27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Climate Variation and Geomorphic Processes Since 1900 in the Central Mojave Desert

Richard Hereford
U.S. Geological Survey, Flagstaff, Arizona

The climate of the Mojave Desert region is perceived as static and unchanging. In the 20th century, however, precipitation has varied from drought (1893-1904 and 1942-77) to relatively wet (1905-1941 and 1978-98). This research, a component of the USGS Recoverability and Vulnerability of Desert Ecosystems Project, addresses climate variability in the Mojave Desert and its affect on the physical landscape-the substrate of the desert ecosystem. Three components of landscape change amenable to geologic study are the frequency of surface runoff, sediment yield, and channel alluviation in large washes. Generally, sediment yield and runoff frequency increase during wet climate episodes and decrease during dry episodes while channels aggrade (dry) or degrade (wet). The question is whether historic-age climate variation was sufficient to alter these components of the landscape.

The answer to this question is important for understanding landscape recovery from human and natural disturbances, because the frequency of sediment movement by water is one of the principal processes of geomorphic change. Aridity aside, evidence supporting the effectiveness of water in sculpting the desert landscape is seen in the numerous washes and countless rills and gullies. The rate at which the desert landscape recovers from disturbance, therefore, is directly related to the frequency of surface runoff and sediment movement. Moreover, the frequency of runoff directly influences the availability of surface water and shallow subsurface water. Regional estimates of runoff frequency help model the availability of surface water, replenishment of shallow aquifers, erosion, sediment yield, and surface stability.

Two types of alluvial deposits provide stratigraphic evidence of historic-age landscape change in the central Mojave Desert. These are ponded alluvial deposits that accumulated upstream of artificial barriers and alluvium in large washes that forms floodplains and terraces. Ponded deposits accumulate where railbeds cross piedmonts and the mouths of small drainage basins. In the first case, the deposits record the frequency of surface runoff in the alluvial washes of the piedmont; these are large drainage basins measured in km2. In the second case, they record sediment yield and the frequency of hillslope runoff from small basins measured in thousands of m2. Alluvium in large washes of the region accumulates episodically on floodplains or floodplain-like surfaces. Establishing the chronology of deposition and erosion of these floodplains may document a temporal and perhaps causal link to historic-age climate variation.

High intensity, geomorphically significant precipitation has varied episodically during the 20th century. High intensity precipitation was relatively frequent from the early 1900s to 1940s and again from the late 1970s to 1998, whereas precipitation frequency was low in the intervening period of the early 1940s to mid-1970s. Dry years occurred during the two wet intervals, but they were typically less frequent and of shorter duration than during the dry period. Likewise, wet years occurred during the dry period, however, these were of short duration and regional precipitation was suppressed.

This variation in precipitation was coincident with changes in the desert landscape (Hereford and Webb, 2001). During the wet periods, relatively frequent high-intensity precipitation increased the occurrence of hillslope runoff. This increased sediment accumulation in about 70 percent of the ponded sites during the early to mid-1900s and again in the late 1900s. During the dry period, the frequency of runoff-producing precipitation was reduced for several decades. This in turn decreased the frequency of overland flow, as suggested by the stratigraphy of the ponded sites. Precipitation variability evidently affected the alluvial washes as well, because a long-term change of intense precipitation alters the likelihood of large floods. Large floods were probably more frequent during the wet periods, resulting in channel incision with little sediment storage. These recent changes in precipitation were evidently large enough to alter hillslope runoff and channel processes. The desert landscape and climate, despite the overall aridity, cannot be viewed as static. The landscape is dynamic, changing detectably in only a few decades. As elsewhere in the Southwest (Swetnam and Betancourt, 1999), the mid-century drought and subsequent wet interval probably affected the ecology of the desert.

References

Hereford, Richard, and Webb, R. H. 2001 Climate variation since 1900 affects geomorphic processes and raises issues for land management. In Reynolds, R.E., ed., The changing face of the east Mojave Desert. California State University, Desert Studies Consortium, p. 54-55.

Swetnam, T. W., and Betancourt, J. L. 1998. Mesoscale disturbance and ecological response to decadal climate variability in the American Southwest. Journal of Climate 11: 3128-3147.