The Conowingo dam has put the Susquehanna River back on American Rivers' list of the most endangered rivers in the United States.

Prompted by the listing, environmentalists and Hogan administration officials joined Wednesday in voicing their concerns about the impact of the Maryland dam on the Chesapeake Bay – and about pending federal legislation that would strip states of leverage in dealing with pollution from such facilities.

This is the seventh time since 1988 that the Susquehanna has made the Washington-based environmental group’s list of the nation’s most endangered rivers. American Rivers pointed out that the Conowingo hydroelectric facility has for decades kept vast amounts of nutrient and sediment pollution from flowing into the Bay, but that trapping capacity has been largely used up.

Exelon Corp., which owns the dam, is asking the Federal Energy Regulatory Commission to renew its license to operate the facility for another 46 years. American Rivers called this a “once in a lifetime” opportunity to force the dam’s owner to restore the dam’s pollution retention capacity and improve fish passage.

Under the federal Clean Water Act, Maryland can block Exelon from getting a new license if the state determines the facility does not meet water quality standards. FERC's review of the license renewal is on hold as state and federal agencies negotiate with Exelon over improving fish passage and the buildup of nutrients and sediments behind the dam.

But a bill pending in Congress would take away states’ role in relicensing hydro facilities. It passed the House last year and is now in the Senate.

“We cannot let the hydropower industry avoid its responsibility for protecting the environment at the expense of our fish, wildlife, water quality and outdoor recreation,” American Rivers President Bob Irvin said. “For the millions who depend on the river and for generations to come, we must act now to save the Susquehanna.”

The group contends that Exelon is profiting from exclusive use of the river, so it should invest some of its revenue in upgrading fish lifts at Conowingo and dealing with water quality problems related to the dam.

"The Hogan administration is putting a priority on the Conowingo Dam and the Susquehanna River to restore and protect the Chesapeake Bay, said Ben Grumbles, Maryland’s environment secretary, “so any effort to reduce Maryland's important tools and incentives should be opposed.”

Sen. Ben Cardin and other Democratic members of Maryland’s Congressional delegation likewise issued statements expressing their concern for the health of the Susquehanna, the source of half the fresh water entering the Bay — and for much of its pollution as well.

Rep. John Sarbanes charged that the federal legislation would undo important environmental protections.

“If we don’t act to block this harmful, special-interest bill, then all of the progress we’ve made to reduce pollution in the Chesapeake Bay could be undone,” Sarbanes said.

Exelon spokeswoman Deena O’Brien countered that the dam is not the source of the nutrient and sediment pollution entering the Bay. O’Brien cited the Lower Susquehanna River Watershed Assessment, a joint federal-state study, which found that it comes from upriver.

The Conowingo Dam has been providing “clean reliable electricity” since 1928 and is Maryland’s largest source of renewable power, O’Brien said. “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,” she said.