Water hickory is a medium to large tree with a long, clear trunk, buttressed base, and narrow, irregular crown.
Leaves are alternate, feather-compound, 9–15 inches long, with 7–15 leaflets; leaflets lance- to egg-shaped, curved, 2–5 inches long, with long-pointed tip; margins finely to inconspicuously toothed. Upper surface dark, smooth; lower surface hairy, especially along veins and stalk. Yellow in fall.
Bark is gray-brown, often red-tinged, splitting into long, loose, platelike shaggy scales, brittle.
Twigs are slender, red-brown or gray, hairy, end bud slightly flattened, covered with tiny yellow scales.
Flowers April–May; male and female flowers separate on same tree; male catkins hairy, solitary or in threes; female flowers 2–6 per spike.
Fruits September–October, pear- to egg-shaped nuts, often clustered, 1–1½ inches long, 1 inch wide, noticeably flattened compared to other hickory nuts, with a thin, 4-winged husk having bright yellow scales, splitting at the base. Nuts flat, reddish, bitter.
Similar species: Pecan is similar but has a broader native range in Missouri than water hickory, and although it appreciates moist bottomland soils, pecan is not a swamp tree. Its nuts are sweet, larger, cylindrical (not flattened as in water hickory), with irregular black markings on the shell. Its bark isn't shaggy like water hickory. Water hickory and pecan can hybridize and produce offspring with intermediate characteristics.