A conflict is brewing over who controls the tap on the largest tributary to the Chesapeake Bay.

The Susquehanna River Basin Commission - which regulates water use on the Bay's largest tributary - will decide in May whether it should begin regulating present and future withdrawals by Baltimore, which is located outside the river's drainage basin. The city says the commission has no right to regulate its withdrawal. The outcome could ultimately affect water quality in the upper Chesapeake, the health of grass beds in the upper Bay, and efforts aimed at restoring shad on the Susquehanna.

"During low flow periods, the salinity of the ocean advances up the Bay and can change the local environment of species such as oysters and crabs, making them vulnerable to predators or disease," said SRBC Chief engineer John McSparran. "Maintaining the natural flow of the river becomes important to protecting these biological resources." To maintain flow, the SRBC - a compact representing New York, Maryland, Pennsylvania and the federal government - generally requires that major water users make arrangements to replace water they take during droughts.

SRBC regulations require that anyone who consumes more than 20,000 gallons of water per day must compensate for their water use during times of low flow.  Compensation can take a variety of forms, from constructing upstream reservoirs that release water during droughts to offset water use, or paying the SRBC to, in turn, purchase water from upstream federal reservoirs which is then released into the river to maintain minimum flows.

Baltimore, historically, has been exempt from those regulations. But the SRBC, eying a consultant's report prepared for the city which calls for taking up to 50 million gallons of water daily from the river and building capacity to withdraw as much as 250 million gallons a day, is worried about the impact of such withdrawals on the river.

Much of the concern focuses on the "Conowingo Pool," a 90-foot-deep reservoir behind the Conowingo Dam, which is where the city's water pipe is. Water from the pool is used by the Peach Bottom Nuclear Power Plant in Pennsylvania, two other electric generating facilities, and one public water supplier. The pool is also used recreationally by an estimated 65,000 people a year.

During an April public hearing on the issue, city officials denied they had any immediate plans to increase withdrawals from the river. But if they do, they said the SRBC does not have the authority to regulate the action.

"The city of Baltimore does not plan, and has not proposed, an increase in its diversion from the Conowingo Pool," said Jeral Milton, an attorney for the city.

But, citing a history of agreements running from the 1920s through the early 1960s, she said the city has the right to access up to 250 million gallons of water a day. "The city's right to divert water from the pool is recognized and guaranteed prior to the [SRBC] compact," Milton said.

The SRBC was created in 1971. Although Maryland is a member of the commission, the state has no authority over Baltimore's action because the law giving state agencies the power to regulate water use specifically exempts Baltimore.

A pipe completed in 1966 that connects Baltimore to the Conowingo Pool has the physical capacity to withdraw up to 137 million gallons a day. But the city has only intermittently withdrawn water from the pool, and instead takes most of its water from a series of reservoirs on other rivers. Over the past 34 years, its average daily withdrawal has only been 5.26 million gallons a day.

Compared to the average Susquehanna flow of 23 billion  gallons a day, Baltimore's needs may seem, almost literally, like a drop in the bucket.

But during dry times, flow can drop to less than one-tenth that level.

At those times, SRBC officials are concerned that an uncompensated withdrawal of 50 million gallons a day by Baltimore could - under some circumstances - lower the water in the Conowingo Pool by half-a-foot a day, significantly affecting other water users.

The longest drought on record for the Susquehanna lasted 114 days in 1930.

A 1964 drought lasted 110 days.

City officials strongly disputed that the increased withdrawals would pose a threat to other water users, and they promised to work with SRBC to study impacts of any withdrawals. "The city remains willing to cooperate with the SRBC," Milton said.

But nearly 20 speakers at the hearing - all except those representing the city - expressed concern about any increased withdrawals that were not offset.

For example, the Susquehanna River has been the the location of a multimillion effort to restore shad through the construction of fish lifts at dams on the river, as well as a massive stocking program.

Richard St. Pierre, a fisheries biologist with the U.S. Fish and Wildlife Service, said reduced water levels during late summer - frequently the time of low flows on the river - would hinder the migration of shad out of the river. "The increased level of withdrawal would occur at the worst possible time from the standpoint of anadromous fish," St. Pierre said.

Mike Fritz, the acting Living Resources Subcommittee coordinator for the Bay Program, said increased salinities could impact beds of underwater grasses in the Susquehanna Flats - one of the largest grass beds in the Bay - at the mouth of the river. Restoring grass beds has been a major priority of the Bay Program because they help filter water and provide important food and habitat for waterfowl, blue crabs, juvenile fish and other species.

Cindy Dunn, executive director of the Pennsylvania chapter of the National Audubon Society, said withdrawals affecting water levels could affect bald eagles living on the river, migrating shorebirds that rely on the lower Susquehanna, local wetlands and the upper Bay. All those impacts needed to be researched, she said.

Fran Flanigan, executive director of the Alliance for the Chesapeake Bay, noted that in past Bay restoration activities, downstream participants had been "very aggressive" in seeking cooperation from upstream areas. "Now is the time for those downstream parties to collaborate with upstream parties," she said.

State environmental officials from both Maryland and Pennsylvania - about two-thirds of the Conowingo Pool is in Pennsylvania - said this would threaten water quality, in part because lower flows would reduce the river's ability to assimilate pollutants.

Other upstream cities and water users expressed concern that they would have to bear the brunt of additional water cutbacks to help maintain downstream flows if the city of Baltimore did not have to replace water it takes out, as other water users do.

The SRBC at its May 21 meeting is expected to make a determination about whether the city's current and future withdrawals from the river have the potential to impact the river and other users. If it does, it would open the door for SRBC to regulate the city's water withdrawals.

The 444-mile long Susquehanna River supplies more than half of the freshwater to the Chesapeake, and about 80 percent of the freshwater into the upper Bay.

SRBC seeks to protect basin's water supply

Faced with concerns that demands for its water will increase in the future, the Susquehanna River Basin Commission in March adopted a new policy to discourage out-of-basin requests for the Susquehanna's water.

Under the policy, anyone outside the basin seeking to divert water from the watershed must show "clear and convincing evidence" that it has made a good-faith effort to conserve its own water.

In addition, the commission will consider a variety of factors before allowing such a diversion. Among them are the needs of Susquehanna and its users; potential impacts on the Chesapeake Bay; cumulative impacts on environmental, social and recreational values; potential economic impacts on the Susquehanna River basin; whether land use and natural resource planning is being carried out within the importing basin; and other issues.