DENVER—As ominous storm clouds gathered in western Colorado over a large area blackened by a recent wildfire, torrential rain fell, and the charred land stripped of vegetation gave way, sending a rush of mud and boulders tumbling down steep canyon walls and onto a major highway. The July 29 mudslides stranded more than 100 people in their vehicles overnight and caused extensive damage that closed Interstate 70, capping several weeks of perilous conditions in a scenic canyon carved through the mountains by the Colorado River. It marked the latest in a string of closures over the past two years for an area that serves as a key transportation corridor between the Rocky Mountains and the West Coast. Each forced long detours for semitrailers that deliver fuel and food, and inflicted economic pain on businesses that cater to tourists in the popular summer destination of Glenwood Springs. Though no injuries were …