The heavily used Georgetown section of the Chesapeake and Ohio Canal will be at least partly closed to strollers, joggers and cyclists - and unavailable for boats - during a two-year, $2.5 million repair and reconstruction project that the National Park Service hopes to start next March.

The accumulated damage to the 140-year-old canal's store walls and locks will not be fully evident until an NPS contractor has drained the 1 1/4 miles between Rock Creek and roughly Foxhall Road and has removed up to four feet of silt from the canal bed.

Park officials already know, however, that the old waterway leaks a lot, that some retaining walls are very unstable and that some beams and stones have broken loose in Lock 4 at Thomas Jefferson Street.

Tearing up the whole stretch for two years "is the only way to do it once and do it right," said Rich Huber, a preservation officer from the NPS Denver service center who manages the repairs.

NPS plans have already aroused concern in several quarters. Contractors have been reluctant to bid for the job, Huber acknowledged, because of uncertainties about the project's scope and the logistical challenges of using heavy equipment in the narrow canal bed and moving machines and materials over or around the four Georgetown locks.

Merhcants at The Foundry mall have complained that emptying the waterway will hurt their business by disrupting the canalside plaza between 30th and Thomas Jefferson streets and interrupting the popular canal-boat rides there.

Some members of the C & O Canal national historical park's advisory commission expressed worries at a meeting last week about how the canal's appearance and atmoshpere may be changed. In other canal repairs, commission member Ken Rollins said, the NPS Denver center has taken "the view that if they can't do a perfect restoration with orginal materials, they won't try to maintain the historic appearance at all."

NPS officials insist they want the rebuilt canal to look much the same. Huber hopes to do some of the rebuilding with old stones retrieved from the silt. New stones, where needed, will be "the same color and texture," he said.

However, the canal corridor in Georgetown is already being changed by the booming development there. The Foundry is only the first of more than a half-dozen private projects seeking to build on the charm and popularity of the canal.

National Capital Parks planner John Parsons has met often with developers on questions of access and aesthetics.

He has three objectives, Parsons said. One is to promote new pedestrian routes in the area "and relieve the pressure on the towpath, which gets very crowded at times."

The second aim, Parsons said, "is to protect the variety of spaces and feelings in that area - and to keep it low-key. We don't want every block to be paved with bricks. And the third objective is to be sure that, if developers have access to the park, they use it in ways that don't intrude."

One plan to use a narrow section of towpath for access to a new restaurant was rejected, Parsons said. He is happier about the Weissberg Development Corporation's plan to include a canal-side walkway in its Flour Mill project at 33rd street.

Parsons is also enthusiastic about the Western Development Corporation's plans for the old multi-story buildings that flank the canal west of Wisconsin Avenue. The developers could have demolished those buildings and "wiped out the canyon-like feeling there," Parsons said. Instead, the structures are being rebuilt for stores and housing, with two high-level walkways across the canal and a stairway to the park from Wisconsin Avenue.

The same NPS officials say the timing is also likely to bring the canal project under fire from opponents of the Georgetown developments as well as citizens concerned mainly about the park. The canal advisory commission decided last week to review the NPS plans in depth at its next meeting, to be held Dec. 16 in Georgetown.