The giant sediment reservoirs at Conowingo Hydroelectric Dam — glorified sand buckets that collect millions of tons of grainy pollution near where the Susquehanna River enters the Chesapeake Bay in Maryland — are nearly full, and research shows that too much of the sediment is splashing over the dam’s wall.

This is bad news for wildlife and a multibillion-dollar federal and state plan to clean the troubled waters of the nation’s largest estuary. Sediment smothers the bay’s life-sustaining grasses. Worse, it carries nutrient pollution that depletes oxygen, leading to dead zones where oysters and fish die.

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.

But if the reservoirs fill and all of the sediment pours over, phosphorus would increase by 40 percent and nitrogen by 2 percent.

Nitrogen comes from lawn fertilizers and urine that finds its way into the bay in large quantities from sewer overflows. Phosphorus comes from farm animals that defecate when farmers allow them to cool off in streams, rain that washes away animal waste at feed operations, and raw human waste from sewage overflows in cities such as the District.

Vast amounts of new sediment and nutrient pollution would all but cancel out the benefits of the pollution diet plan engineered by the Environmental Protection Agency, the District and six bay watershed states — Virginia, Maryland, West Virginia, Delaware, Pennsylvania and New York.

Taxpayers in Virginia and Maryland will pay as much as $20 billion to upgrade sewer facilities, to build conservation buffers for farmers and for landscaping to reduce runoff. Protecting the waters from dam sediment is not a major undertaking in the plan.

“Even though they are doing a lot of good things to limit the amounts,” Hirsch said, referring to states and the EPA, “it’s not going to have a good effect on the bay because a lot of material is coming in from the dam.”

The USGS study found that above the dam, the amount of phosphorus and nitrogen has been declining. The nutrient level has increased in Maryland after water passes over the dam and its collected sediment.

Last year, before they knew the problem had worsened, Maryland Gov. Martin O’Malley (D) and Col. David E. Anderson, commander of the U.S. Army Corps of Engineers’ Baltimore District, started a three-year, $1.4 million effort to study the sediment problem.

“We need to start addressing it now because we’re starting to see those impacts,” said Bruce Michael, director of resource assessment for Maryland’s Department of Natural Resources. Michael is on a task force of state and federal officials who plan to meet Sept. 24 in Baltimore to continue exploring alternatives to deal with sediment.

Dredging behind the dam is an option, along with controlled releases at times of year when it would have the least negative impact.

One thing is clear: It will not take a storm the size of Lee to send sediment surging into the bay if the problem is unsolved. Smaller storms would have greater impact, Hirsch said.

“Once we develop some of these options, we will try to run models” to determine how they might work, Michael said.