Hi - Our commercial horticulture office has received a lot of inquiries asking why river birches have yellowing/dropping leaves at this time. You're not alone! The spring season was wet and cool and ideal for flushing out a lot of growth on birch trees. When the temperatures started soaring higher, the birches responded by dropping a lot of excess leaves. The trees will survive; just keep them watered if we get an extended period of dry weather. It is not uncommon for birches to show this symptom at this time of year.
There are other issues that may cause leaf yellowing in birches. For more information on what to watch for, see the following publication.