Sediment remaining from colonial era land use still impacts water quality today

The forest canopy was thick, the flowing stream tranquil and the air cool and quiet. An adventurous team of University of Delaware graduate students sifted through streambank sediments underlain by mysterious layers of dark buried soils. Digging into remnants of colonial civilization in the mid-Atlantic, the student researchers were surrounded by large rocks, decomposing logs and long rusted nails as they overturned beech and maple leaves from centuries past. They sought to discover whether these age-old sediments and land use legacies impact erosion and pollution in streams and rivers today. Are we haunted by what’s left behind?

Previous research by pioneering geologists Robert Walter and Dorothy Merrits has underscored the transformation of valley-bottom and fluvial systems caused by European colonization in the 1600s. Early colonial settlers cleared old-growth forests for agriculture causing widespread soil erosion. They were also prolific builders of mill dams.

“Mill dams were built by early settlers on rivers and streams to harness water power for activities such as grinding grain, cutting lumber and forging metal,” said Shreeram Inamdar, a professor in the Department of Plant and Soil Sciences and director of the Water Science and Policy graduate program. “These structures were created every few miles and by 1840 there were more than 65,000 mill dams across 872 counties in the eastern U.S as per Walter and Merritts.”

While many of the historic dams have been breached or removed over time, the remnants are still visible at many locations. More importantly, the legacy sediments—sometimes tens of feet thick—remain in river and stream banks and continue to contribute to contemporary sediment erosion. These sediments may also contribute to nutrient pollution and leaching into the water supply, potentially undercutting conservation gains made in upland agricultural sources.

Led by Inamdar, the student research team included Alyssa Lutgen, Grant Jiang, Nate Sienkiewicz and Katie Mattern—all master’s thesis students—who each studied a specific complementary aspect of legacy sedimentation. The water science and policy graduate students, all now alumni of the College of Agriculture and Natural Resources, published their findings in four different peer-reviewed journals. Collectively, they found that while nutrient concentration is lower in legacy sediment than in agricultural soils, legacy sediment exports accounted for half of the watershed suspended sediment load in the Big Elk creek watershed. Additionally, precolonial soils are host to bacterial communities that vary considerably by location. They also found that legacy sediments could be a source or sink of phosphorus depending on the stream conditions. Finally, the team monitored the removal of legacy sediment as part of an ongoing $4.2 million floodplain restoration project and concluded that water quality improvements may take time and scientists must take overall soil health into consideration in watershed management and mitigation plans.

“This excellent research of the graduate students has not only provided important insights into a vexing problem but has also spawned new research and grants,” said Inamdar.