Tamarisk (Tamarix spp.), an introduced shrub or small tree, has invaded riparian areas throughout the western United States. Tamarisk invasion has been studied extensively in the Southwest, but there is little information on its performance at the northern margin of its naturalized range. We measured the canopy cover, density, height and age of tamarisk and plains cottonwood (Populus deltoides) in 50 plots at 25 sites along the Bighorn, Powder, and Yellowstone rivers in southeast Montana near the northern edge of tamarisk's western North American range. Tamarisk commonly formed thickets on open, low terraces and along overflow channels but was less dense beneath a cottonwood canopy. Tamarisk stems routinely died back to the ground, and the oldest live stems were generally much younger than the plants. Tamarisk 30 to 40 years old in our study area usually attained heights of only 4 m or less. Height and number of live stems of tamarisk plants were 16% and 44% lower respectively under a tall cottonwood canopy. Cottonwood grows faster than tamarisk, eventually shading it and causing its decline. We believe that tamarisk will be only an understory shrub in most eastern Montana riparian forests, declining as cottonwoods form a closed canopy. Minimizing the spread of tamarisk in riparian areas in Montana can best be accomplished by managing for cottonwood.