WILLIAMSPORT -- What Gen. Stonewall Jackson's sappers couldn't achieve with mortars and shells has come dangerously close to being accomplished by the relentless push of water on the Potomac River near here.
Two stone dams built a decade earlier than the Civil War that create 10 to 12 miles of smooth water immensely popular with boaters have been eroded severely enough to warrant fears of their collapse.
As a result, U.S. Navy divers are in the midst of unusual, tricky and dangerous work to assure that the dams -- blandly named Dams No. 4 and No. 5 -- remain safe for many more years.
The dams are the last of several built in the 19th century to provide water for the Chesapeake & Ohio Canal, which parallels the rocky river in Maryland.
"Stonewall Jackson tried to blow up Dam No. 5 during the Civil War, but couldn't do it," said Patty Manown, a state Department of Natural Resources spokeswoman.
Damage caused by the Confederate general's troops was repaired within a few days. Confederates were hoping to breach the dam to sabotage the Union's transporting of supplies on the nearby canal, said Keith Whisenant, chief ranger at the C&O; Historical Park.
More than a century later, the Navy divers marvel at the dams.
"They sure knew what they were doing 140 years ago when it came to building these dams. As construction workers, we're amazed," said Navy Chief Ron Skurkis, whose divers have worked on underwater projects around the world. "Some of these stones are 18 inches to 2 feet thick. They're 5 feet long and 3 feet wide."
The Navy divers -- Seabees stationed in the Norfolk, Va., area -- are at work because large holes were discovered in the dams during an "intense underwater inspection" last year, said Terrie Savering, assistant superintendent of the C&O; park.
"It's almost like a cave under [Dam No. 5]," Brad Iarossi, chief of Maryland's Dam Safety Division, said of the damage. "Divers could almost stand in the hole. Deterioration is a slow process. It's been like that for some time."
Navy divers began work three weeks ago on Dam No. 5, which stretches 811 feet (more than one-tenth of a mile) across the Potomac about seven miles upstream from Williamsport and are expected to be finished within a few weeks.
The Seabees were brought in because of their specialized construction skills, as well as for the chance at a training mission.
Counting the Navy's work and materials being used, the dams' repair will cost about $350,000. But using the Seabees, instead of private contractors, is saving the federal government an estimated $600,000, Mr. Iarossi said.
The task is complex. Working in water 5 or more feet deep -- then going beneath the dam footings on their downstream sides -- divers use underwater equipment to do the same kind of foundation and reinforcement work that construction workers do land.
The divers enter voids beneath the dam and install pipes so that concrete, specially mixed for underwater application, can be pumped into these areas.
Mr. Skurkis said voids in Dam No. 5 vary in size from 4-by-4 to 15-by-15 feet. He said that divers must be careful when pumping concrete so they don't shift stones within the 20-foot-wide dam.
Original plans called for repairs on both dams to be completed this summer, but work at Dam No. 5 has been more difficult than expected. Thus, work on Dam No. 4 -- about 12 miles upstream from Shepherdstown, W. Va. -- will not be completed until next year.
"We didn't expect the river bottom itself to be so arduous," Mr. Skurkis said. "There are large stones -- some of them the size of cars -- on the river bed around the dam, and that makes it difficult for divers to use underwater hydraulic tools."
High water also has hampered the effort. On the dams' downstream sides, turbulence from the river spilling down as far as 20 feet makes maneuvering difficult for divers. Underwater visibility extends only 2 to 4 feet.
The dams were built with stone foundations and filled with rubble and concrete-like mixes, Ms. Savering said. Conflicting accounts date the construction of Dam No. 5 sometime in the 1850s and Dam No. 4 a few years later. They replaced earlier dams -- timber cribs filled with stones -- that settlers built in the late 1830s.
"They're really magnificent structures," Mr. Iarossi said.
The last barge traveled the C&O; Canal in the early 1900s. Today, the dams largely serve recreational purposes, creating 10 to 12 miles of slack water upstream for power-boaters, water-skiers, jet-skiers and anglers from Baltimore, Washington and West Virginia.
"There's extremely high boat traffic up there," said Bud Reaves, a natural resources manager for the state's boating administration.
On weekends and holidays, he said, "it's absolutely a zoo out there."
Concerned about public safety, Maryland boating officials issued emergency boating restrictions last summer, curtailing boating a half-mile up and downstream from the dams.
The restrictions will remain in effect until the repairs are finished, Ms. Manown said.
If the dams were to burst, danger for people upstream and downstream could be considerable. Boats might capsize and swimmers and waders downstream might not be able to reach higher ground because of a rapid rise in water, Mr. Iarossi said. But no homes downstream would be affected.
"I support what they're doing," said Charles Boward, a Hagerstown boater and member of the Western Maryland Sportsmen's Club. "They've been in and out of there doing repairs over the years. I knew they were in bad shape."