Lythrum salicaria L.
Purple Loosestrife is a perennial, grows to 8' that prefers damp soil. Leaves are simple, lance-shaped, have smooth margins and are opposite or whorled.  Stems branch, ending in flower stalks.  Flowers are rose-purple in color with long vertical recemes having 5-7 petals and numerous stamens.  Roots are large and adapted to aquatic sites.  Can grow dense enough to impede water flow in canals or ditches
Photos and information adapted from "Weeds of the West" by Tom Whitson