The Colorado River’s largest reservoirs stand nearly three-quarters empty, and federal officials now say there is a real danger the reservoirs could drop so low that water would no longer flow past Hoover Dam in two years. That dire scenario — which would cut off water supplies to California, Arizona and Mexico — has taken center stage at the annual Colorado River conference in Las Vegas this week, where officials from seven states, water agencies, tribes and the federal government are negotiating over how to decrease usage on a scale never seen before.