This week’s Georgetown Time Machine comes courtesy of the Library of Congress. It’s a photo of a boy relaxing on the canal gate paddle at what I believe to be Lock 3 of the C&O Canal.
The listing gives the photo a data of 1909. If that date is correct, then this photo would depict a time when the canal was in steep decline but nonetheless still functional. Operations on the canal did not come to an absolute end until a flood in 1924. It was effectively abandoned at that point.
The reason I believe the bridge depicted is the Thomas Jefferson St. bridge is the fence on the left advertising wood (and I believe coal). That would match up with the large lumberyard that was once on Thomas Jefferson St. seen in this more recent photo:
I could be wrong about the location, though, as this could also just as easily by Lock 4 with the bridge being 31st St. The only clear buildings are the rowhouses on the left, which are long gone from either street. Which do you think it is?
The Georgetown Metropolitan 
I think you are correct. If the date 1909 is correct I think it would be too early for our building, but the later photograph clearly shows the Allen-Mitchell building next to the warehouse just south of the bridge. across the canal is the upsloping pathway to the 31st bridge. I could not match the windows on the gray building just beyond the boy which i thought at first could have been ours. the runup to the TJ bridge is shorter i believe and matches the earlier photo compared to the 31st street path. great fun trying to figure this out.