Plants annual or perennial; rhizomes 1 mm diam. Culms often arching to decumbent, cylindric, ridged when dry, 0.09-0.65 m × 0.5-1.5 mm. Leaves 3-4, 1(-2) cauline; sheath fronts not pinnate-fibrillose; blades 1-2, proximally thickly C-shaped in cross section, distally flat, rudimentary to longer than sheath, 1-200 × 0.2-1 mm, smooth or margins distally spinulose. Inflorescences capitate or of 1 spikelet or with 1 or 2 branches to 15 mm; proximal bract erect, like leaf blades, 5-15 cm. Spikelets 1-20; 6-20 × 3 mm; scales pale orange-brown to colorless, midrib region sometimes greenish, ovate, 2.2-3(-3.5) × 1.5 mm, awn sparsely spinulose, margins ciliolate, midrib keeled distally, apex entire, rounded, awn 0.2-0.5 mm or to 1 mm on proximal scale. Flowers: perianth absent; anthers 0.3-0.5 mm; styles 3-fid. Spikelet achenes blackish brown, nearly equilaterally sharply trigonous, ovoid to obovoid, 1.3-1.8 × 1-1.4 mm, with 10-20 mostly sharp ridges; beak 0.1 mm. Amphicarpic achenes trigonous, 2.2-3 mm including stout, tapered beak 0.3 mm, with many obscure to evident horizontal ridges. 2n = 50.
Fruiting late summer-fall, spring-fall (California, Texas). Damp soils to emergent, freshwater ponds, ditches, often drying, disturbed, and sandy areas; 0-2200 m; B.C.; Calif., Colo., Ill., Kans., Mo., Nebr., Ohio, Okla., S.Dak., Tex., Utah, Wyo.; Mexico (San Luis Potosí, Tamaulipas).
The distribution of Schoenoplectus saximontanus is very scattered (local).