Kadua fluviatilis is a scandent shrub that emits a strong odor when bruised. It features cylindrical stems that are slightly flattened, ranging from 3 to 25 decimeters in length, with short lateral branches, all of which are glabrous. The leaves are simple and oppositely arranged, typically spaced apart. The leaf blades are variable in shape, from elliptic-oblanceolate to elliptic-lanceolate, sometimes broadly elliptic-oblanceolate, broadly elliptic, or ovate. These leaves measure (3.8–) 8–17(–23) cm in length and (1.6–)3–5(–6) cm in width. The apex of each leaf is acuminate to caudate and somewhat falcate, with a cuneate base. The leaves are glabrous and firm-chartaceous with entire margins. The lateral veins are raised on the lower surface, and the higher-order venation is conspicuously reticulate. Petioles range from (0.5–)1–3(–4) cm in length, and stipules are broadly deltate, measuring 5–6 mm long and adnate to the petioles. The flowers occur singly or up to five in reduced, axillary, cymose inflorescences. The peduncles are usually 5–8 mm long and closely adnate to the stem, while the pedicels are quadrangular and typically 30–55 mm long. The hypanthium is quadrangular-turbinate, approximately 3-4 mm long. The flowers are bisexual and pistillate, with calyx lobes that are deltate to narrowly ovate, ranging from (2–)12–18 mm long and 3–6 mm wide, and somewhat enlarging in fruit. The corolla is white, fleshy, and waxy, salverform in shape, with a tube (13–)22–30 mm long. The lobes of the corolla are 8–20 mm long and spread at anthesis, each tipped with a slender appendage. It is insect-pollinated, with stamens that alternate with the corolla lobes and anthers that open by longitudinal slits. The ovary is inferior or partly so, with few to numerous ovules on fleshy placentas. The fruit is a woody capsule, broadly turbinate to turbinate-globose and strongly quadrangular or winged, measuring 8–13 mm long and 9–13 mm in diameter. The seeds are translucent reddish-brown, cuneiform, sharply angled, and minutely reticulate. It inhabits mesic to wet forests between 60 – 1220 meters.