Sp. Pl. 2: 969. 1753.
Leaves connate at base, ± linear, tapering strongly from near base or spatulate, 2.5–9 × 0.3–5 mm, 1+-veined. Stem and leaf scales present. Inflorescences: bracts caducous, sometimes persistent. Pedicels 0–0.2 mm in fruit. Flowers usually 1 staminate and 1 pistillate in 1 axil opposed by 1 pistillate; styles erect or spreading; pollen yellow. Schizocarps obovoid, usually widest distal to middle, 0.8–1.4 × 0.6–1.4 mm, longer than wide, sometimes as long as wide; mericarps black, not swollen, not winged, winged only at apex, or winged throughout but widest at apex, wings straight, 0.03–0.1 mm wide. 2n = 20.
Phenology: Flowering May–Oct.
Habitat: Fast-flowing rivers and streams, backwaters, ditches, swamps, Sphagnum bogs, lakes, ponds, springs, seepages.
Elevation: 0–3600 m.
Greenland, St. Pierre and Miquelon, Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., Nunavut, Ont., P.E.I., Que., Sask., Yukon, Alaska, Ariz., Calif., Colo., Conn., Del., Idaho, Ill., Iowa, Maine, Mass., Mich., Minn., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.Dak., Oreg., Pa., R.I., S.Dak., Tex., Utah, Vt., Wash., Wis., Wyo., Mexico (Chihuahua, San Luis Potosí), Eurasia, introduced in Australia.
Callitriche palustris has been included on the Composite List of Weeds maintained by the Weed Science Society of America (http://wssa.net/wssa/weed/composite-list-of-weeds/); its inclusion seems unwarranted.