Marble Canyon is the section of the Colorado River canyon in northern Arizona that extends from Lee's Ferry to the Little Colorado River, often regarded as the start of the Grand Canyon. A major river-running route, it also marks the western boundary of the Navajo Nation and is crossed by Navajo Bridge on U.S. Highway 89A. Though its name suggests marble, John Wesley Powell chose it for limestone polished to a marble-like appearance. The canyon is geologically distinctive, shelters rare species including Pediocactus bradyi and the California condor, and became a focus of opposition to a proposed dam project that was abandoned in 1968.