From Mike High's The C&O Canal Companion:
The pier in the middle of the canal [at mile 9.9] was once part of a pivot bridge. The knoll left by the road embankment offers a view of the remains of the circular pivot mechanism. The bridge was built in 1941 to provide access to a Civilian Conservation Corps camp. The canal company had long since ceased operations, but a pivot bridge was still needed because the National Park Service was operating barges in the canal in this area.
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According to the Civilian Conservation Corps Alumni web site, this would correspond to project S-68, company 325-C, founded 10/11/1933, of Bethesda MD.