Plants: large, occasionally medium-sized, in loose to moderately dense mats, light to dark green, yellow-green, or brownish yellow. Stems: to 10(–15) cm, creeping or ascending, terete-foliate, irregularly to regularly pinnate, branches to 20 mm, straight to slightly curved, terete-foliate. Stem: leaves erect-appressed, often rigidly spreading in sympodial stems, closely imbricate, broadly ovate, broadest at 1/7 leaf length, concave, not plicate, slightly, or rarely moderately plicate, 1.5–2.5 × 0.8–1.5 mm; base rounded, broadly long-decurrent; margins plane or recurved at places, serrulate throughout (often serrate in sympodial stems); apex gradually tapered, broadly acute, rarely short-acuminate; costa to 50–70% leaf length, strong, terminal spine indistinct or absent; alar cells short-rectangular, strongly enlarged, 20–50 × 15–30(–35) µm, walls thin, region abruptly delimited, large, of 8–15 × 8–15 cells, somewhat saclike, reaching from near margin 50% distance to costa; laminal cells linear, 50–100 (–150) × 6–11 µm; basal cells 15–35 × 11–15 µm, region in 3 rows. Branch: leaves more spreading, ovate-lanceolate, 2–3:1; costal terminal spine present; alar region conspicuous or not; laminal cells enlarged across base. Sexual: condition dioicous. Seta: red-brown, 2–2.5 cm, rough. Capsule: inclined to horizontal, red-brown or dark brown, ovate to elongate, curved, 2–2.5 mm; annulus separating by fragments; operculum long-conic. Spores: 13–18(–22) µm.
Soil in wet places, wet semiliquid peat in alder and sedge swamps, alluvial sand banks along streams, temporary flooding depressions in xeric areas, wet mountain tundra, rock, logs, affected by temporary flooding. low to high elevations (0-3400 m). Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., Que., Sask., Yukon, Ala., Alaska, Ariz., Ark., Colo., Conn., Del., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., Europe, Asia, Africa, Atlantic Islands, Pacific Islands, Australia, Subantarctic Islands.
Brachythecium rivulare occurs in most states and provinces but is absent in the southeastern United States. Although B. rivulare was reported from California, no collection was confirmed (many specimens proved to be atypical phenotypes of B. frigidum). The plants form whitish green soft mats near spring water but become rigid and olive brownish when affected by strong currents. In all cases, B. rivulare is somewhat dendroid and distinguished by the peculiar alar group that is somewhat saclike, composed of strongly enlarged thin-walled cells reaching to the margins; in B. frigidum and B. rutabulum, that may consist of a fairly large pellucid group in leaf corners, but the region never reaches the margins where cells are relatively narrow and rectangular in several rows and often rich in chloroplasts. This character is sometimes difficult to find in phenotypes that have extensive sympodial branching and are also regularly pinnate.