The topic of River Birch Trees dropping yellow leaves in summer has been discussed at great length on this forum and elsewhere across the internet, and I understand that typical causes are leaf blight or drought, but I have a tree that doesn't appear to be a clear case one way or the other. After a very wet late spring, the weather has turned hot (90's and even a couple of 100's). While I have continued to water with sprinklers a couple of times a week, the leaves from the River Birch on the west side of my house started dropping leaves at an alarming rate. The same species on the east side also dropped yellow leaves, but has since stopped and only lost a small percentage. It now appears that 30% of the leaves on the west tree have dropped or are yellow and yet to drop.

Because of the percentage of leaves affected, and because my other tree seems to be fine, I'm leaning towards a blight diagnosis. I'm hesitant to do nothing though, as the tell-tale spots of blight don't seem to be prevalent on yellow leaves. Does anyone have any ideas based on the description or image?

Any thoughts or suggestions would be much appreciated.