The lower Colorado is the most-threatened river in America, a conservation advocacy group said in its annual report published Tuesday. The nonprofit American Rivers had placed the entire Colorado River and upper river atop its list of “most-endangered rivers” in previous years. But this is the first time the lower Colorado, which supplies Las Vegas with 90 percent of its water via Lake Mead, has been designated as in danger. “The main criteria we use is whether there’s a key decision point in the year,” said Amy Kober, a spokeswoman for the group. In the case of the lower Colorado,