With its gardens, historic bridges and scenic waterway, it’s hard to imagine that the area known today as the Back Bay Fens was once described as “The foulest marsh and muddy flats to be found anywhere in Massachusetts” by a city engineer in the 1870s. Hear the story of how Frederick Law Olmsted set about to transform the area and the subsequent changes made to his landscape in the century that followed. The walk, led by Emerald Necklace Conservancy docents, includes stops at the Kelleher Rose Garden, a 17th-century Japanese temple bell and a historic gatehouse designed by H.H. Richardson. Call 617-522-2700 for updates on weather-related cancellation.