Perennial, densely tufted with a short rhizome. Culms 40–80 cm tall, 1–2 mm in diam., internodes and nodes glabrous. Leaf sheaths smooth, glabrous, shorter or longer than intermodes, margins membranous, sometimes ciliate, especially at mouth; leaf blades linear, stiff, often involute, 5–15(–20) cm × 4–6 mm, glabrous on both surfaces; ligule 0.4–0.7 mm. Panicle narrow, 4–15 cm; central axis scabrid-hispidulous; branches 2–6 cm, erect or narrowly ascending; usually solitary; pedicels 1–3 mm, scabrid. Spikelets 3.5–4.5 mm, purplish; glumes smooth or almost so along veins; lower glume 2.6–3.5 mm, 5-veined; upper glume 5-veined; lower floret staminate; upper floret 2.6–3.4 mm, lemma shortly awned; awn straight, 0.3–1.5 mm; callus hairs usually ca. 1/2 length of lemma. Fl. and fr. Jul–Nov.

This is a rather small, slender variant from the Arundinella hirta complex, typically lacking conspicuous, stout, scaly rhizomes and apparently confined to wet places by streams. Arundinella hirta var. ripar-ia (Honda) T. Koyama, from Japan, is very similar and also occurs on rocky stream banks, but the upper floret has a longer awn to 3.75 mm.

Arundinella hirta (Thunberg) Tanaka var. depauperata (Rendle) Keng (Claves Gen. Sp. Gram. Prim. Sin. 230. 1957; A. anomala Steudel var. depauperata Rendle, J. Linn. Soc., Bot. 36: 341. 1904), described from Hubei, is another slender variant from rocky streamsides that will key out to A. fluviatilis. It differs from that species by its stoloniferous habit, leafy vegetative shoots with concealed nodes, and short callus hairs only 1/4 as long as the fertile floret. Occasionally specimens of typical, more robust, broad-leaved A. hirta also occur with short callus hairs. Possibly none of these slender variants merits separate specific status, but they do present a recognizable habit and occupy a particular ecological niche.

* Watersides, land flooded during summer, rock fissures, between stones; 200–500 m. Guizhou, Hubei, Hunan, Jiangxi, Sichuan.