Published online 17 January 2008 | Nature | doi:10.1038/news.2008.448

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What does a natural stream look like?

The legacy of mill dams muddies the water for river restorers.

Rivers are constantly shaped and re-shaped by human activities.R. WALTER, D. MERRITTS

Ecologists working to restore streams in the eastern United States have been using a misguided ideal, according to new research.

The picturesque notion, supported by many ecologists, that a stream untouched by human hands meanders in a single S-shaped channel with high vertical banks seems to be wrong. Instead, this shape is an artefact of the thousands of small mill dams built on eastern streams between the seventeenth and nineteenth centuries, say Robert Walter and Dorothy Merritts of Franklin and Marshall College in Lancaster, Pennsylvania.

The team poured over old maps, looked at historical documents, visited hundreds of streams and used light detection and ranging (LIDAR) techniques to get a sense of the lay of the land underneath modern vegetation. In some places, they used a backhoe to expose sedimentary layers and check on the geological history.

The researchers conclude that streams in the Piedmont region of the eastern United States — just to the east of the Appalachian mountains — were more like swamps than streams when Europeans first arrived. The water didn't run in a single channel, but rather was in branching streams, pools and mud, they report in Science1.

By the late eighteenth century, many streams had been dammed (with dams as wide as whole valleys, because the streams were so spread out), and they turned into a necklace of mill ponds, one every four kilometres or so. Meanwhile, deforestation on the high ground increased the water supply and inflow of soil. Mill ponds collected thick layers of sediment on their bottoms.

When steam power started to displace hydropower for milling, forging and mining, many of these dams were breached. The resulting bursts of fast-flowing water cut a channel through the sediment in the old ponds, creating the meandering shape thought of as 'natural' today.

“These characteristic meandering streams in wide valley bottoms with steep-cut banks are not normal,” says Walter.

Muddied waters

Similar work has been done in the Pacific Northwest, and the team adds that they think the same process might have taken place in Europe. “By the 1700s, there were 80,000 mills in France,” says Walter.

If this reconstruction of events is true, then millions of dollars have been spent trying to return streams to an artificial state: their condition after old dams came down, rather than before they went up.

In a project tentatively planned for summer 2008 in Pennsylvania, Walter and Merritts are working with restorers to try to remove all that modern sediment and strip valleys down to the Holocene wetlands beneath. They think this will allow old marshlands to return, decreasing sediment and nutrient load in the streams and preventing some of the problems seen today from excessive sediment dumped into the sea.

Everything changes

Sean Smith, who reviews river restoration proposals for the Maryland Department of Natural Resources in Annapolis, says that the work has already had an impact on the field. “There are already proposals that are basically valley-dredging, where they want to scrape down to the pre-colonial form,” he says.

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In some such projects, restorers have been surprised and pleased to find that wetland plants buried under loads of sediment for hundreds of years are still viable and starting to re-sprout.

But the change in thinking isn't necessarily a good thing. Margaret Palmer, a river and restoration ecologist at the University of Maryland in College Park worries that the effect of the research will be to replace one rigid paradigm with another — neither of which take into account the changing nature of the landscape. “Everything changes. We have cleared trees; we have dramatically changed the amount of water in these streams. If our goal is to decrease sediment load we should focus on that and not worry about making the stream look the way it did at pre-settlement time, because nothing else is the same as it was pre-settlement,” she says.

Dave Rosgen, a well-known river restorer in Fort Collins, Colorado, agrees. “What I suggest is that we don’t try to make restoration match a ‘pristine’ condition, because rivers have to be stable in the present conditions that they are in.” 

  • References

    1. Walter, R. & Merritts, D. Science 319, 299-304 (2008). | Article |
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