Alluvial Fans and Archaeological Sites on the East Range of Fort Huachuca Military Reservation, SE Arizona

Introduction

In the American Southwest, a ubiquitous and recognizable geomorphic landscape feature is the bajada, consisting of a series of broad, laterally coalesced alluvial fans. These alluvial aprons emerging from mountain fronts are a characteristic feature of the Basin and Range physiographic province (Nations and Stump 1996). The East Range of the Fort Huachuca Military Reservation is on the middle to lower portion of a vast southwest-to-northeast-trending bajada emanating from the Huachuca Mountains in the San Pedro River Valley of southeastern Arizona. (Figure 1). As modern erosion of the bajada accelerates and becomes more pervasive, numerous archaeological sites in the area are being severely impacted. The physical nature of erosional impacts is manifested through the removal of confining sediments, transport of artifacts and accelerated weathering processes

Holocene alluvial deposits in the East Range, as well as many areas of southeastern Arizona, display evidence of multiple episodes of downcutting and arroyo formation during the last 10,000 years (Bull 1991; Cooke and Reeves 1976; Cooke et al. 1993; Haynes and Huckell 1986). However, these deposits are the most important physical components of the East Range because they have created the dynamic bajada landform, which has been dissected by mountain streams and periodic runoff, forming steep-sided canyons and numerous deep arroyos. This landform, and its diverse environment, offers a multitude of natural resources such as mesquite pods, seeds, firewood, cacti, game, and other exploitable resources that have attracted human populations who have been occupying and adapting to the dynamic characteristics of bajada landscape since at least 12,000 B.C. Archaeological surveys of the East Range have documented 223 sites on the bajada, evidence of this rich cultural heritage. Many of these sites, however, are being impacted by the natural erosional processes affecting bajada landscapes, exacerbated by impacts from historic and recent human activities. To counteract these effects, Fort Huachuca is implementing effective management practices in terms of environmental sustainability, landscape equilibrium, range and woodland preservation, wildlife administration, and the management and preservation of archaeological resources.

The first step in implementation requires a thorough understanding of soil properties, the geomorphic landscapes they occupy, and their susceptibility to erosion (Figure 2). As shown in Figure 2, soil areas with slight erosion potential are located primarily in the south and central portion of the East Range and support perennial grasses that "bind" the soil; soil areas with moderate erosion potential are in the western, and eastern portions of the East Range where sparse shrubby vegetation is the dominate vegetation type; and soil areas with severe to very severe erosion potential, including soil areas with a moderate to high content of gypsum, a chemical that is partially soluble in water that can rapidly develop subsurface piping conduits, are located primarily on the lower northeast, east, north, and west portions of the lower bajada. Soils along drainage pathways, which consist of loose and unconsolidated sediments subject to frequent flooding and shifting, have severe to very severe erosion potential.

Erosion Potential

In the arid Southwest, the processes of erosion on the landscape are complex, ranging from climate change (e.g., Antevs 1955) to subsistence and settlement practices (agriculture, ranching, land-clearing, road-building) that lead to increased erosiveness of storm flows and the resulting erodibility of valley-floor materials. Figure 3 illustrates several of the interrelated processes and environmental factors involved in the initiation of arroyos (Cooke and Reeves 1976). Although prehistoric arroyo cutting can be attributed to climate change, the historical possibilities (human land-use changes), are much more complicated because increased erosiveness of flows and the resulting erodibility of valley-floor materials can occur as a result of different factors and in several different ways. The erosion observed in the Fort Huachuca area is based on five major factors that determine, either directly or indirectly, the degree of erosion at any given location. These include; (1) position on the landscape; (2) substrate material; (3) precipitation characteristics; (4) vegetation; and (5) human activities. In the last 100 years, due primarily to human land-use changes, the erosional regime in the East Range has significantly changed, and numerous interrelated factors including removal of vegetation cover on the valley floor, steepening valley side slopes, alteration of valley-bottom soils, and an increase of subsurface piping ducts have contributed to these changes.

Erosional Classes

As a result of previous geomorphological research in a portion of the East Range, Nials (2004) devised a generalized model to explain the observed erosion, identifying five major factors that determine the degree of erosion at a specific location. Subsequently, ACS compiled and synthesized geomorphological and archaeological data to provide site condition assessment and management recommendations for all 223 sites in the East Range (Droz 2008). To facilitate comparisons between planning areas, Nials' model was modified to become applicable to any given location on the bajada (Figure 4). Figure 4 illustrates the landscape position on the bajada, the probable erosion classes for each landscape position and infiltration and runoff conditions. For each site, the author evaluated the following factors: position on the landscape, substrate materials and soil properties, precipitation characteristics, vegetation, and human activities Assessment of these five major factors allowed for the development of a rough "predictive" model using six classes of site erosion to provide a means of assessing overall landscape conditions throughout the East Range (Table 1). (For a more detailed description of the six erosional classes see (Droz 2008)).

Deposition or non-erosional areas (Erosion Class 0) typically occur on wide interfluve areas or alluvial bottoms. Vegetation is dominated by grasslands. The dominant processes are alluvial deposition and aeolian deposition of a sandy aeolian mantle. The potential integrity of archaeological sites and features in this class is typically good. This environment is characterized by rapid deposition and the probability of deep burial of sites.

Minimally eroded locations (Erosion Class 1) tend to occur in broad, flat interfluve areas with no significant runoff concentration. Sandy aeolian sediments are present. Vegetation is dominated by grasslands. Ground surface topography exhibits small rills and shallow deflation basins. The potential integrity of archaeological sites is highly variable but good to fair in the upper part convex slopes.

Slightly eroded conditions (Erosion Class 2) also tend to occur in interfluve areas near the convex slope boundary in areas dominated by relatively low-energy aeolian and sheetwash processes. The sandy aeolian mantle is essentially complete. Vegetation is dominated by grasslands. Pebbles are pedestaled. The potential integrity of archaeological sites is variable but in locations near rills, there may be a slight tendency for the downslope movement of artifacts.

Moderately eroded locations (Erosion Class 3) can occur anywhere, but tend to occur on convex slopes and in the upper portion of straight slopes. This erosional class is characterized by a number of distinctive characteristics. Vegetation is dominated by spiny plants, slopewash and rillwash are the dominant processes, a pavement of pebbles and/or cobbles is present, and clasts of indurated caliche become significantly more abundant. The potential for disruption of the spatial integrity of artifacts within sites is high in this environment.

Severely eroded locations (Erosion Class 4) tend to occur near the base of straight and convex slopes where alluvial bottoms are incised by arroyos. Gullies are deep and abundant. Vegetation is dominated by spiny shrubs and have densities of less than 15 to 20 percent. Root crowns are exposed and pedestaled rocks are present. The integrity of archaeological deposits is severely compromised. Additionally, "stretching" of features in downslope directions is pronounced.

Extremely eroded conditions (Erosion Class 5) also tend to occur near the base of straight and convex slopes where the adjacent alluvial bottom has been incised by arroyos. Alluvial bottoms are typically incised and and gullies eroding these locations may be graded to the local "master arroyo". Pleistocene sediments are present. The integrity of archaeological manifestations has been severely compromised in most locations, if not destroyed.

Fort Huachuca Assessment

Figure 5 illustrates the erosional classes as colored dots representing archaeological sites ranging from extremely eroded to stable. The erosion potential of soils were also illustrated on Figure 5 to potentially assess spatial landscape patterns revealing potential highly eroded areas of erosion where archaeological sites may be severely altered potentially obliterating the integrity of the site's surface. Figure 5 reveals three general areas of soil erosion potential on the East Range. Highly erodible land covers the west, central, and east portions of all planning areas. Moderately erodible land mimics the spatial distribution of highly erodible land, but is restricted to major drainage pathways. The stable or non-erosional areas are in the south-central portions of the East Range.

The spatial distribution of archaeological sites and their erosional classification upon the bajada landscape reveals several distinctive patterns. Extremely eroded sites are prominent along tributary drainage pathways and near the confluence of tributaries that drain into the San Pedro and Babocomari rivers. Severely and moderately eroded sites follow a similar spatial distribution on moderately to highly erodible land. The distribution of minimally eroded, slightly eroded, and stable sites correlates to non-erosional areas. By analyzing and evaluating the erosional classes which incorporate the five major factors that contribute to erosional degradation, Fort Huachuca has begun to apply immediate measures to slow or limit the effects of active erosion on unstable archaeological sites and preserve the remaining stable ones on the East Range bajada.

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