Long Channel Drive, Long Channel Circle and Tidewinds Way offer housing that abuts Little Seneca Lake, as seen from the air in Boyds, Md. (Katherine Frey/The Washington Post)

Only twice in its 29-year history — during droughts in 1999 and 2002 — have authorities released water from Little Seneca Lake, about five miles south of Clarksburg, Md. Otherwise, it has been a 3.9 billion-gallon emergency backup water supply for the Washington region, more useful for boaters and other patrons of Black Hill Regional Park.

But concerns about climate change and new construction, and their possible long-term impact on the water supply, have pushed the man-made lake into the debate over the future of the northern Montgomery County town.

Little Seneca is fed in part by the Ten Mile Creek watershed in the Clarksburg area, where Pulte Homes wants to build a mix of up to 1,000 single-family houses and townhouses on 538 acres west of Interstate 270. On the other side of the highway, Peterson Cos., the developers of National Harbor, envisions 450,000 square feet of retail.

Both companies have been planning their projects for years. But Montgomery’s original 1994 blueprint for Clarksburg calls for a pause in construction to evaluate the impact of development on Ten Mile, one of the county’s last high-quality streams. This week, the County Council began considering a proposal by the Montgomery Planning Board to revise the blueprint in a way that could significantly limit growth. A final vote is expected sometime next month.

Opponents say that even at the reduced scale of construction recommended by the planning board, the projects are untenable. In their view, storm-water runoff from new expanses of impervious surface could drive increased amounts of sediment and pollutants into the creek — and into Little Seneca — limiting its storage capacity and future usefulness as an emergency water supply. Environmentalists have rallied around the health of the watershed and the reservoir, declaring it a defining issue for a council whose members face a June primary.

“Your decision will brand you as faithful trustees of the future of the town and the region or as negligent custodians of the public interest,” Royce Hanson, former chairman of the Montgomery County Planning Board, told council members at a hearing last month. “There are some things that are just intrinsically important and should be protected.”

Developers, county officials and the researchers they’ve hired contend that the environmental concerns are overblown, even bogus. A $200,000 study commissioned by the planning board concluded that runoff from the proposed development would be no more harmful to Ten Mile Creek’s biology than the level of pollutants generated by existing farms.

Officials point out that Ten Mile Creek accounts for only about a third of Little Seneca’s water and that the reservoir is well protected by forebays that capture most of the sediment before it enters the lake. A recent study by the Washington Suburban Sanitary Commission, which operates Little Seneca, found that it loses only about a tenth of one percent of its capacity annually to sediment.

“If we can protect Ten Mile Creek, we should be able to protect the reservoir,” said Mary Dolan, chief of functional planning and policy for the county.

Lewis Birnbaum, president of Pulte’s Mid-Atlantic division, is more blunt, calling concerns about the reservoir “fear mongering” by those who want no development in the area. “Anyone who says it’s a threat to the emergency water supply is trying to create excitement,” he said.

Maintaining the Potomac

Although Little Seneca has been used only twice to augment the flow of the Potomac River — which supplies 80 percent of the region’s drinking water — there is evidence to suggest that it could play a larger role. In 2010, studies by the Interstate Commission on the Potomac River Basin found that climate change could have a serious impact on the level of the Potomac.

Commission researchers modeled 18 scenarios with various configurations of temperature, rainfall and storm frequency in 2040. In many of them, the levels of streams feeding the river basin drop by as much as a third.

In six of the scenarios, Little Seneca and another backup water supply, Jennings Randolph Lake in Western Maryland, are heavily used, or even emptied, in an effort to maintain the Potomac’s flow.

The WSSC has sidestepped questions from the council about the possible effects of the Pulte and Peterson projects. But at a planning board hearing last fall, the agency’s environmental group leader, Mohammad Habibian, said: “We expect that with population growth and the strengthening of the climate change, the reservoir’s use as a backup source water will significantly increase. As such, WSSC strongly recommends that the Little Seneca Lake be protected against sedimentation, which will reduce water availability for release when needed.”

County planners acknowledge that they never asked their consultants, Biohabitats and Brown and Caldwell, to study possible impacts on the reservoir. That’s why Hanson and two former Montgomery Council members, John Menke and Scott Fosler, are calling on the county to study the impact of future development on Little Seneca.

“We’re not saying this is definitely going to be devastating,” Fosler said. “What we’re saying is, we don’t know and the county officials don’t know, either.”

1960s drought

Little Seneca was created as part of a regional strategy to protect against shortages of drinking water. After severe drought in the 1960s, there was concern about Washington’s heavy reliance on the Potomac for its supply.

The Army Corps of Engineers proposed building a series of 16 reservoirs along the river basin, which appalled many local officials.

As an alternative, the Corps along with leaders from Montgomery, Fairfax County and the District collaborated to create a shared regional system of resources and backups. Nearly 80 percent of the region’s raw water still comes from the river, but the supply is also supported by the Occoquan Reservoir in Northern Virginia and two reservoirs along the Patuxent River — Triadelphia and Rocky Gorge — operated by the WSSC.

The jurisdictions also agreed to create two lakes as emergency reservoirs: Jennings Randolph, on the West Virginia border, and Little Seneca, formed in 1985 when an earthen dam was built across a portion of Little Seneca Creek. Neither is a direct source of drinking water; they are intended to keep the Potomac at a level where it is possible to draw water during serious drought.

Fosler said the system has worked well.

“But what about the next 50 years?” he asked. “What about the next 10 years?”