The terrain of Death Valley itself should not be described without first identifying it as being part of, and surrounded by, the Great Basin, or the Basin and Range Province. The fact that it is situated within this tremendous area, surrounded by other ranges and basins, and is itself the deepest, hottest, and driest of all the Great Basin depressions, accounts in part for the present study.
The physiography of Death Valley has contributed to its climate, its scenic beauty, its geologic interest, its ecology, and its economic and human history in such a way that the composite picture becomes one of a unique and superlative desert area, to be recognized as such and set aside by Presidential proclamation in 1933 as a national monument.
Since this study has been projected by the National Park Service, the 2 million acres of land within the boundaries of the monument comprise the terrain of principal consideration. On the other hand, this arbitrary line in many instances leaves an incomplete biota within the monument boundary, thus forcing a major consideration of the contiguous areas involved.
Death Valley is approximately 190 miles long and from 5 to 20 miles wide, with about 550 square miles below sea level, reaching the lowest land point in this hemisphere at 282 feet near Badwater.
Mountain ranges surround the valley, rising to elevations of 5,000 to 11,000 feet, and there are practically no foothills between them and the valley floor. These mountains are composed principally of metamorphosed sedimentary rock, With the entire span of geologic time represented in the exposures of one ancient layer after another. The mountains arose, however, in relatively recent times, and volcanic activity, faulting, folding, tilting, and uplifting continued through the Pleistocene epoch with such force that erosion was not allowed to become a predominant force until the ice age ended. So the mountains are still new, steep, and relatively devoid of soil.
What might in a less arid climate have become a cismontane soil supply now comprises a third geological feature of Death Valley, the alluvial fans. Composed of billions of tons of detritus spreading out from the mouths of hourglass canyons on both sides of the valley, the fans are still being formed and reformed as floodwater periodically strips the mountains of their supply of decomposing rock which could have become earth.
The entire region may be characterized as one of extreme variability over incredibly short distances. The altitudinal variation is, of course reflected in the climate. When the then world record of 134° F. was reached below sea level at Furnace Creek Ranch on July 10, 1913, bighorn might have been feeding at 11,000 feet on Telescope Peak in the mild eighties.
During the summer, temperatures of over 120° F. are not uncommon, and nighttime temperatures remain relatively high. In 1959, July recorded an average maximum of 120.1°.
January is consistently the coldest month, with a steady rise in temperature through July and a steady fall through December.
The lowest official temperature of 15° F. was recorded at Furnace Creek Ranch January 8, 1913, the same year as the record high.
Summer and winter, the lowest temperatures occur just before sunrise, with an average differential of 27.2° between 24-hour maximum and minimum readings.
The average number of clear days in a calendar year is 283, and the recorded high is 351. Absence of clouds, the low-density ground cover, and the generally light-reflecting quality of the terrain contribute to an intensity of insolation affecting all life in the area.
The extremely variable rainfall is not adequately conveyed by the official weather records, for the only two stations in operation during the time of this study were less than 4 miles apart and at the same elevation, below sea level. The recorded rainfall averages around 2 inches per year, with a maximum of 4.2 inches in 1941 and a minimum of 0.09 in 1953.
But since, in general, precipitation is correlated with elevation, and since elevations can vary so much in such short distances, it becomes apparent that an accurate record of the rainfall affecting bighorn habitat would involve more weather stations and personnel than are likely to be available for some time. Precipitation can, for instance, be heavy throughout the high mountains and yet none happen to hit the existing weather stations on the valley floor. And the opposite, of course, happens. During an extremely dry year, an isolated cloud burst can catch a weather station, raising the weather chart to "normal" but adding nothing to the vegetation of the sheep range.
The general concept of desert rainfall in North America, including that of Death Valley, is one of two sources and two seasons: winter rains of cyclonic origin from the Pacific, moving across mountain ranges with the rain shadow deepening as it moves eastward, and summer storms from the Gulf of Mexico.
A correlation has become apparent between the general pattern of precipitation and the pattern of prevailing wind. In the summers, when there is high occurrence of mountain rainfall, over 90 percent of the wind has been southerly. While winter winds may bring dust storms from either the south or the north, the prevailing currents appear to come from the north and west, their direction being altered locally by the north and south orientation of the mountain ranges.
Very little work has been done on meteorology specifically for this area, but, presumably, the above generality would be sound despite the local contention that "Death Valley makes its own weather."
Snow is common in the winter to all the surrounding mountains down to the 4,000-foot level, but seldom achieves a pack deep enough to affect winter foraging. Only once in history has snow lain on the valley floor.
It is commonly believed that a relative humidity of zero is to be expected in the summer, but during the period of this survey the lowest determination made was 3 percent, with a summer average of about 10 percent.
That the aridity of Death Valley is commonly exaggerated is indicated in a report from the Scripps Institution of Oceanography (Gorczynski, 1940): "The Mojave and Colorado Deserts represent the driest area in North America; Bagdad, California, has 70% aridity co-efficient, Death Valley 57%, Yuma 42%, Phoenix 28%, and Tucson 22%, as compared with the middle of the Sahara approaching 100%."
Vegetation, of course, is predominantly xerophilous (i.e., of a type adapted to live in dry soil, sand, or on rocky ridges; characterized by thickening of epidermis, reduction of leaf surface), with a limited, but, to the bighorn, important growth of phreatophytes (deep-rooted plants that obtain their water from the water table or from the layer of soil just above it) in areas of live, perennial groundwater supply.
Because of the high salt content of most of the ground and water at lower levels, and especially on the valley floor, many of these plants must be highly salt resistant.
Away from the relatively few ground-water areas, the vegetation is inclined to be uniformly sparse, the plants widely spaced by the demands of their root systems in the shallow soil and rounded by the equal exposure to light from all sides.
With a few exceptions such as the creosotebush (Larrea tridentata) and pigmy cedar (Peucephyllum schottii), sometimes known as spruce-bush, the vegetation is light in color, ranging from the silver-white of desertholly (Atriplex hymenelytra) to the dark grays of blackbrush (Coleogyne ramosissima), and the yellowish green of rabbitbrush (Chrysothamnus).
With the exception of willows and mesquite in ground-water areas, the Black and Funeral Mountains on the eastern side of Death Valley are devoid of trees. The higher elevations from about 6,000 feet up, in the Grapevine Mountains, the Cottonwoods, and the Panamints, support the usual high-desert communities of drought-resistant conifers.
Except for those in the high areas, the plants of the region must be prepared to withstand severe drought conditions, lasting from several months to several years. During these observations we have watched perennial plants lie dormant for at least 6 years, growing drier, more brittle and brown month by month, until even we thought they were dead. No annuals of any kind made an appearance the entire time and yet when in the seventh year the rains came the perennials grew green again and the annuals appeared as if by magic.