Reservoirs at the Conowingo have captured sediment for more than 60 years, amassing 160 million tons, along with household garbage, street junk, an occasional carcass and rotting wood.
As they fill, the reservoirs designed to protect the bay are “becoming a source of pollution,” said Robert Hirsch, a hydrologist for the U.S. Geological Survey and lead author of a recent study that found more sediment than ever splashing from behind the dam into the Chesapeake.
“The way to look at it is that in the past these reservoirs were a very helpful trap for so much of this material coming out of the Susquehanna basin. They are now no longer a very effective trap,” Hirsch said.
The mass of sediment awaits another weather event such as Tropical Storm Lee
, which sent a record 4 million tons of sediment surging into the Chesapeake, turning it brown with pollution.
Along with the sediment already collected at the dam, an additional 3 million tons a year arrives via the Susquehanna, starting in New York state. Last year, Maryland and federal officials projected that the last of the three lakes that serve as the dam’s reservoirs would reach capacity in 15 to 20 years. This year officials said that final lake closest to where the river meets the dam could fill in about 10 to 15 years.
Sediment enters the Susquehanna along its network of 49,000 miles of creeks, streams and brooks carrying dirt and dust from construction projects, lawns and farms.
The reservoirs were engineered to trap two-thirds of the Susquehanna’s sediment. The rest — 1 million tons — creeps over in the roughly 18 million gallons of water per minute flowing to the Chesapeake.
But the study found that as the reservoirs fill, an additional half-million or more tons of sediment has been going over the wall in recent years. Left unchecked, the amount of yearly sediment could increase to as much as 3 million tons.
In a worst-case scenario, a sediment transformation could prevent sunlight from penetrating the bay. Underwater grasses that nourish waterfowl, serve as a nursery for juvenile fish and hide marine life such as the iconic blue crab could perish.
The Chesapeake Bay is on a pollution diet that partly stakes its success on reducing the amount of sediment entering the bay from all sources to 1 million tons per year by 2025. The cleanup also calls for a reduction in nitrogen and phosphorus, nutrient pollutants that cause the bay’s frequent summer dead zones.