Rivnuts from Homestead, PA on 10/25/2021 8:14:58 PM:
I was surprised by the detour at McMahon's Mill on the Towpath between mile markers 88 and 89 . It does appear among the list of detours and closures on the National Park Service C&O Towpath website. The particular detour is however clearly described (and I failed to read) at:
https://www.nps.gov/choh/planyourvisit/mcmahon-s-lock-42-towpath.htm
It is a combination of some grassy paths and country roads totalling about 3 miles and has a couple of short steep, gravelly sections which may require walking a loaded bike.