Fifty-four years of ephemeral channel response to two years of intense World War II military activity, Camp Iron Mountain, Mojave Desert, California

During World War II, U.S. Army personnel lived, trained, and executed mock battles on low gradient piedmonts (~2°) in the Mojave Desert. For example, Camp Iron Mountain (established in 1942 by General George S. Patton, Jr., and used until 1944) housed up to 20000 Army personnel at any specific time. The camp is located on the large alluvial piedmont that extends from the Iron Mountains and is drained by shallow ephemeral channels. At this camp, we made 18 detailed topographic maps in order to compare drainage networks of six undisturbed control plots and 12 plots disturbed by army activities. There are significant differences between the morphometery of small-scale, ephemeral drainage networks on control plots, on plots bisected by stone-walled walkways, and on plots down gradient of former army roads. Control plot channels are wider (2.05 ± 1.48 m) and deeper (8.8 ± 4.5 cm) than channels in walkway plots (width: 1.19 ± 0.71 m, depth: 7.4 ± 4.1 cm) and in road plots (width: 1.18 ± 0.61 m, depth: 7.2 ± 6.7 cm). The military’s modification of the landscape affected subsequent channel originations and orientations. Channel heads were found in 76% of the compacted and smoothed walkways. In walkway plots, 80% of walkways caused the orientation of channels to deviate from the steepest piedmont gradient by more than 20°. After more than 50 years, road berms still act as local drainage divides. Down gradient of each intact road berm, there is a wide (20–40 m) zone in which no channels exist. Where channels have developed below intact road berms, they are smaller than channels in undisturbed control plots. Down gradient of breached road berms, wide, braided channels are common. Fifty-four years after camp abandonment, the channel network at Camp Iron Mountain has yet to recover, primarily because rock alignments and road berms continue to influence drainage patterns and local gradients. Nichols, K.K., and Bierman, P.R., 2001, Fifty-four years of ephemeral channel response to two years of intense World War II military activity, Camp Iron Mountain, Mojave Desert, California, in Ehlen, J., and Harmon, R.S., eds., The Environmental Legacy of Military Operations: Boulder, Colorado, Geological Society of America Reviews in Engineering Geology, v. XIV, p. 123–136. 123 kknichol@zoo.uvm.edu rates of geomorphic processes and the rate at which natural systems recover from disturbance. Because many desert processes are so slow, results of short-term monitoring are often inconclusive (Abrahams et al., 1984). A longer-term perspective may be gained by studying areas where military activity occurred over a known and discrete time frame. Such data may allow identification and implementation of training protocols that minimize environmental impact. From 1942 to 1944, the U.S. Army established twelve temporary base camps and living quarters in the Desert Training Center (Fig. 1A). Camp Iron Mountain (~ 6 km) was built on a low-gradient piedmont surface extending > 10 km from the Iron Mountains to Danby Lake playa in the Mojave Desert (Fig. 1B). At this camp, the Army constructed an extensive (4 km by 1.5 km) grid of roads (Fig. 1C). During road construction, berms (30–40 cm high) were built on both sides of each road. Troops outlined walkways, tents, and roads with angular granitic clasts ranging in size from 10 to 20 cm (Fig. 2). After two years of continuous use, the Army dismantled the camps in May 1944 (Henley, 1989). Today, the only evidence of occupation is the presence of road berms, rock alignments, rock designs, and other artifacts, in various states of deterioration. The Iron Mountains are Cretaceous granite (Miller et al., 1981). The piedmont meets the mountain front at a sharp angle and is dominated by granular granitic alluvium (grus); median grain size is 2 mm. The piedmont slopes uniformly ~2° to the east where Camp Iron Mountain is located (Fig. 1B). Shallow, 10-cm deep ephemeral channels form gentle channel-terrace topography and dissect the surface. The Iron Mountain piedmont receives an average of 7.9 cm of precipitation a year (National Oceanic and Atmospheric Administration, 1982). Late summer thunderstorms and midwinter cyclonic events produce the majority of precipitation. Most precipitation infiltrates the grus. Runoff occurs only during rare, high-intensity rainfall events. Iverson et al. (1981) determined that runoff events in the western Mojave Desert probably occur about once per decade in undisturbed areas and more frequently in disturbed areas. As overland flow moves down the piedmont, some of the flow infiltrates the unsaturated grus (Bull, 1991; Ritter, 1978). In areas where no channels exist, sheetwash is the dominant overland flow mechanism (Horton, 1945). Sheetwash continues until flow depth is great enough so that: (1) a critical basal shear stress is reached and channel incision begins (Horton, 1945; Montgomery and Dietrich, 1994), or (2) raindrops cannot obliterate incipient channels as they begin to concentrate flow (Dunne, 1980). Small channels then form and transport sediment until the sediment load becomes greater than the transport capacity. When deposition occurs, caused by flow loss (infiltration) or a decrease in local gradient, many of the small channels diffuse into the planar surface. Such discontinuous channel morphologies dominate inside Camp Iron Mountain. Historic photographs document that constant foot, vehicle, and tank traffic destroyed most, if not all, of the small channels during the occupation of Camp Iron Mountain (Fig. 2). Thus, we can reasonably assume a surface absent of channels inside the camp when our experiment began in May 1944. We assume that road berms and rock alignments were intact at the time of evacuation. Because the area is remote and the nearest populated town is more than 55 km away (Desert Center, California, population 120), post-army activity is limited to natural processes (overland flow and bioturbation) and occasional off-road vehicle (ORV) use. Around 1980, a fence was constructed around Camp Iron Mountain that stopped all ORV traffic (Bureau of Land Management, 1986). Camp Iron Mountain provides a unique opportunity to investigate the redevelopment of an ephemeral channel network after two years of intense military training. We collected data 54 years after camp abandonment, in May 1998. In this chapter, we use our observations and data, in conjunction with previous research illustrating that human influence increases soil density (Prose, 1985; Webb et al., 1986), soil compaction, and soil surface smoothness (Iverson, 1980; Iverson et al., 1981), to develop a process model of channel response to military activity on lowgradient arid landscapes.