Betula occidentalis, the water birch or red birch, is a species of birch native to western North America, in Canada from Yukon east to Northwestern Ontario and southwards, and in the United States from eastern Washington east to western North Dakota,[citation needed] and south to eastern California, northern Arizona and northern New Mexico, and southwestern Alaska.

It typically occurs along streams in mountainous regions,[1] sometimes at elevations of 2,100 metres (6,900 feet) and in drier areas than paper birch.

[2] It is a deciduous shrub or small tree growing to 14 m (46 ft) high, up to 25 centimetres (10 inches) thick.

The fruit is 2–3 cm (3⁄4–1+1⁄4 in) long and 8–15 millimetres (1⁄4–1⁄2 in) broad, composed of numerous tiny winged seeds packed between the catkin bracts.

[1] A 2023 study sequenced chloroplast genomes of species from the genus Betula for phylogenetic analysis.