In Utah, the Colorado flows primarily through the slickrock country, which is characterized by its narrow canyons and unique folds created by the tilting of sedimentary rock layers along faults. This is one of the most inaccessible regions of the continental United States. Below the confluence with the Green River, its largest tributary, in Canyonlands National Park, the Colorado enters Cataract Canyon, named for its dangerous rapids, and then Glen Canyon, known for its arches and erosion-sculpted Navajo sandstone formations. Here, the San Juan River, carrying runoff from the southern slope of Colorado's San Juan Mountains, joins the Colorado from the east. The Colorado then enters northern Arizona, where since the 1960s Glen Canyon Dam near Page has flooded the Glen Canyon reach of the river, forming Lake Powell for water supply and hydroelectricity generation.