The West could be facing a water shortage in the Colorado River that threatens a century-old agreement between states that share the dwindling resource.

That possibility once felt far off, but could come earlier than expected. One prominent water and climate scientist is sounding the alarm that the Colorado River system could reach that crossroads in the next five years, possibly triggering an unpredictable chain-reaction of legal wrangling that could lead to some water users being cut off from the river.