Bloomberg: The Colorado River may shrink to its lowest level in at least 500 years because of global warming, threatening water supplies to California and six other states, researchers at the U.S. Geological Survey said. A “modest” 0.86 degree Celsius (1.5 degree Fahrenheit) increase in this century could trim the average flow of the river — the primary water supply for residents in much of the U.S. Southwest — to the low end of a range marked between 1490 and 1998, USGS scientist Gregory …