Those trees are actually tamarisk, an invasive and non-native plant. Riparian lands in the Western U.S. have been severely impacted by many human-related actions, but none so much as the introduction of tamarisk. The Colorado River Corridor is an ecosystem that has been out of balance for several decades, and now the tamarisk has spread to such an extent that it has effectively altered the natural functions and processes of the ecosystem.
In an effort to control the problem, the harmless tamarisk or saltcedar leaf beetle (Diorhabda elongata) has been released along the Colorado Riverway. Beetles are an effective biological control used to reduce or eradicate tamarisk. The beetles are a natural control agent from areas in the world where tamarisk originated. After much study, the beetles were brought to the U.S. in an attempt to stem the tide of the tamarisk invasion.
For more information on the tamarisk and beetles, click here.