HESPERIA, Calif. — Federal engineers have found that a dam protecting the high desert communities of Victorville, Hesperia, Apple Valley and Barstow falls short of national safety standards and could erode and collapse in an extreme flood, inundating thousands of people. Officials for the U.S. Army Corps of Engineers said Tuesday that they had raised the risk factor for the Mojave River Dam from “low” to “high urgency action” because of “performance concerns” discovered at the 48-year-old structure, which joins a growing inventory of California dams showing signs of severe stress.