Birches need lots of water. Maybe it will make it if you give it what it needs.

Dan

Various of my trees shed leaves all during the growing season with some turning yellow before dropping. It is normal, especially if the leaves come from someplace on the tree where they get very little or no sunlight. Unless it is a fairly large amount it most likely is not anything to be concerned about.

Dan is right, birch need water and don't do well in extended dry periods....which means they thrive here where cool moist soil is the rule, but that doesn't help you :)

Start watering deeply once a week until you've had some rainfall if this tree is important to you. 12 or 13 years is exceptionally short lived for any tree including birch, it hasn't reached its natural lifespan.

Thanks to all for your help! Sorry that I could not follow your replies more vigilantly. I actually had time to do what Dan suggested. Then covered the weeded zone at the tree's base with its own shedded leaves. ...Then luckily a major shower came along. ...Then the yellowing stopped almost instantly! We love this tree so much; starts to shade the house from the S-W sun, and is a winter haven for redpolls! Many thanks again!