Geologists Upend Environmental Maxim
The results of a study by Franklin & Marshall geologists Robert Walter and Dorothy Merritts, published as the cover story in the Jan. 18 issue of the journal Science, overturns fundamental concepts about how streams form and function in the mid-Atlantic region, with implications for stream-restoration efforts.
In their paper, Natural Streams and the Legacy of Water Powered Mills, the authors present evidence disproving the accepted model of the meandering river as the result of a natural process. The article captured the attention of the national scientific and consumer media, including coverage by the New York Times and a story on National Public Radio (NPR).
The authors argue that the classic meandering stream form with eroding banks and self-formed floodplain in the mid-Atlantic region is actually an artifact of post-European settlement and land-use change over the last 300 years. Deforestation of the land for agricultural uses from the 17th century through the 19th century unleashed large volumes of eroded soil from watersheds in the eastern United States. These fine sediments were trapped behind tens of thousands of dams that were constructed to create millponds.
The research shows that the model of the meandering river form as the result of a natural process is a misinterpretation for many streams in the eastern United States, and that the form arose only after extensive human intervention.
Some 60 years ago, researchers studying meandering streams in southeastern Pennsylvania and northern Maryland, such as the Brandywine River and Seneca Creek, saw moderately high banks of mostly silt and clay, or mud, with gravel at the bottom, said Merritts, professor of earth and environment, who focuses on streams, rivers and other landforms, and on the impact of humans and geologic processes on landscape evolution.
Over time, they observed that as these streams cut away at one bank, they deposited coarse material on the other side, forming gravel bars, Merritts said. They erroneously concluded that the form and composition of the valley bottom were the result of streams swiping back and forth, eroding in one place and depositing in another, over tens of thousands of years.
Instead, Merritts and Walter found that Colonial milldam construction and millpond sedimentation were rapid and pervasive. This process inundated, buried and sequestered pre-settlement wetlands and altered regional stream functionsprobably within two generations of European settlement.
Our field, laboratory and historical research shows that stream valleys were once lined with millponds, which silted in to form broad, flat bottom lands, and that the modern, incised, meandering stream is an artifact of human manipulation of stream valleys for waterpower, said Walter, an associate professor of earth and environment, who specializes in chemical analysis and dating of rocks, minerals and sediments.
Under natural conditions, a stream grades to base level, that is, the level of the water body to which the stream flows. However, Merritts said, If you build a series of dams along a small, low-gradient stream in this region, you create a series of local base-level controls, and the stream will grade up to those dam heights.
Streams that have high banks composed of fine sediment are the result of a stream incising, or cutting down, to its original base level after breaching one or more of those dams, Merritts said.
Walter said the research relied on various field, laboratory and historical research methods, including light-detection and ranging (LIDAR) topographic surveys, geochemical and geochronological analyses of sediment and 19th-century maps of the study area showing the mill locations, dam positions and millpond sizes. Geologists rarely have a historical record that we can use to test our hypotheses, he said.
The study has implications for stream-restoration efforts in the eastern United States. A common strategy, called natural stream channel design, aims to slow erosion of stream banks by reshaping stream bends and installing boulders, yet the strategy is based on false assumptions about streams in the mid-Atlantic region.
Our research could, and probably should, change the way people look at and restore streams in the mid-Atlantic region, Walter said.
Merritts and Walter collaborated with LandStudies Inc., environmental consultants based in Lititz, Pa., to study an alternative strategy, floodplain wetland restoration.
At a site on Lititz Run in Lancaster County, workers removed a partially breached dam that stretched across the floodplain, as well as much of the legacy sediments from the Colonial period, exposing the earlier wetlands.
Within a year, native plants have sprouted that, so far, are out-competing invasive plant species in the valley, Walter said. Not only is this a good flood-control measure, it also has the potential to reduce the volume of nutrient-rich legacy sediments that flow downstream, contributing to the pollution of Chesapeake Bay.
The authors, who married during the course of the five-year study, said their complementary expertise, experience in other parts of the world and close relationship helped to propel this project.
It has been an essential part of our life together, Walter said. Our private banterin the car, during dinnerhas generated a lot of our best ideas, which we test through the scientific process. Without our mutual rapport, I dont think either of us would have gotten to this point with this project.
Merritts said, Together we were able to throw away the old ideas that had been around for decadesthey were good ideas, but they didnt quite fit togetherand reinterpret the evidence in a completely new way. Together we had the confidence to persuade the scientific community to look at this landscape in a new way.
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