Sonoran Desertscrub – Lower Colorado River Valley
- Scientific Name: Sonoran Desertscrub – Lower Colorado River Valley
Named for its location surrounding the lower Colorado River in parts of four states, this is the largest, hottest, and driest subdivision. It challenges the Mohave Desert’s Death Valley as the hottest and driest place in North America. Summer highs may exceed 120°F (49°C), with surface temperatures approaching 180°F (82°C). The intense solar radiation from cloudless skies on most days and the very low humidity suck the life-sustaining water from plants, water that cannot be replaced from the parched mineral soil. Annual rainfall in the driest sites averages less than three inches (76 mm), and some localities have gone thirty-six months with no rain. Even so, life exists here, abundantly in the rare wet years.
The terrain consists mostly of broad, flat valleys with widely-scattered, small mountain ranges of almost barren rock. There are also seas of loose sand and the spectacular Pinacate volcanic field (see plate 14). The valleys are dominated by low shrubs, primarily creosote bush (L. tridentata) and white bursage (Ambrosia dumosa). These are the two most drought-tolerant perennial plants in North America, but in the driest areas of this subdivision even they are restricted to drainageways. Trees grow only along the larger washes. The mountains support a wider variety of shrubs and cacti, but the density is still very sparse. Columnar cacti, one of the indicators of the Sonoran Desert, are rare (virtually absent in California) and are restricted to valley floors. Annual species comprise over half the flora, up to ninety percent at the driest sites; they are mostly winter growing species and appear in large numbers only in wet years.
This is the only part of the Sonoran Desert that extends into California, where most residents call it the Colorado Desert. North of a sagging line between the Coachella Valley (Palm Springs) and Needles, California, it merges almost imperceptibly into the lower Mohave Desert.