The Colorado River — which supplies about two-thirds of San Diego County’s drinkable water — on Tuesday was named the most endangered river in the U.S. by a leading conservation group. American Rivers’ annual report, published since 1984, ranks the 10 most threatened rivers nationwide. The group said it tries to spotlight rivers that are subject to influential policy decisions, not necessarily the most polluted. This year, it chose the lower portion of the Colorado River for greatest attention based on ongoing concerns about dwindling flows due to increasing water consumption and adverse impacts from global warming.