This adventive perennial plant is about ½2½' tall, branching occasionally. The light green stems are hairless and more or less ribbed; they are more or less erect, but occasionally sprawl. The alternate compound leaves are oddly pinnate (simple or double), often with a ternary arrangement of leaflets (usually in groups of 3, but sometimes in groups of 5 or more). In outline, a compound leaf is triangular, broader at the base than at its tip, its size up to 12" long and 8" across. Each leaflet is ovate, ovate-lanceolate, or ovate-oblanceolate, and about 1" long and ½" across. Usually the terminal leaflet has a short petiole, while the lateral leaflets are sessile. The margin of each leaflet is coarsely dentate, sometimes with 1 or 2 lobes that are cleft. Its base can be wedge-shaped or rounded and asymmetric. The petioles of the compound leaves are rather long and stout; there is a membranous sheath where the base of each petiole joins the stem. The foliage of Pacific Water Parsley is hairless. Occasionally, some of the upper stems will bolt and produce compound umbels of small white flowers; these umbels are flat-headed at the top. Each compound umbel is about 3" across when fully developed, and consists of about 8-12 umbellets. Each umbellet has about 8-12 small white flowers that are individually about 1/8" across. There are about 8-12 bractlets underneath each umbellet; they green, linear, and spread outward. At the base of each compound umbel, there are 2 or more bracts that have an appearance that is similar to the bractlets, except they are larger in size. Each flower has 5 notched white petals, 5 white stamens, and a central pistil. The blooming period (in Illinois) occurs from mid-summer to early fall, and lasts about 1-2 months. The compound umbels of flowers are produced sparingly during this time period. Each flower is replaced by a single elongated seed. The root system consists of fibrous roots and slender white runners that lie slightly below the ground surface. This plant often forms colonies.