DENVER, Colo.—Mudslides from heavy rains caused “extreme damage” to a major interstate and left it blocked with piles of boulders and logs, Colorado transportation officials said Sunday, as forecasters warned of more flash floods in the coming days across the Rocky Mountain and Great Basin regions. The flood risk was elevated for many areas of the West where recent wildfires burned away vegetation and left hillsides more susceptible to erosion, the National Weather Service said. Interstate 70 in Glenwood Canyon, Colorado, was closed with no word on when it might reopen after being pounded by flash floods over a three-day period. Lanes in both directions remained blocked by debris that flowed out of the burn scar from a wildfire last year in the Grizzly Creek area. The torrent of rocks that came tumbling down the canyon’s steep walls smashed apart sections of the concrete roadway and sheared off a long …