River birch is a medium-sized tree with an irregular, broad, spreading crown. It often has multiple trunks.
Leaves are alternate, simple, somewhat triangular to egg-shaped, 1½–3½ inches long, 1–2 inches wide; margins densely toothed; upper surface shiny dark green, lower surface whitish, very hairy.
Bark is distinctive: reddish-brown or grayish, peeling into conspicuous papery strips exposing a light pinkish-tan or cinnamon-brown inner bark; pores elongated.
Twigs on new growth are very hairy, gradually turning smooth; in the second year, dark red, slender, shiny, dull reddish-brown.
Flowering is in April–May, male catkins formed the year before, becoming 1–3½ inches long; female catkins formed from buds of the previous year, about ½ inch long, upright; flowering as the leaves appear; wind-pollinated.
Fruit mature in May–June, barrel-shaped, 1½ inches long, ½ inch wide, the nutlet about 1/8 inch long.