Visionary engineers, tough local development regulations -- and a stroke of luck -- might have spared Loch Raven Reservoir from filling in with sediment these last 75 years, extending the life of the water supply and saving taxpayers from costly dredging.
Officials with the Maryland Geological Survey, working with federal and local agencies on the most comprehensive study of area reservoirs, found the capacity of Loch Raven has not significantly diminished since 1923 -- contrary to the fears of environmentalists.
"The reservoir is healthy from my standpoint," said Richard Ortt, an engineer with the geological survey, who participated in the study of the Gunpowder River Watershed that supplies drinking water to millions of residents.
The geological survey also is mapping Pretty Boy Reservoir, the smaller holding reservoir that flows into Loch Raven.
The findings provide good, if somewhat surprising, news to environmentalists who had feared that decades of development and agricultural runoff had caused silt to build up within the reservoirs.
"It did surprise me," said Charles E. Conklin, president of Gunpowder Valley Conservancy. "I would have suspected there would be extensive sediment."
Conklin credited Baltimore County's laws controlling runoff for preserving the integrity of the reservoir, while engineers credit the reservoir's designers and nature.
In the early part of the century, with Baltimore's population growing and its water supply in Lake Roland threatened by sediment, engineers began searching for a site for a reservoir. They soon settled on the Gunpowder Falls and its deep valley in Baltimore County.
In 1910, engineers John Freeman and Frederick Stearns proposed damming the Gunpowder Falls 237 feet above sea level, creating a reservoir of more than 20 billion gallons.
"But they were concerned the reservoir would fill in quickly," said William S. L. Banks, a hydrologist with the U.S. Geological Survey.
They designed pools upstream to help catch the sediment they knew would flow into the water, and predicted that the enormous reservoir would be able to meet the city's water needs for at least 35 years.
"They were much ahead of their time in designing the reservoir," Ortt said.
The engineers were lucky. The Gunpowder's deep, narrow valley helped transport the sediment quickly to the pools, where it collects before the water enters the wide mouth of the reservoir. And a buffer of forest has helped protect the reservoirs.
To determine the health of the watershed, state, federal and local agencies are spending two years and $750,000 to investigate the capacity of the reservoirs and the level of pollutants in the streams that feed them.
Last year, the Maryland Geological Survey mapped the bottom of the Loch Raven Reservoir using sound waves. The U.S. Geological Survey followed with seismic recordings used to locate the original bottom before the reservoir was expanded in 1923.
Results of both studies were compared with old survey maps created before the farms, forests and mills were flooded by the reservoir in 1914.
Researchers found that up to 30 feet of silt and mud have filled in the upper part of the reservoir near Warren Road. But in the main body of the reservoir, the sediment is less than 10 feet -- an amount considered normal after decades of storms and erosion.
Officials are testing core samples that were made to determine when the sediment was deposited in the reservoir. Scientists are trying to explain why most of the deposited material is coarse gravel and not the loose silt that would be expected from the runoff from construction sites.
The entire watershed study, including the report on pollutants, is to be finished by December 1999.
Pub Date: 11/19/98