This week on Now and a Long Time Ago, GM stops by the C & O Canal at Wisconsin Ave. The old photo is labeled by the Library of Congress as being taken somewhere between 1890 and 1940. Given the industrial scene, GM is going to guess the shot was taken on the earlier end of that time frame.
Also, the photo is labeled “C & O Canal from Wisconsin Ave.”. But it doesn’t really look that much like the view from Wisconsin Ave. in either direction. GM guessed that if it’s either one, it’s the east side, so that’s what he matched it up with. Anyone have a better guess?
The Georgetown Metropolitan 
I’m thinking that the building on the left is on the western end of Cady’s Alley and the old picture is looking east from the bridge that extends 34th St.
RobRob’s guess seems accurate. The building on the right side of the canal, with its gable end-on to the camera, looks somehow familiar: I have a feeling it may have been part of the Capital Transit shops complex that was later taken over by Army Communications, and was eventually rebuilt as Georgetown Park. This would put it between 34th Street and Wisconsin. An interesting aside: note that there is laundry hanging out to dry from the building on the left.
From the angle of that left-hand building, it seems that the canal is not running parallel to M St at this point. Looking into the distance, it looks like the canal also makes a slight leftward turn, which be at about 33rd St. For what it’s worth, one of the smokestacks may be the one still in place on Grace St. today.
This Google Street View shot at the end of the Key Bridge is as close as I could get. http://goo.gl/maps/S7hs
If you look closely at the right-hand side of the old shot, you can almost make out the Washington Monument, too.