I just looked at the comments from yesterday and realized that I had misunderstood the question:
Regarding the question about the pathway going over the bridge visible in the aerial photo from 1921, I don’t think he was asking about Harris Gulley. Go back to that photo and zoom in at the bottom of the photo, just left of center (and to the right of the plane’s wing). There’s a bridge crossing what appears to be a drainage ditch in Hermann Park.
Luckily, I know the answer to this one too. It was just a ditch, probably dug by some farmer who lived along the unpaved road that is Main Street today. Rains caused brutal flooding along that road and although the ditches didn’t really help during a big inundation, they probably gave some relief during more moderate rainfalls. The farmhouse below was actually on Institute property:
Here’s another peek, just for fun:
There was a whole maze of these ditches in the area, including, at first, on the Institute side of the road. You can see it here during construction:
And a cleaned up, more sophisticated version a bit later:
In this shot you can just make out the ditch and little bridge over it at the Community House (later replaced by Autry house) in 1920:
The ditches on the Rice side disappeared fairly soon because of the much more sophisticated underground drainage work, but on the other side of Main there were only a few farms and Main Street Park so they lasted longer. I believe that most of them were filled in or paved over around the time the trolley tracks were ripped up and Fannin was built in its current configuration, but little pieces lingered for quite a while.