Tamarisk, a shrub or low tree that was artificially introduced into the American Southwest in the late 1800s, has spread throughout the Colorado Plateau region by occupying islands, sand bars, and beaches along streams. Historical photographs show that tamarisk spread from northern Arizona to the upper reaches of the Colorado and Green Rivers at a rate of about 20 km/yr. Detailed studies on the Green River in Canyonlands National Park, Utah, show that the plant has trapped and stabilized sediment, causing an average reduction in channel width of 27%. Photogrammetric analysis of historical ground photography, including photos from John Wesley Powell's 1871 expedition, and recent aerial photographs supplemented by field surveys provided quantitative data. Expanded islands and channel-side bars exhibit allometric relationships as they change, apparently maintaining a balance between turbulence and friction. Overbank flooding is common on the tamarisk-stabilized features.