Tamarix disrupts the structure and stability of North American native plant communities and degrades native wildlife habitat, by outcompeting and replacing native plant species, salinizing soils, monopolizing limited sources of moisture, and increasing the frequency, intensity and effect of fires and floods. wikipedia 
While it has been shown that individual plants may not consume larger quantities of water than native species, it has also been shown that large dense stands of tamarisk do consume more water than equivalent stands of native cottonwoods.
There is an active and ongoing debate as to when the tamarisk can out-compete native plants, and if it is actively displacing native plants or it just taking advantage of disturbance by removal of natives by humans and changes in flood regimes.
Research on competition between tamarisk seedlings and co-occurring native trees has found that the seedlings are not competitive over a range of environments, however stands of mature trees effectively prevent native species establishment in the understory, due to low light, elevated salinity, and possibly changes to the soil biota. Thus, anthropogenic activities that preferentially favor tamarisk (such as changes to flooding regimes) are associated with infestation.
Tamarix has taken over large sections of riparian ecosystems in the western United States that were once home to native cottonwoods and willows, and are projected by some to spread well beyond the current range.