The Colorado River pours thick tan sediments around Montague Island and into the Sea of Cortes. The sediments gradually take on a greenish appearance as they diffuse southward.
North of the rivermouth is the Gran Desierto de Altar, part of the Sonoran Desert. The desert’s vast sand dune field appears mostly tan here, with the exception of a dark brown circular area in the upper right quadrant.
This part of the desert is the location of the Pinacate Peaks, a group of volcanic peaks and cinder cones. The tallest of the peaks is Cerro del Pinacate (also called Volcan Santa Clara), elevation 3,904 feet (1,190 m).