That feeder stream is called "Klingle valley stream" (no surprise given the street name there). I sampled it several times per year when I worked for DC Water Quality 2008-2010. It's one of the few streams that has caddis (case-makers) and a couple of mayfly nymphs! It's one of the *nicer streams in the district for WQ across the board.
For a while the zoo had a huge black PVC pipe running from their parking lot on top of the hill down into the stream -- but they had permission to do so. I remember finding this huge pipe during sampling one morning and being kind of angry about it, but the zoo was doing some good things with the storm drains up there (haystacks, etc.) so no biggie.
I think you mean Rock Creek there though -- I've heard of people fishing there, but I've never done it.
Gene