Plants perennial, glabrous or indistinctly papillose-pubescent especially on leaf blade veins abaxially, with creeping or fusiform rhizomes. Stems erect, branched above middle (only in inflorescence), 60-100 cm. Leaves: ocrea deciduous or partially persistent at maturity; blade with lateral veins ± equal in size, oblong or oblong-lanceolate, 20-40(-50) × 8-15(-18) cm, more than 3 times as long as wide, base broadly cuneate, obtuse, or weakly cordate, margins entire, flat, apex acute, subacute, or acuminate. Inflorescences terminal, occupying distal 1/ 2 of stem, often dense, narrowly paniculate. Pedicels articulated in proximal 1/ 2, filiform, (5-)12-15(-17) mm, articulation indistinct, scarcely visible. Flowers 10-20 in whorls; inner tepals ovate-deltoid, 4.5-7 × 3.5-7 mm, widest in proximal 3, base truncate or weakly emarginate, margins erose to weakly serrate or indistinctly denticulate in basal part, apex acute to acuminate; tubercles absent. Achenes brown, 2.5-4 × 1.5-2.5 mm. 2n = 120. Flowering late spring-summer. Along streams; 2500 m; Ariz.; N.Mex., Mexico (Sonora). Rumex orthoneurus has been reported from northern Mexico (M. Fishbein 1993). The species is in the Center for Plant Conservation's National Collection of Endangered Plants.