Plants perennial, glabrous or indistinctly papillose-pubescent, with fusiform or creeping horizontal rhizome. Stems erect, branched above middle (only in inflorescence), 60-100 cm. Leaves: ocreae deciduous or partially persistent at maturity; blade normally oblong-lanceolate, 20-45 × 8-10 cm, normally more than 3 times as long as wide, base broadly cuneate, truncate, or weakly cordate, margins entire, flat or indistinctly crisped, apex obtuse or broadly acute. Inflorescences terminal, occupying distal 1/ 2 of stem, often dense, narrowly paniculate. Pedicels articulated in proximal 1/ 2, occasionally almost near base, filiform, 3-11 mm, articulation indistinct or weakly evident. Flowers 10-20 in whorls; inner tepals ovate-deltoid or ovate-triangular, 4-7 × 3-5 mm, widest at or near base, base truncate or weakly emarginate, margins erose to minutely dentate at least near base, apex narrowly acute; tubercles absent. Achenes deep brown to reddish brown, 3-4 × 1.5-2.2 mm. 2n = 120.

Flowering late spring-summer. Along streams and rivers in montane, subalpine, and alpine zones; 1700-3000 m; Colo., Utah.

Rumex pycnanthus was reported (as R. subalpinus M. E. Jones) from White Pine County, Nevada (Mont E. Lewis 1973) and may occur in that state; according to J. T. Kartesz (1987, vol. 1), that record was based on misidentification of R. californicus.