Also, it's a clump tree with 4 main trunks and is planted in full sun.

I did some more research and think my problem may be Chlorosis. Although I don't know why I'd have this problem this year and not the past 2 years. The only thing different between this year and the previous years is we put Preen down around the tree this year when we mulched the bed. Could this have caused the problem?

If it is Chlorosis, how do I fix it?

Preen says it's safe around birches, according to their website. That is, if you used Preen Weed Preventer, which comes as granules you shake out on the ground. If you used their weed killer, which is a liquid spray, then it might have had an effect.

Chlorosis can be caused by overwatering as well as alkalinity or iron deficiencies. Are you sure you're not overwatering it?

We used the Preen granules, so I guess we're safe there.

Definitely not overwatering. If anything I was afraid I was underwatering considering how dry it's been.

I guess I should take a soil sample to get tested. I just don't want to lose the tree.

What you have is what everyone has going on with their river birches this time of year. They're not planted in the right type of place, they are in the landscape not in the river bottom so the trees are dropping leaves to cope with the heat and drought.

quirkyquercus - you are right. I took some soil and branch clipping with yellow leaves to local nursery last night. The very knowledgeable guy there (man if only you could hook a monitor and printer up to his brain!) said I need to water it more due to how dry it is this year. He said they're usually pretty disease tolerant (which I had learned from This Old House and is the reason I choose this variety). I bought a soaker hose this morning that I'm going to coil around the tree and bury in the mulch so I can do real deep root waterings. Thanks for the help.

Mine do the same thing -- it's dry soil. They're really trees for moist soil. I notice it's the first spring leaves that fall. Are the newer, larger leaves at the tips doing just fine? If so, I wouldn't worry at all.

Just run a hose to the tree's base, reduce the flow to a trickle and let it run for 6 hours every few weeks unless you get rain.

Heritage is a tough birch and it can take heat well.

I sure wish it would rain. We're so far behind in rain, and following a dry winter and dry growing seaosn last year, I expect some arboral casualties here in MN.

I am fortunate to have stands of river birch growing in the area as it's one of my favorite trees. One of them popped up behind my fence so now it belongs to me. They seem to do great when they choose their own location. The 'little king' I planted in my yard dropped about 30% of it's leaves a couple weeks ago. It looked like fall then abruptly stopped as if it knew to drop precisely a certain amount of leaves all at once. Now it looks like normal again. And it was getting water.