Griffiths, Proceedings of the Biological Society of Washington 27(6): 26, 1914
Opuntia riparia grows in low, wide thickets in southern AZ that are rarely more than waste high, but occasional stems may reach 5-ft tall. Pads are often elongate, averaging about 8-inches 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. Many spines are an inch 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. 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 (near Tucson, AZ and below). Read the original description of O. riparia.