A tree 30 to 100 ft high, forming a thin, spreading, round-topped head of branches, pendulous at the ends; young branches at first covered with pale hairs which soon fall away. Leaves ovate, oval, or rounded, ordinarily 2 to 4 in. long, about two-thirds as wide; usually pointed (sometimes rounded) at the apex, tapered at the base, unevenly or doubly toothed; dark shining green (but at first very hairy) above; paler, yellowish, and permanently downy beneath. Male catkins two to seven in a cluster, opening on the naked shoots early in spring, each catkin 3 to 5 in. long; stamens two, rarely three. Fruits 1⁄3 to 3⁄4 in. long, three to seven together.
Native of western N. America. The leaves occasionally approach the diamond shape indicated by the name, and on vigorous shoots are up to 5 in. long. According to Jepson, this alder keeps to streams which do not run dry, forming files of trees in mountain gorges which are 'to the traveller a reliable sign of water'. It is very rare in cultivation, the plant supplied for it in this country and on the continent being, as a rule, A. rubra.