DENVER — The drought-stressed Colorado River carried even less water than expected this summer, increasing the odds of a shortage in the vital river system in 2020, federal water managers said Friday. The U.S. Bureau of Reclamation said the chances of a shortfall in Lake Mead, the river’s biggest reservoir, are now 57 percent, up from the 52 percent projected in May. The river and its tributaries serve 40 million people and 6,300 square miles of farmland in Mexico and the U.S.