Griffiths, Proceedings of the Biological Society of Washington 27(6): 26, 1914
Opuntia riparia is a prickly pear cactus that grows in low, wide thickets in southern AZ. It rarely reaches waist high, but occasional cactus stems may reach to 1.2 m tall. Pads are often elongate, averaging about 20 cm long. The dark areoles are noticeably large and white spines are present in most of them. There are 2 to 6(8) spines per areole with one or two small ones that are deflexed. Spines on this Opuntia are an 1 to 2.5 cm or more in length, stout, thick, and angular in cross section. Ovaries and fruits are somewhat to distinctly elongate, with ovaries usually turbinate and fruits usually ovoid (sometimes tapering/narrowed toward the base). The seeds are not large but are larger than those of O. engelmannii, a large prickly pear. Flowers are a rich bright yellow. O. angustata and O. riparia are similar and may in fact represent the same taxon. O. riparia occurs further south than O. angustata (near Tucson, AZ and below). Read the original description of O. riparia.