Plants large or very large, green, brownish, or yellowish. Stems regularly radially pinnate; axillary hair apical cell elongate-rectangular to short-linear. Stem leaves rounded ovate, broadly rounded ovate, or rounded ovate-cordate; costa single, 74- 105 µm wide near base, ending shortly before apex; alar region sharply delimited, oblong or sometimes triangular, from margins 35-60(-65)% distance to costa; medial laminal cells 84-195 × (7-)8-13µm. Sexual condition dioicous. Spores 13-21 µm.

Submerged in somewhat nutrient-rich lakes, ox-bow lakes, small bodies of water, floating or among reeds or sedges along shores; low to high elevations; N.W.T., Yukon; Alaska; n Eurasia.

Calliergon megalophyllum is probably overlooked in many areas since it usually grows submerged. The species is sometimes confused with large phenotypes of the autoicous C. richardsonii, but normally has a costa that is single rather than branched or 2-fid and reaches almost to the leaf apex instead of ending well before. In addition, the alar groups are slightly smaller and the axillary hairs generally have shorter cells, at least in their distal part, than in C. richardsonii.