It appears that your river birch is infected with the spiny witchhazel gall aphid (Hamamelistes spinosus). During its life cycle the aphid alternates between two hosts: witch hazel (Hamamelis spp.) and birch (Betula spp.), and among the birches it has a particular affinity for river birch (Betula nigra), hence the aphid’s common name, "river birch aphid.” The aphid produces raised ribs or "corrugations" on the upper leaf surface that match deep furrows between the veins on the lower leaf surface where they live, and they cover themselves in a waxy, white, material, which you have captured in your photograph. Affected leaves usually turn yellow and may prematurely fall off of the tree. In most cases the aphid does not require treatment. Here is some additional information about this insect and other pests and diseases that affect birches.

http://media.clemson.edu/public/esps/pdfs/to17.pdf

http://www.clemson.edu/extension/publications/entomology/turf-ornamental/river-birch-aphid-to17.html

https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG58_IPM_Birch.pdf