The Chesapeake Bay in Maryland is poised delicately as the end point of the increase in chemical and sediment run-off from natural forces and climate change. Heavier than usual rain storms, often resulting from increasing global temperatures, greatly contribute to the severity of increased drainage and water.
Even though the Bay’s shorelines have slowly eroded for thousands of years from wave action and natural sea-level rise, Dr. Dorothy Merritts, the Harry W. & Mary B. Huffnagle Professor of Geoscience Department of Earth & Environment for Franklin & Marshall College, explains how intense rainstorms exacerbated by a warming climate hasten this activity.
This increased erosion and pollution is exacerbated by watershed action taking place hundreds of miles away. According to Dr. Merritts, many watersheds, an area of land draining or shedding water, in New York and Pennsylvania are “degraded.” They are filled with sediment originating from dam construction and farms from more than one hundred years ago, she explains. She adds while there are many healthy reaches of streams, many are impaired.
The streams carrying this sediment and chemicals begin in New York and Pennsylvania as well as Maryland. She says, “Many streams far from the Bay contribute sediment. There are overlapping processes occurring with climate change. It is true the frequency and severity of storms has impacted the Bay by depositing more sediment.”
Merritts references the Ellicott City flooding in May 2018. During that incident, torrential rain dropped between six and 12 inches on the Ellicott City and Catonsville areas of Howard and Baltimore Counties. This event happened only two years after a similar flood in 2016. These incidents caused catastrophic damage, especially in historic Ellicott City, a city on the banks of the Patapsco River, one of five rivers flowing into the Chesapeake Bay. Both floods were considered 1,000-year floods, meaning in any year, there is only a 1-in-1,000 (or 0.1 percent) chance of happening.
“There was an immense amount of sediment going into Patapsco River,” she said. She went there shortly after the flood. “It was astonishing to see that storm’s impact. This continued activity would devastate this town.”
There is an involved mechanism to how storms contribute to pollution of the Bay, she says.
According to Merritts, the “Incision” of streams, even small ones, can deepen over time and widen from increased rainfall. This creates intense power and “shear stress,“ or a higher “stream bed load” carrying suspended sediment. The excess minerals in these loads” wreak havoc–carrying more nutrients from farm fields.
“There are vast networks of blue lines (small streams) draining during big storms carrying these materials into the Bay.”
Increased incisions are problematic, says Merritts. The main contributors are old mill dams, some from colonial days.Once there were 15,000 in Pennsylvania. They fail during big storms, water cuts through old sediment and some abandoned dams include mud behind them. Therefore, they are a problem now because of stronger storms initiated by a warming climate.