River Birch leaf problem
I had a 10-12' River Birch planted 2 years ago. I noticed in spring of 2015 a few of the leaves looked bad. This spring the condition is much worse. The affected leaves have bumps over the entire leaf and maybe a little fungus going on. Yet, 50% of the leaves are perfectly normal. It is planted in full sun and soli is good (although I had a Tri-color Beech at this same location that refused to grow roots and had to be replaced).
This looks like Spiny Witch-hazel Gall Aphid, a common pest of birch. Beneficial insects and predators usually give sufficient control so no pesticides are recommended. See our publication for more information http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG58_IPM_Birch.pdf