The Legacy of Little Dams

This week the New York Times ran an article about how environmental history is changing the way geologists consider natural river flow. It turns out that many waterways that scientists thought were natural may have already been shaped by human industry in the eighteenth century. Reporter Cornelia Dean wrote:

The researchers [at Franklin and Marshall College] examined historical records and maps, geochemical data, aerial photographs and other imagery from river systems in Pennsylvania and Maryland. They discovered that beginning in the 1700s, European settlers built tens of thousands of dams, with perhaps almost 18,000 or more in Pennsylvania alone.

In a telephone interview, Dr. [Dorothy J.] Merritts described a typical scenario. Settlers build a dam across a valley to power a grist mill, and a pond forms behind the dam, inundating the original valley wetland. Meanwhile, the settlers clear hillsides for farming, sending vast quantities of eroded silt washing into the pond.

Years go by. The valley bottom fills with sediment trapped behind the dam. By 1900 or so the dam is long out of use and eventually fails. Water begins to flow freely through the valley again. But now, instead of reverting to branching channels moving over and through extensive valley wetlands, the stream cuts a sharp path through accumulated sediment. This is the kind of stream that earlier researchers thought was natural.

“This early work was excellent,” Dr. Merritts said, “but it was done unknowingly in breached millponds.”

Merritts’s paper with Robert C. Walter, “Natural Streams and the Legacy of Water-Powered Mills,” appears in the 18 Jan 2008 issue of Science. Its abstract reads:

Gravel-bedded streams are thought to have a characteristic meandering form bordered by a self-formed, fine-grained floodplain. This ideal guides a multibillion-dollar stream restoration industry.

We have mapped and dated many of the deposits along mid-Atlantic streams that formed the basis for this widely accepted model. These data, as well as historical maps and records, show instead that before European settlement, the streams were small anabranching channels within extensive vegetated wetlands that accumulated little sediment but stored substantial organic carbon. Subsequently, 1 to 5 meters of slackwater sedimentation, behind tens of thousands of 17th- to 19th-century milldams, buried the presettlement wetlands with fine sediment.

These findings show that most floodplains along mid-Atlantic streams are actually fill terraces, and historically incised channels are not natural archetypes for meandering streams.

I have no idea whether geologists have thought the same model applied in New England, but British-American farmers and small mill-owners were undoubtedly shaping that landscape even longer than in Pennsylvania.

(Aerial photo of the Assabet River in Concord above by Steve Dunwell, creator of Massachusetts: A Scenic Journey and other books from Back Bay Press.)