The National Park Service is currently rehabilitating the Memorial Bridge which will last until 2021. The south sidewalk is currently closed to pedestrians and cyclists. There is a detour that requires crossing busy roadways multiple times.
A video demonstrating the detour and additional information is available from the National Park Service at https://www.nps.gov/gwmp/learn/management/pedestrians-bicyclists.html
Here’s the video outlining the detours: