Reconstruct/Stabilize Historic Stone Wall & Towpath McMahons Mill to Lock 42

A stone retaining wall holding up a dirt pathway, winding between a river and a rock cliff
A portion of the towpath and retaining wall that is part of the project

NPS | Joe Reed, Park Engineer

The towpath between miles 88 and 89 is closed. The McMahons Mill parking lot (mile 88) is closed.

The park plans to rehabilitate a historic stone retaining wall and stabilize the towpath along a 0.9-mile stretch between McMahons Mill (mile 88) and Lock 42 (mile 89) in Williamsport, Maryland. This section of the park is intermittently covered with water during river flooding, causing resource damage and requiring bicycle and pedestrian detours.

The project aims to lessen high-water event impacts to the towpath, historic features of the park, and park visitors by stabilizing this entire section and raising the elevation of the lowest segments of towpath and retaining wall. Currently, Potomac River high waters, driven by floods, cause sections of the towpath in this area to be impassable many times each year, requiring a 3-mile detour for hikers and bikers on narrow, winding county roads.

A photo of a map showing a project area and future visitor detour.
McMahons Mill Project Area Detour Map

NPS | Joe Reed, Park Engineer


The towpath between miles 88 and 89 is CLOSED. To the left is a map showing the project area and the detour that is in place. Beginning at mile 88, this detour diverts traffic from the towpath, up a hill near McMahons Mill, and along an old access road. Visitors are asked to follow the signage and dismount bicycles along this stretch.


What is the need for the project?

The historic stone wall has been weakened by water impacts from the Potomac River, particularly during storm events, and vegetation growing through the structure. Portions of the historic stone wall are currently washed out or missing. Periodic flood events continue to weaken the stone wall and create sinkholes, washouts, and slope failures in the towpath. While some of these deficiencies have been temporarily fixed with short-term repairs, they will continue to occur and worsen until portions of the trail are completely cut off.

What does the project include?

The project consists of work removing vegetation growing within the stone wall and next to the towpath, reconstructing missing or washed out portions of the historic stone wall, stabilizing and building up other portions of the stone wall, and moving some portions of the towpath more inland. It also includes re-establishing the towpath with a width to allow for safe, limited passage of emergency/maintenance vehicles, as well as raising the towpath to an elevation that would reduce closures caused by minor flood waters. Even after repairs are completed, moderate-to-major flooding would still likely overtop the towpath.

Similar to the 2010 – 2012 work completed on the portion of the towpath between McMahons Mill to Dam #4, the National Park Service (NPS) anticipates the contractor to rely heavily on barges and boats to perform the work from the river. Contrary to the work in 2010 – 2012, no concrete bridges will be used for the stabilization work.

A portion of a dirt path surrounded dense, green vegetation is covered with high river water
2017 high water event covering a portion of the towpath: The completion of the towpath and retaining wall work will improve safety and reduce the impacts of high-water events to the towpath.

NPS | Joe Reed, Park Engineer

Where will the project take place?

This project will take place on approximately 0.9 miles of the C&O Canal towpath from McMahons Mill (mile 88) to Lock 42 (mile 89), approximately 10 river miles downstream of Williamsport, MD, and four miles upstream from Dam #4.

When will the project begin and end?

The project began in August 2021 and is anticipated to continue through winter 2022/23, weather dependent.

How will this project impact park visitors?

To facilitate the work, the McMahons Mill Parking Lot (mile 88, near the end of Avis Mill Road) is CLOSED. There is no public parking available along Avis Mill Road or Briar Patch Lane.

Alternative parking can be found upstream in Williamsport, MD, (miles 99.32 and 99.71) and downstream at Dam #4 (mile 84.41).

How will this project benefit park visitors?

Once the project is complete, the towpath is less likely to experience as many high-water event closures, allowing for more frequent and safe passage for visitors. The existing towpath will be stabilized, increasing the resiliency of this fragile length of the towpath and preserving the continuity of the 184.5-mile-long trail. Many existing impairments - such as sinkholes, washouts, deposited river sediment, and encroaching trees/roots - will be addressed, resulting in a safer and better visitor experience.

How will this project impact natural and cultural resources?

Every project the NPS executes undergoes review by a multidisciplinary team to ensure compliance with Section 106 of the National Historic Preservation Act (NHPA), National Environmental Policy Act (NEPA), and other federal, state, and local laws.

An environmental assessment (EA) was prepared by the park, which can be found on Planning, Environment and Public Comment (PEPC) website (

What is the park doing to protect those resources during the project?

NPS resource protection experts, engineers, and the contractor’s team will be monitoring the stone and towpath work along this stretch of the towpath, along with contractor’s operations and visitor safety during the project with the goal of safely protecting the environment surrounding the project. Every project undertaken by the NPS is conducted in compliance with the National Environmental Protection Act (NEPA), the National Historic Preservation Act (NHPA), and other regulations. Please see the Environmental Assessment (EA) prepared by the park for further details.

How much does the project cost?

The design build contract for the rehabilitation of 0.9 miles of towpath and supporting masonry walls between McMahons Mill to Lock 42 near Williamsport, MD, was awarded in May 2021 for $18 million and was funded from the National Park Service’s Line Item Construction Program.

Who can I contact if I have further questions regarding the project?

Additional questions are to be directed to the park headquarters at 301-739-4200.

A dirt path that is low to a river and has a rock cliff on the other side of it.
An example of a low-lying portion of the towpath that is frequently covered during high water events.

NPS | Joe Reed, Park Engineer

Last updated: February 12, 2022

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Mailing Address:

142 W. Potomac St.
Williamsport , MD 21795



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