Generations of sediment choking Chesapeake Bay | GUEST COMMENTARY

Near the geographic center of the Chesapeake Bay Watershed, a narrow creek winds through a small rural Pennsylvania valley. Here, in the early 1700s, settlers built a dam that unwittingly damaged one of nature’s best water pollution filters — valley bottom wetlands — ushering in an era of water quality decline throughout the region.

The 20-foot dam powered a grist mill and formed a pond extending more than a mile upstream, large and deep enough (as much as 20 feet) for people to boat, fish, skate or swim. Bucolic, indeed, but the peaceful lake scene described here camouflages an environmental quandary that continues to play out. Damming this and numerous other valleys for milling prevents streams from flowing cleanly into the Chesapeake Bay, and frustrates efforts to improve the bay’s health. The root cause of this problem was not fully known or appreciated until recently.


Today, the milldam and pond are gone, but a huge environmental footprint remains along the valley. Up to 20 feet of legacy sediment and mud produced by centuries of agriculture activities deposited in the pond behind the dam, deeply buried the critical wetlands that once existed on this site. Fine layers of mud, representing sediments flushed from generations of farm slopes that poured into the millpond, are now exposed in high, eroding stream banks because the dam failed during a large storm in the 1920s.

The scene above is replicated thousands of times over throughout the 64,000-square-mile Chesapeake watershed, which spans six Mid-Atlantic states and Washington, D.C. During the 17th to 19th centuries, milldams and ponds were built across valleys for water power throughout the eastern United States. Many streams were so clogged with milldams that entire valley bottoms were transformed from extensive, native, and resilient wetlands with small, stable intertwined streams into a series of linked ponds. We know this from our primary research, but also from maps, photographs and other documents found in historical archives.


Among the nation’s first and most influential modern environmental laws is the 1972 Clean Water Act, which approaches a milestone anniversary this year. One of the most significant applications of the act to date — the Chesapeake Bay Total Maximum Daily Load for nitrogen, phosphorus and sediment — still lags behind its goals for improved water quality, as nutrient and sediment pollution (previously filtered by the sediment-buried wetlands) pose very real threats to Bay ecosystems.

In the 20 years since our research connected historic stream impediments, such as the milldams in Pennsylvania, to the current health of the bay, the scientific community has come to view land use and water quality through a new lens: Valley bottoms in the eastern United States have been completely altered by hundreds of years of sediment and nutrient accumulation caused by the human transformation of entire landscapes.

Last year, thanks to a $1.25 million grant from the Richard King Mellon Foundation, we launched the Chesapeake Watershed Initiative at Franklin & Marshall College in Lancaster, Pennsylvania. Its mission is threefold. Our first goal is knowledge generation through primary research. Second, is outreach to stakeholders with this additional knowledge — farmers, land developers, local and state government officials, businesses and manufacturers — to build support for, and increase adoption of, cost-effective and science-based legacy sediment restorations to make meaningful impacts on water quality. Third, and equally critical, is education. Unnoticed for centuries, water quality problems caused by legacy sediments and associated nutrients remain underappreciated, and their full scope is challenging to comprehend without a frame of reference that stretches back several hundred years. We need programs at all education levels to inform the generations who will continue the stewardship of the watershed.

In this Mid-Atlantic region, we must understand that while a single thread of a meandering stream with high eroding banks might seem natural, it is, in fact, completely unnatural in this landscape. These highly altered valley bottom landscapes and their legacy sediments are artifacts of the industry of 17th to 19th century European settlers and their descendants who cleared vegetation, tilled soil and built milldams by the thousands as soil erosion rates increased rapidly. Until our research showed otherwise, a mud-caked meandering stream was thought to be the standard by which stream restoration practices should be gauged.

In 2011, we restored a stream and valley bottom in south central Lancaster County, where the erosion of legacy sediment — not farmland production — caused an overabundance of the sediment and nutrient runoff (particularly phosphorus) that cloud the bay’s waters and leave the estuary polluted. We removed more than 20,000 tons of legacy sediment pollution, uncovering and restoring the ecological functions of the buried wetland ecosystem. Extensive scientific monitoring three years before and six years after restoration documented profound reductions in sediment and nutrient loads.

While initially expensive, stream and wetland restorations at legacy sediment hot spots exhibit tremendous economies of scale and enduring environmental benefits. Actually, these types of restorations are highly cost-effective in comparison to other restoration options for water quality improvement, such as planting cover crops and riparian buffers. We now know that farmers adopting best management practices like these will only take us so far to demonstrate measurable progress, and achieving an enduring impact will require greater intentionality to fix a problem that began centuries ago. There is no one-size-fits-all solution to all this, but clearly, legacy sediment hot spot restoration is an important, new and previously missing, piece of the puzzle.

This 50th anniversary of the Clean Water Act is a critical time to renew efforts to improve Chesapeake Bay water quality and to adopt new practices based on knowledge gained over this time. To do this, we need policymakers in state capitals, watershed organizations, farmers, landowners and all citizens to commit to science-based, economically-sound management of valley bottoms, thus making clean water, and not sediment, a legacy of our stewardship.

Dorothy Merritts ( is a professor of geosciences, Robert Walter ( is a professor of geosciences and Patrick Fleming ( is an assistant professor of economics and public policy at Franklin & Marshall College in Lancaster, Pennsylvania.