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The Science of Stream Restoration - NYTimes.com <br />providecl important fish habitat. "We are now spending millions of dollars to compensate for all <br />the wood we took out earlier," Dr. Dietrich said. <br />In Pennsylvania, Dr. Merritts and Dr. Walter say, efforts to restore stream flow by removing <br />dams ignored not just the sediment piled up behind them, but also the original landscape, in <br />many cases not meandering streambeds but swampy valleys over which shallow water flowed <br />in sheets. <br />After dams were built - as many as 8,00o in Pennsylvania - water accumulated in millponds, <br />and the sediment it carried settled to the bottom. When waterpower fell out of favor in the late <br />i9th and early 2oth centuries, the dams deteriorated until they failed or were removed. <br />Freed to flow more swiftly, streams began incising channels through the beds of silt. The fine <br />material eroded rapidly, sending tons of sediment - much of it carrying agricultural chemicals <br />like nitrogen and phosphorous - downstream to the Susquehanna River and, ultimately, <br />Chesapeake Bay. <br />One day recently, Dr. Walter and Dr. Merritts visited restoration sites in the Lancaster region. <br />On one stream, a property owner had planted trees to stabilize stream banks. But the trees had <br />to send roots through almost five feet of accumulated sediment before reaching the water table, <br />a feat most were unable to accomplish before dying of thirst. <br />Dr. Merritts and Dr. Walter recommend simply removing the sediment and exposing the valley <br />floor, as was done in a restoration project near Lititz, Pa. The project, financed by the State of <br />Pennsylvania, working with LandStudies Inc., a restoration concern, involved removing up to <br />25,000 to 30,000 cubic yards of silt, enough to fill thousands of dump trucks. Luckily, said <br />Ward Oberholtzer, a partner in LandStudies, it was easy to dispose of because farmers love to <br />http://www.nytimes.com/2008/06/24/science/24str...l?_r=1&oref=slogin&ref=science&pagewanted=print (4 of 7) [6/24/2008 12:37:57 PM]