Scouring Ancient Landscapes for Clues to Cleaner Water

Having made their mark on the natural landscape the last few hundred years, humans are now undoing some development -- notably with regard to old milldams and sediment-filled ponds that have negatively affected water quality.

To find clues to the environments of valley bottoms during Pennsylvania's pre-historic era -- well before the building of dams and millponds -- Franklin & Marshall College geology major Erin Markey, who graduated in May, is analyzing core soil samples from a 2,500-acre marshland in Chester County, Pa., that has remained unchanged for millennia. It is part of a watershed twice as large, and the samples are 13,000 to 18,000 years old. 

Once ubiquitous and a source of waterpower, the thousands of 18th-, 19th- and 20th-century millpond dams along the state's streams are being dismantled, but the sediment that built up over time is causing problems, washing freely downstream without the scrubbing effect of wetlands that yield clean water, said Dorothy Merritts, Markey's project adviser and the Harry W. & Mary B. Huffnagle Professor of Geosciences at F&M.

Markey is examining seeds from the soil cores of the Great Marsh's valley bottoms to determine what type of plants once grew when the tract was a dry, windy tundra and later when it transformed into warm wetlands. From this research, she can extrapolate the types of plants that grew in other area waterways before sediment-collecting millponds were built.

"The damming drastically changed those valley bottoms," Markey said. "Now that the dams are being removed, people are trying to figure what the environment in those valley bottoms was like."

Merritts, who chairs the Earth and Environment Department, said restoring wetlands or wet meadows where dams once stood is about restoring water quality locally, as well as downstream in estuaries such as the Chesapeake Bay.

Dating back to the 1700s, Pennsylvania once had an estimated 18,000 millponds providing waterpower for everything from grain and textile processors to lumber mills, Merritts said. "Think of it like oil," she said. "It's the same with water. It was the only major source of power."

Great Marsh, though, never had a dam or millpond. As a wetland, it endures -- as it has for millennia -- despite extreme weather events such as hurricanes and, more recently, human activity. In fact, scientists refer to Great Marsh as the "Rosetta Stone," a key to understanding wetlands, Merritts said.

"It might be a very resilient type of ecosystem and landscape," she said. "At Great Marsh, there is development all around. There has even been farming. As long as you don't have a dam that allows the sediment to build up, and as long as you keep it a wetland, it can survive all that. It has some of the highest water quality in the state."

With Great Marsh in mind, a few years ago Merritts and Associate Professor of Geosciences Bob Walter led a team that restored a wet marsh in Lancaster's Willow Valley to its 18th-century condition. She and her colleagues removed a dam and two centuries of millpond sediment that was later used in a brownfield reclamation project.

The primary goal of Markey's research is to document the characteristics of the plant seeds from when the valley bottoms transformed into wetlands. She looks at seed length, width and shape to figure out what kind of plants grew from the seeds.

"Preliminarily, I've been finding seeds from wetland plants going back 12,000 years," said Markey. She used radiocarbon dating research that had been collected two-years earlier by Visiting Scholar Candace Grand Pre '03 to document seed age. "Every plant has a different kind of seed, and every seed tells you what conditions in that time period were like." 

The research has so intrigued Markey that she is returning to F&M this summer to spend a year as Merritts' assistant, studying geomorphology -- how landscapes change