Answer: What was the name of that stream?

When you're trying to track someone down these days, a reasonable first starting place would be their G+, Facebook, or Flickr social postings.  Or, in my case, since you know I have a Home Page, you might check there.  If you do the obvious query, you'll find both my G+ update and my conference travel update. 

My site:  https://sites.google.com/site/dmrussell/ 
tells you I'm probably in Paris, and checking my G+ post confirms it.  I posted a CHI2013 event yesterday afternoon.  

Now that you know I'm in Paris, let's look at the parks and see if any could possibly look like the one in that photo.  Here's the photo.  The key features we'll want to look for in the map are a large green park with a long boulevard running through it, with a large freeway-like thing and a traffic circle near one corner.  
  

And here's the relevant map of Paris:  

Although there are a few parks, only one, the Bois de Boulogne (on the left side near the red label of N185) has a road that looks like the one in the photo, cutting a big slash through the parkland.   (This is a little easier to see if you click on the map image above to see it in full resolution.)  

At this point we suspect that it's the Bois de Boulogne, so let's search for that and read up a bit.  

[ Bois de Boulogne ] 

Leads to the Wikipedia article, which suggests that there is a stream running through it, which sounds even better.  

A good double check here would be to look for the conference location with: 

[ CHI 2013 conference ] 

and find the conference page that reveals that the conference is being held at the Palais des congrès de Paris.  Map that out in Google Maps, and you'll see it's right on the edge of the Bois de Boulogne.  There's also a conference hotel by the Palais that's positioned exactly right to create my photo.  

Once you look for the word "stream" in the article on the Bois, you'll see that the stream called the Ruisseau de Longchamp (1855) is the major (artificial) stream in the park. It flows through the Pré-Catelan area of the park, under the alley of Reine Marguerite, then to the Mare des Biches, one of the oldest natural ponds in the park, then to another reservoir and the Grand Cascade.

A bit of history from Wikipedia: 

[The architect's ] plan called for long straight alleys in patterns crisscrossing the park and, as the Emperor had asked, lakes and a long stream similar to the Serpentine in Hyde Park. Unfortunately, Varé bungled the assignment. He failed to take into account the difference in elevation between the beginning of the stream and the end; if his plan had been followed, the upper part of the stream would have been empty, and the lower portion flooded. When Haussmann saw the partially finished stream, he saw the problem immediately and had the elevations measured. He dismissed the unfortunate Varé and Hittorff, and designed the solution himself; an upper lake and a lower lake, divided by an elevated road, which serves as a dam; and a cascade which allows the water to flow between the lakes. This is the design still seen today.

And while there are many trees in the Bois, the one that caught my eye was the Redwood tree, the California state tree.  

Hope you had fun doing this... I certainly did!   (I also checked other maps of the Bois, confirming my find that this was the *artificial* stream that I'd run beside.)  

Search on!