This is fascinating: Apparently geologists have spent decades assuming that the shapes of Mid-Atlantic-state rivers were natural -- when they're actually man-made.

Basically, one of the problems with studying rivers in the US is that so many have been warped by commercial and residential development that it's hard for us to know what the stream ought to look like, naturally. The closest to "natural" that the geologists could identify were rivers of the mid-Atlantic states -- which move in ribbon-like channels through silty banks. They assumed, for decades, that this ribbon-like shape was the Platonic solid.

But it turns out that those ribbon-straight rivers were in fact affected by human development, as two scientists -- Robert C. Walter and Dorothy J. Merritts -- report in Science today. As the New York Times reports:

In a telephone interview, Dr. 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."

(Image above by Fhantazm, via his Creative Commons Attribution-NonCommercial-ShareAlike license!)