Two geologists at Franklin and Marshall College are suggesting that the models upon which stream restoration efforts have been based may be flawed because they were deverloped by studying "non-natural" streams, many of which are in the Mid-Atlantic states.
Here is the New York Times' 18 January 2008 article by Cornelia Deans.
Dr. Robert Walter and Dr. Dorothy J. Merritts, in a recent Science paper, say the streams studied by their geological predecessors were not “natural archetypes” but rather the artifacts of 18th- and 19th-century dam building and deforestation.
The article continues:
The researchers 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. 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.”
She said she and Dr. Walter believed their work had important implications for stream restoration. For one thing, she said, evidence so far suggests that removing the overlay of sediment may encourage streams to return to a truly natural state. But also, she added, restoration “requires much more consideration of what we are trying to restore, and what might actually be a sustainable approach.”
NPR also has an excellent story by John Nielsen on Walter's and Merritts' work.
The two smaller pictures - of a dam and Merritts - are from the NPR story. The larger one of Walter is from his F&M home page.
Here is an excerpt from that story:
David Montgomery, a geologist at the University of Washington (UW), says the paper is also a cautionary tale for everyone involved in river restoration projects.
For example, says Montgomery, "if you are trying to restore rivers in a way that will benefit fish and other organisms — if you're not just trying to make them look pretty — the best model for that is the system in which they evolved and thrived."
Basically, if you want to put Humpty Dumpty back together again, you need to know exactly what he looked like in the first place.
Merritts and Walter say it's likely that the landscape underneath a lot of winding rivers was dominated by broad wetlands full of tangled water channels. Walter says those broad wetlands are now all but nonexistent in the East. Whether they will be restored on a large scale is an open question, he says.
Ain't science wonderful!
The other issue is what are we restoring to? All watersheds are now different. Changes in climate, impervious area, and plant/animal species means these streams can't go back to some pre-human form. A stream in the middle of the city is never going to go back to how it was like before the city existed. Streams need to be restored so they are stable systems (in terms of ecology, geomorphology, hydrology) for present conditions. Of course the other currently popular methods (Rosgen) just prescribe some "correct" stream form based on questionable variables.
Rosgen Method: http://www.epa.gov/watertrain/stream_class/
http://landscape.ced.berkeley.edu/~kondolf/topics/restore/restore.html
Posted by: Chris M | Monday, 28 January 2008 at 08:42 AM