The Case of the Missing Dirt

Whether or not geology’s Powers That Be accept “The Anthropoceneâ€? as an official epoch (see the January 31 entry in The Gist), we humans are leaving quite a mark on the geologic record. Pardon the shameless shilling for my colleagues at Smithsonian.com, but some of the stories in their new “Ecocenter: The Landâ€? package illustrate why The Anthropocene is a useful concept. From agriculture to deforestation to sprawl, we’re changing the planet arguably as much as any errant asteroid ever did.

(The Anthropocene sounds fairly nonjudgmental, doesn’t it? Until now, my favorite name for these modern times came from a cartoon. It shows a couple of guys in space suits digging up a ruined building with a sign on top that says “Tanning Salon.â€? The caption? “Future archaeologists unearth yet another relic from the Age of Vanity.â€?)

It’s hard to get one’s head around how human activities add up on a global scale. Did you know that we move many times more dirt than all natural processes combined? Wind and water erosion, landslides, sand storms—you name it, bulldozers beat it.

What surprised me recently was just how long humans have been moving mountains. Or valleys, in this case. Researchers from Franklin and Marshall College in Pennsylvania figured out that the streams we Easterners love to fish and canoe in and hike along look nothing like their natural state. Today streams have steep banks and gravel riffles and defined channels. But 500 years ago they would have been wide, shallow and marshy.

What reconfigured the entire Eastern Seaboard’s waterworks? Mills. Robert Walter and Dorothy Merritts looked at old photos, property deeds, censuses and other historic records and found that Eastern rivers had mill dams blocking their flow every few miles, most built between 1600 and the early 1900s. The mills ground grain, sawed lumber, ran iron forges. Mill owners built dams that pooled water behind the mill and kept the mill wheels turning reliably. When coal and oil made mills obsolete, they broke down, washed away, and people pretty much forgot about them.

The study seems to answer a longstanding question in geology: where did all the sediment go? Scientists knew that logging and poor agricultural practices caused massive erosion, but it didn’t end up in the oceans. Now we know, thanks to Walter and Merritts’ analysis of satellite images and stream beds: the sediments were backed up behind thousands and thousands of dams. When streams finally breached abandoned dams, they cut through meters-deep sediment and carved the steep channels we see today.

It’s a surprising finding, yet one that makes perfect sense if you look at place names (my street, miles from any modern stream, has the name “Millâ€? in it) or notice all the crumbling stone mills when you hike around in second-growth forests. Now the question is what to do with the information: should stream restoration projects aim to recreate 16th-century swamps? (How to rebuild streams is contentious enough already, as I learned from “Fish Storyâ€? in the August issue of Smithsonian magazine.) If so, will that decrease our risk of catastrophic flooding?

The research was done on the East Coast, but the lessons apply elsewhere: rivers in the Pacific Northwest were once frequently blocked by treefalls; now they also run faster and deeper than before. And Europe? They’ve been damming their rivers for about a millennium.

Do you have a favorite example of why we should—or shouldn’t—call the modern era The Anthropocene? Post your thoughts here.

Laura Helmuth is a senior editor at Smithsonian magazine.

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Laura Helmuth is the science and health editor at Slate. Previously, she was a senior science editor at Smithsonian magazine.