In Arizona, floodwater has washed away a portion of a fence along the Mexican border, affirming predictions made by the barrier’s environmental critics. Built between 2007 and 2008, the fence at the Organ Pipe Cactus National Monument has been criticized for dividing wildlife populations and habitats and diminishing the range for endangered animals by as much 75 percent. Experts predicted the barrier, which forms an unnatural dam during rainfall, could present a risk to its own structural integrity. On Sunday, it took two-and-a-half inches of rain to wash away 40 feet of the fence.