Alisma: Answers to key questions leading to this genus. 

Emergent leaves linear-lanceolate or elliptic to ovate; inflorescences panicles with whorls of branches; pistils and achenes arranged in a single ring around margin of flat receptacles, usually fewer than 20; flowers perfect; stamens few, 6-9; NOT [Emergent leaves mostly sagittate or hastate; inflorescences usually racemes with whorls of flowers, rarely panicles; pistils and achenes spirally arranged on convex receptacles, very numerous; flowers imperfect; stamens 7-30]

Triviale: Answers to key questions leading to this species (in genus Alisma). 

Leaves usually emergent, occasionally submersed or floating; blades of emergent leaves to 17 (35) cm long, and to 12 cm wide, narrowly-elliptic to ovate; outer (abaxial) edge of achenes usually 1-grooved, sometimes 2-grooved with rounded ridge between them; NOT [Leaves submersed, floating or emergent; blades of emergent leaves (2) 4-6 cm long, to 1.5 cm wide, linear-lanceolate to narrowly elliptic; outer (abaxial) edge of achenes with a slight ridge between 2 shallow grooves]

Fruiting heads 4.1-7 mm wide; achenes 2.1-3 mm long (ours usually ca. 3 mm); petals 3.5-6 mm long; NOT [Fruiting heads 2-4 mm wide; achenes 1.5-2.2 mm long; petals 1-3 mm long]