Before the Stony Brook became entirely covered for its run through Jamaica Plain in the late 19th century, it ran in an open channel until it reached the point shown in the picture above, where it went underground into the Boston sewer system. The massive grate which acted as a water filter collected all sorts of debris which needed to be cleaned off at regular intervals to prevent flooding during times of heavy rain, which is what the men above are doing. Before being diverted into the sewer, the Stony Brook would flood in a fairly regular basis, including a major flood in the mid-1880s which prompted the covering of the entire channel.
I'm still trying to determine the location of the Stony Brook Gate House--does anyone have any thoughts? More about the Stony Brook sewer can be read here and here. You can see several animations of how the Stony Brook Sewer works. Don't worry, it isn't as disgusting as it sounds.
p.s. I apologize for the poor scan.