Some of the leaves in my river birch are turning yellow and dropping to the ground. Why?

FAQ
Question: 

Some of the leaves in my river birch are turning yellow and dropping to the ground. Why?

Answer: 

Hot, dry weather may be responsible for the leaf drop on the river birch. River birches shed some of their leaves in hot, dry weather.  Leaves in the interior of the river birch turn yellow and drop to the ground.  Healthy, well-established trees should not be seriously harmed by hot, dry weather.  The hot, dry weather poses a more serious risk to trees in poor health and those planted in the last three to five years.  Recently planted trees should be watered approximately every seven to ten days during dry weather.