The Colorado Desert is a part of the larger Sonoran Desert in the southeastern part of the U.S. state of California and the northeastern portion of the Mexican state of Baja California. It encompasses approximately 2,500 sq mi (40,000 km²) east of Los Angeles and San Diego, extending from the San Bernardino Mountains east and southeast to the Colorado River, from which it takes its name.
The region is essentially the northwest extension of the Sonoran Desert to the southeast. The region includes the heavily-irrigated Coachella and Imperial valleys on the north and south side of the Salton Sea respectively. It is crossed by several mountain ranges, including the San Jacinto, Santa Rosa, Little San Bernardino, and Chocolate mountains.
In this region, the geology is dominated by the transition of the tectonic plate boundary from rift to fault. The southernmost strands of the San Andreas Fault connect the northern-most extensions of the East Pacific Rise. Consequently, the region is subject to earthquakes, and the crust is being stretched, resulting in a sinking of the terrain over time.
Joshua Tree National Park is located on the northern edge of the region and includes many of the unique species and habitats of the region. Santa Rosa and San Jacinto National Monument, created in 2000, encompasses much of the San Jacinto and Santa Rosa mountains along the western side of the region.
Big Morongo Canyon Preserve is located in the Little San Bernadino Mountains and spans the Mojave and Colorado Deserts. It was identified as an area of concern by the Bureau of Land Management in 1982.
Anza-Borrego Desert State Park, the second largest state park in the contiguous United States, covers 600,000 acres (2400 km²) from the edge of the coastal mountains east of San Diego to the Salton Sea and south almost to the US-Mexico border.