HUMAN BEINGS HAVE A KNACK for starting things. But not for stopping them.

That truth is at the center of a story that’s entering its endgame along Arizona’s waterways. The short version: A century and a half ago, a type of exotic shrub arrived in North America. It quickly spread, populating riparian areas where plants native to this land once thrived. Twenty years ago, an insect that devours the leaves of that shrub was brought to the American Southwest in an attempt to restore order. But now, that transplant is spreading fast — and, as a result, more of our region’s ecosystem is at risk.

Below the surface, though, the story of the tamarisk and the tamarisk leaf beetle is one of good intentions and unforeseen consequences. Of events set in motion long ago and landscapes changed forever. And of what can happen when we take away Mother Nature’s keys and put ourselves behind the wheel.

“It touches on so many issues beyond rivers, plants and species,” says Greg Beatty, a Phoenix-...