may all have their negative consequences, Thorson wants us to keep in mind that Thoreau found “beauty in even the most devastated corners of nature,” and that his “positive attitude can help us brace for the global changes heading our way.” The Thoreauvian lesson of the Anthropocene, Thorson warns in his gently technocratic conclusion, is that we must learn to let “science lead the law when it comes to environmental management,” for only science can guide us safely through whatever lies ahead. (Though he doesn’t mention them, Thorson’s position is close to the Ecomodernists a group of technologists and social scientists who are quite sure that they can engineer a “good, or even a great, Anthropocene” for us, if only we let them.)

The irony is that though Thoreau’s work may very well tell us a great deal about the Anthropocene, Thorson’s Thoreau — a Thoreau stripped of his literary, ethical, political, and philosophical complexity — doesn’t, and the problem lies with Thorson’s disinterest in all of Thoreau’s nonscientific work. In Walden’s Shore, Thorson described a Thoreau — “my Thoreau” — who was in need of “resuscitation”: a hardheaded scientist unconcerned with society, economics, politics, literature, economics, or psychology, who lived for objective data alone. It is true that one benefit of such ruthless simplification is clarity, and Thorson’s relentless pruning allows him dozens of glittering observations into Thoreau’s own world that have previously been missed. For instance, there is a passage in the essay “Walking” that has long puzzled scholars in its uncharacteristic (for Thoreau) celebration of Manifest Destiny, in which Thoreau tells us that every time he went for a walk, he inevitably headed southwest, because “the future lies that way.” To many scholars, this has seemed to be a veiled embrace of the American sea-to-shining-sea project. But Thorson points out that one of Thoreau’s favorite walking routes was along the Concord River, which, due to the underlying geology, runs to the southwest. Such simple material facts of Thoreau’s daily life, Thorson shows, often get lost in the thicket of humanistic criticism.

But in jettisoning everything except for his scientific practice, Thorson leaves us with an impoverished Thoreau, one stripped of both connections to his time and relevance to ours. Thorson is clearly uncomfortable, for instance, with Thoreau’s more radical, socially engaged side: there’s almost no mention of “Civil Disobedience,” one of the founding texts of an American anarchist tradition, with its declaration that government “can have no pure right over my person and property,” nor of the politically scorching “Slavery in Massachusetts,” which concludes: “[M]y thoughts are murder to the State.” Nor is there much about Thoreau’s ringing embrace of John Brown after Brown’s Harpers Ferry raid in 1859. Indeed, the only mention of Thoreau’s politics comes in a brief dismissal of Thoreau’s antislavery writing as “abolitionist rant.”

One of the currents just below The Boatman’s surface is a narrative of a politically enraged young man who dreamed, in 1849, of direct action, of taking a crowbar to the dam that was at the center of the flowage controversy and liberating the river, but who eventually mellowed, exchanging the crowbar for the slide rule, revolutionary notions for dispassionate, scientific facts. It’s as if an Earth First! twentysomething of today grew up, went to grad school, and became an EPA field scientist dutifully taking water samples. “The younger Thoreau was a boatman with wild, agitated, idealistic ideas,” Thorson writes. “The older Thoreau was a boatman whose mature flow of thoughts was slower.”

But when the rest of Thoreau’s work is brought back into focus, we find a strident critic as well as an accomplished scientist, a master of the slide rule and the crowbar, one of the earliest Americans to realize that our landscapes, our economy, and our politics are all indivisibly bound together. Far from mellowing as he aged, Thoreau remained scathingly acerbic in his criticism of American market-oriented society. A few months after he finished his river survey, he wrote a blistering critique of capitalism thinly veiled as a pleasant bit of nature writing called “Huckleberries.” “What sort of a country is that where the huckleberry fields are private property?” Thoreau asked, then elaborated:

When I pass such fields on the highway, my heart sinks within me. I see a blight on the land. Nature is under a veil there. I make haste away from the accursed spot. Nothing could deform her fair face more. I cannot think of it ever after but as the place where fair and palatable berries, are [sic] converted into money, where the huckleberry is desecrated.

All environmental crises, Thoreau well knew, are also social crises, and a devastated corner of nature could only have as its complement a devastated corner of humanity. It is hard to see how this Thoreau can be reconciled with the ecomodern optimist who would embrace the Anthropocene as an age of scientific beauty, or science as the savior of our warming world. If Thoreau is to be our guide to the Anthropocene, then we must also be ready to accept his radicalism, his skepticism of “man’s improvements,” and his call for, as he puts it in “Walking,” “[a] people who would begin by burning the fences and let the forests stand!”

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Daegan Miller is a writer and landscape historian. His first book, This Radical Land: A Natural History of American Dissent, is due out from the University of Chicago Press in February 2018.