Water rushes through open bays on the Conowingo Dam on the Susquehanna River near Conowingo, Md., in 2011. (Patrick Semansky/AP)

A national conservation group has named the Susquehanna No. 3 among America’s most endangered rivers, saying the Conowingo Dam’s ability to prevent pollutants and sediment from flowing into the Chesapeake Bay is threatened.

The group American Rivers warns that the Conowingo’s capacity for trapping those materials is exhausted. The 88-year-old dam, owned by Chicago-based Exelon, sits about 10 miles north of the Susquehanna’s mouth to the Chesapeake.

State leaders joined the group in raising concerns about the dam and its impact on the health of the river and bay. They are calling on the U.S. Senate to reject legislation they say would allow Exelon to avoid complying with state water quality standards.

Ben H. Grumbles and Mark J. Belton, the state’s secretaries of the environment and natural resources, respectively, spoke in support of American Rivers on Wednesday. They said the state is heeding the concerns as Exelon seeks a new federal license to operate the dam.

“The state is committed to addressing the potential environmental damage caused by the Conowingo Dam reaching capacity, and is partnering with the federal government and Exelon to address fish passage and water quality concerns as part of the relicensing process,” Belton said, appearing alongside officials from American Rivers and the Chesapeake Bay Foundation in Annapolis.

“There is no doubt that the Susquehanna River is an endangered river,” foundation Vice President Kim Coble said.

The Susquehanna runs more than 400 miles from Coopers­town, N.Y., to the Chesapeake, draining a 27,000-square-mile area that includes half the state of Pennsylvania.

It provides more than half of the bay’s fresh water but also brings heavy loads of nitrogen from farm runoff. The nitrogen feeds algae blooms that cloud bay waters, blocking sunlight from reaching underwater grasses; when the blooms die, they suck oxygen from the water, creating dead zones and killing fish en masse.

The federal legislation, known as the North American Energy Security and Infrastructure Act of 2015, includes a long list of measures addressing the reliability, resiliency and security of the nation’s electricity grid.

Rep. John Sarbanes (D-Md.) urged the Senate to reject the resolution, which has passed in the House.

“The health of the Susquehanna River — and of the Chesapeake Bay — depends on the safe and environmentally conscious operation of the Conowingo Hydroelectric Dam,” he said.

“Today, we have an opportunity to ensure that the dam continues to limit nitrogen, phosphorus and sediment pollution to the Bay.”

An Exelon spokeswoman said the company is working with stakeholders to help protect the bay’s health.

“Our goal is to keep Conowingo operating, while continuing to work with key stakeholders to ensure the long-term health of the Lower Susquehanna River and the Chesapeake Bay,” spokeswoman Deena O’Brien said in an email.

The other rivers on this year’s endangered list are:

The Apalachicola, Chattahoochee and Flint rivers, which form a basin across Alabama, Florida and Georgia; the San Joaquin River in California; the Smith River in Montana; the Green and Duwamish rivers in Washington; the Pee Dee River in North Carolina; the Russell Fork River in Kentucky and Virginia; the Merrimack River in New Hampshire and Massachusetts; the St. Lawrence River in New York; and the Pascagoula River in Mississippi and Alabama.