Introduction
The Great Basin and Mojave Desert are large physiographic congeries comprised of numerous basins and ranges. These geological and biological provinces are products of a complex history spanning hundreds of millions of years. Although much of the pluvial flora and fauna of the area undoubtedly has been lost, where relict ecosystems have persisted some of these species remain. One such relict ecosystem is the Amargosa River drainage, of which Amargosa Canyon is a unique and fragile segment.
Amargosa Canyon lies approximately 65 km. north of the town of Baker, San Bernardino County, California. At the head of the canyon is the small community of Tecopa (fig. 1). At the southern extremity the canyon opens onto a large alluvial plain bordered on the east by Dumont Dunes. The Amargosa Canyon ecosystem includes the Amargosa River from Tecopa Hot Springs to Sperry; springs near Tecopa Hot Springs and Tecopa; and Willow Creek (fig. 2). The canyon varies in width from about 122 m. in the northern portion to approximately 610 m. as it opens up to the south. It is about 20 km. in length. The depth of the canyon is variable with a maximum near the nexus of Willow Creek and the Amargosa River, where canyon walls are approximately 293 m. high.
Figure l.
The Amargosa Canyon ecosystem from
Tecopa Hot Springs to Sperry, Inyo and
San Bernardino Counties, California.
[1] Paper presented at the California. Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].
[2] Jack E. Williams is Staff Ichthyologist, Gail C. Kobetich is Field Supervisor, and Carl T. Benz is Staff Mammalogist at the Endangered Species Office, USDI Fish and Wildlife Service, Sacramento, California.
Figure 2.
The Amargosa River drainage, southwestern
Nevada and southeastern California.
The Amargosa Canyon ecosystem is an "ecological island". Isolated by geographical and climatic factors within the heart of the Mojave Desert, the extant flora and fauna have survived by adapting to the unique environmental conditions of their restricted habitats.
Tecopa Hot Springs is situated on lacustrine deposits of Pleistocene Lake Tecopa (Mason 1948). These spring waters flowing across the alkaline flat support a riparian vegetation characterized by bulrush (Scirpusolneyi ) and salt grass (Distichlisspicata ).
At the head of the canyon, and throughout it to the old mining site of Sperry, the native floral assemblage is representative of recent pluvial periods. Willow (Salixgooddingii ) and mesquite (Prosopis spp.) characterize thickets along the banks of the river. Cattail (Typhalatifolia ), bulrush, salt grass, rushes (Juncus spp.), and arrowweed (Plucheasericea ) create a moderate to dense vegetative barrier through which the river flows. The xeric upper slopes parallel to the river support salt bush (Atriplex spp.), buckwheat (Eriogonum spp.), creosote bush (Larreatridentata ), and others. Sawgrass (Cladiummariscus ) is found around many of the springs and seeps. Tamarisk (Tamarix spp.) is a major exotic component of the flora at the north end of the canyon and along Willow Creek.
Willow Creek is characterized by the richest botanical complex in the canyon system. Stands of cottonwood (Populus fremontii ) and thickets of mesquite, willow, and tamarisk dominate the visual scene, particularly along the creek and around Willow Spring and Willow Creek Reservoir. The spring and reservoir appear to be nearly overgrown with cattail.
Climate
The average annual precipitation during 1973 to 1980 was only 11.5 cm., with peaks occurring during winter and late summer months, as determined from a rain gauge at Shoshone. Extended periods with virtually no precipitation have been recorded during every season of the year.
During the period 1941 to 1970, January temperatures averaged 7.3°C., while July averaged 31.6°C. High summer temperatures, high rates of wind movement, and low humidity all interact to promote high rates of evapotranspiration. A maximum monthly evaporation rate of 502 mm. was recorded at Silver Lake, approximately 40 km. south of Amargosa Canyon (Pupfish Habitat Preservation Committee 1972).
Paleohydrology
The climate of western North America fluctuated widely during later Pleistocene and pluvial times. Pluvial periods were 4–8°C. cooler than at present and appreciably hydric. During these cooler and wetter periods, large lakes and extensive river systems occupied most basins, providing dispersal mechanisms for many aquatic and semi-aquatic organisms (Hubbs and Miller 1948; Mifflin and Wheat 1979; Miller 1946, 1948; Snyder et al . 1964).
The connections and extent of the Amargosa River drainage during later Pleistocene and pluvial times have been extensively studied; however, some differences of opinion still exist. In his studies of the fishes inhabiting southeastern California and southwestern Nevada, Miller (1946, 1948) proposed the name "Death Valley system" for what he believed was a large drainage system including the Amargosa, Mojave, and Owens Rivers, which all drained into Death Valley. Taylor (1980) disputed the existence of a Death Valley system based on his studies of the molluscan fauna. However, it appears clear that the current Amargosa River drainage at one time was occupied by three lakes, Lake Ash Meadows (16 km2 ), Lake Tecopa (254 km2 ), and Lake Manly (1,601 km2 ) (Snyder etal . 1964). At its maximum size, Lake Tecopa extended from north of
Shoshone through the Amargosa Canyon area. Lake Manly occupied most of the Death Valley floor, extending south to Saratoga Springs. Paleohydrographic evidence also indicates an occasional connection between the Mojave River and Amargosa River drainages near the vicinity of Silver Lake (ibid .). Wave-cut terraces, gravelbars, and deltas of pluvial origin provide easily visible evidence of the former water systems. The present Amargosa River drainage is but a shadow of its extent during late Pleistocene and pluvial times.
Hydrology
The headwaters of the Amargosa River occur in Oasis Valley north of the old mining town of Beatty, Nye County, Nevada (fig. 2). The Amargosa River drainage is anomalous in that the river flows southward from Oasis Valley, turns westward after flowing through Amargosa Canyon, and finally northward into Death Valley. The final sink of the 264-km. long river is at Bad Water, Death Valley. Much of the river flow is subterranean most of the year, particularly in summer. Three sections of the river wash—Oasis Valley, Amargosa Canyon, and a small (1.2-km.) section just northwest of Saratoga Springs—have perennial flows.
The largest permanent flow occurs through the Amargosa Canyon area (fig. 3). The river surfaces just north of Tecopa and continues through Amargosa Canyon for about 19 km. to the vicinity of Sperry. The entire ecological balance of the canyon, with few exceptions, depends on the natural surface water flows.
Figure 3.
Aerial view of Amargosa Canyon looking north. Willow Creek is the tributary canyon to the right.
The flow entering the canyon emanates from subterranean flows updrainage and springs in and adjacent to the community of Tecopa Hot Springs. The average discharge of the river at Tecopa during 1961 to 1968 was 2.21 cfs (USDI Geological Survey 1968). The Amargosa drainage occasionally experiences torrential floods, as in a 1965 storm that caused a maximum flow of 950 cfs.
In the northern half of the canyon, several minor springs and seeps feed the river. Willow Spring, the headwater of Willow Creek, flows all year. The surface flow of Willow Creek varies seasonally, and is often dry near its confluence with the Amargosa River during summer.
Total dissolved solids (TDS in milligrams per liter) vary considerably within the river because of differing water qualities of inflow springs. During a May 1981 survey, TDS varied from approximately 1,400 at the head of the canyon to 3,850 in the Amargosa River just above the inflow of Willow Creek.[3] The water quality
[3] Cindy Williams. California State University, Sacramento. Personal communication.
of Willow Creek is good; TDS is typically 700. The pH of the river water and inflow springs ranges from 8.0 to 8.4. Dissolved oxygen is high, approaching saturation throughout the river.
Geology
The geologic history of Amargosa Canyon is not totally understood. The canyon formed as a result of the Amargosa River cutting through an east-west trending uplift along the major axis of Sperry Hills. A Pleistocene lake, Lake Tecopa, was an ephemeral feature just north of the canyon. The outflow channel of Lake Tecopa is presumably the present-day Amargosa Canyon.
Various geological formations, the result of past geologic processes, are presently visible in Amargosa Canyon. At the north end, lacustrine deposits of Lake Tecopa, with associated vertebrate fossils, are visible (Mason 1948). Further down the canyon, erosional forces have exposed very coarse gravel and cobble (fanglomerates), thought to have been deposited contemporaneously with or after the Lake Tecopa sediments (ibid .). Upstream of the mouth of Willow Creek, rocks of volcanic origin such as tuffs, breccias, and undifferentiated volcanic flows of later tertiary origin have been exposed (Troxel 1961). South of the volcanics lie the China Ranch Beds, a series of light-colored, mainly white to brilliantly pink saline claystones with some interbedded gypsum layers. To the south of Willow Creek the geological sequence is reversed.
Downcutting by the river near the inflow of Willow Creek has exposed Precambrian rock, which is the probable basement for much of the canyon. From Tecopa to approximately 4.5 km. south of the Willow Creek confluence, moderate amounts of water flow at the surface throughout the year. Downstream of this area, water usually flows on the surface throughout the winter and spring months and occasionally during the summer. This suggests that the relatively impervious Precambrian rocks lie very close to the surface throughout the northern reaches of the canyon and are responsible for raising the subterranean water flow.