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Great Basin Riparian Areas
Ecology, Management, and Restoration
By Jeanne C. Chambers, Jerry R. Miller ISLAND PRESS
Copyright © 2004 Island Press
All rights reserved.
ISBN: 978-1-59726-294-1
CHAPTER 1
Restoring and Maintaining Sustainable Riparian Ecosystems: The Great Basin Ecosystem Management Project
JEANNE C. CHAMBERS AND JERRY R. MILLER
In the Great Basin, as in other semiarid regions, riparian areas exhibit widespread degradation. It has been estimated that more than 50 percent of the riparian areas (streams and their associated riparian ecosystems) in the Great Basin are currently in poor ecological condition (Jenson and Platts 1990). The ongoing deterioration of these areas is of significant concern to land managers and other stakeholders who value these watersheds for a variety of purposes. Riparian areas are important components of all landscapes, but in the semiarid Great Basin they constitute an especially vital resource. Although they comprise less than 1 percent of the Great Basin, they supply many critical ecosystem services. Riparian areas supply water for both culinary and agricultural uses, forage and browse for native herbivores and livestock, and recreational opportunities. In addition, they serve as the foundation for much of the region's biodiversity. Riparian areas in the Great Basin provide habitat for a wide array of organisms such as butterflies (Fleishman et al. 1999) and Neotropical migrant birds (Martin and Finch 1996), and support a relatively high number of endemic species, including the Lahontan cutthroat trout (Oncorhynchus clarki henshawi), which is listed as threatened under the U.S. Endangered Species Act (Dunham et al. 1997).
Degradation of riparian areas in the Great Basin is the result of complex and interrelated responses of geomorphic, hydrologic, and biotic processes to climate change and natural and anthropogenic disturbances. These disturbances can alter the hydrologic or sedimentologic regime of a fluvial (river or stream) system and produce changes in the physical foundations of riparian ecosystems, such as stream channel characteristics and surface-groundwater interactions. Ultimately, they alter the structure and functioning of riparian ecosystems. In this volume, anthropogenic disturbances refer to all human activities that affect physical and biological processes within a watershed, while natural disturbances include phenomena such as floods, landslides, and wildfires. Although climate change could be considered to be a type of natural disturbance, it operates over longer temporal scales and larger spatial scales than most other forms of natural disturbance. Also, current shifts in climate arguably are related to human activities. Thus, climate change is treated as a special form of disturbance herein.
Much of the research on stream and riparian ecosystem degradation in arid and semiarid regions of the western United States has focused on the effects of anthropogenic disturbances. Consequently, the degradation of these riparian areas has been attributed largely to human activities, and management and restoration strategies have focused primarily on anthropogenic disturbances. In the Great Basin, riparian areas and their associated uplands have been subjected to various anthropogenic disturbances since European settlement of the region around 1860. The most extensive disturbances have been overgrazing by livestock (Kauffman and Krueger 1984; Fleischner 1994; Ohmart 1996; Trimble and Mendel 1995; Belsky et al. 1999) and road construction in the valley and canyon bottoms. Local alterations of hydrologic regimes via dams and water diversions, mining operations, and recreational activities also have had negative influences on riparian ecosystems (Sidle and Amacher 1990; Sidle and Hornbeck 1991). The influences of these disturbances on riparian areas have been well documented and management strategies for mitigating their effects are discussed in numerous locations (e.g., Kusler and Kentula 1990; National Research Council 1992, 2002; Briggs 1996; Kauffman et al. 1997; Williams et al. 1997).
The effects of past and present climate change on stream and riparian ecosystems have received considerably less attention than the influences of anthropogenic disturbance. This is surprising given that arid and semiarid regions like the Great Basin are more sensitive to the effects of both past and present climate change than humid regions. The sensitivity of these regions to climate change has important implications for the types and characteristics of disturbances that riparian areas experience, and the effects of these disturbances on riparian ecosystems. In comparison to humid regions, arid and semiarid regions generally exhibit amplified runoff responses to precipitation change (Dahm and Molles 1991), have higher streamflow variability (Poff 1991; Osterkamp and Friedman 2000), and have more severe flash floods (Graf 1988; Osterkamp and Friedman 2000). In the Great Basin, paleoecological and geomorphic records indicate that there have been significant fluctuations in climate during the Holocene (approximately the past ten thousand years) (Tausch and Nowak 2000), and that these fluctuations have had major effects on disturbance regimes (Miller et al. 2001). Changes in hillslope processes, stream channel pattern and form, surface and groundwater interactions, and riparian vegetation composition and structure over time scales of hundreds of years have all been attributed to Holocene shifts in climate (Chambers et al. 1998; Miller et al. 2001). Perhaps more important from a management and restoration perspective is that the effects of these changes on hillslope processes and landforms have persisted for hundreds to thousands of years. For example, a shift from moister to drier conditions during the mid- to late-Holocene led to accelerated hillslope erosion, sediment deposition on alluvial fans and in valley bottoms, and a depletion of hillslope sediment supplies in upland watersheds of the central Great Basin (Miller et al. 2001). These climate-induced changes still influence geomorphic processes and, thus, channel pattern and form.
The failure of past restoration activities in semiarid riparian areas to meet desired goals has been attributed to a general lack of understanding of existing physical and biotic processes and the causes of disturbance (Elmore and Kauffman 1994; Kauffman et al. 1997; Goodwin et al. 1997), and to the use of small-scale, site-specific approaches that fail to consider watershed scale processes (Roper et al. 1997; Williams et al. 1997). Clearly, developing appropriate management and restoration approaches for riparian areas in the Great Basin requires an understanding of the responses of geomorphic, hydrologic, and biotic processes not only to natural and anthropogenic disturbances, but also to climate change. It also requires knowledge of these processes over sufficiently large temporal and spatial scales.
The Great Basin Ecosystem Management Project
In 1992, a USDA Forest Service Research ecosystem management project, "Restoring and Maintaining Sustainable Riparian Ecosystems," was initiated to address the problem of stream and riparian ecosystem degradation within the central Great Basin. The Great Basin Ecosystem Management (EM) Project uses an integrated, interdisciplinary approach to increase our understanding of the effects of climate change and anthropogenic disturbance on riparian areas, and to elucidate the connections among watershed and channel processes, hydrologic regimes, and riparian ecosystem dynamics. The EM Project is unique in that it addresses temporal scales ranging from the mid-Holocene to the present and spatial scales ranging from entire watersheds to localized stream reaches. The project's process-based and multiscaled approach is used to develop guidelines and methods for maintaining and restoring sustainable riparian ecosystems.
Definitions and Concepts
Ecosystem management uses ecological, economic, social, and managerial principles to maintain, restore, or create ecosystems that are capable of sustaining desired uses, products, values, and services over long time periods (modified from Overbay 1992). Thus, restoration is an integral component of contemporary ecosystem management. The Society for Ecological Restoration International's (SERI) definition of restoration is consistent with the concepts inherent in ecosystem management. Ecological restoration is defined by SERI as the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed (Society for Ecological Restoration International 2002). Ecosystem management has focused on watershed and regional scales, regardless of ecological condition, and emphasized the need to include both larger spatial and longer temporal scales. In contrast, restoration ecology has focused on degraded, damaged, or destroyed ecosystems but increasingly recognizes the need to consider larger scales in developing viable restoration approaches (Naveh 1994; Hobbs and Harris 2001). At the core of both ecosystem management and restoration ecology is the concept of sustainability. Sustainable ecosystems, over the normal cycle of disturbance events, retain characteristic processes including hydrologic flux and storage, geomorphic processes, biogeochemical cycling and storage, and biological activity and production (modified from Chapin et al. 1996 and Christensen et al. 1996). Because riparian areas serve as the interface between upland and stream ecosystems, sustainable stream and riparian ecosystems exhibit physical, chemical, and biological linkages among their geomorphic, hydrologic, and biotic components (Gregory et al. 1991). Thus, for the purposes of this volume, managing and restoring riparian areas is defined as maintaining or reestablishing sustainable fluvial systems and riparian ecosystems that exhibit both characteristic processes and related biological, chemical, and physical linkages among system components (modified from National Research Council 1992). Inherent in this definition is the notion that sustainable ecosystems supply important ecosystem services.
To establish viable management and restoration goals, it is necessary to understand the current ecosystem or restoration potential. A frequent goal of both management and restoration has been to re-create and manage for the predisturbance condition (National Research Council 1992). In many cases, the predisturbance condition has been assumed to be the state of the stream or riparian ecosystem prior to European settlement. Several problems exist with this approach. First, we seldom understand the structure or processes of stream and riparian ecosystems prior to settlement. More important, using pre-settlement conditions as the goal of management or restoration assumes stable or equilibrium conditions over hundreds of years and ignores changes in stream processes and ecosystem dynamics due to changes in climate, hydrology, or land uses (Wade et al. 1998). A more realistic approach is to base management and restoration goals on the current potential of the streams and riparian ecosystems to support a given set of conditions. This approach requires an understanding of the types and magnitudes of biotic and abiotic thresholds that may have been crossed as a result of climate change or anthropogenic disturbance, and of the alternative states that currently exist for these systems (e.g., Hobbs and Norton 1996; Whisenant 1999; Hobbs and Harris 2001).
Objectives
The primary emphasis of the Great Basin EM Project has been on the geomorphic processes, hydrologic regimes, and vegetation dynamics of the systems. A parallel effort by the Nevada Biodiversity Initiative has examined the relationships among faunal distributions and the physical environment, and this volume includes an overview of various approaches for studying and managing faunal distributions in the Great Basin. The results of the EM Project that are presented in this volume were generated to accomplish the following series of interrelated objectives.
1. Reconstruct the vegetation and geomorphic history of central Great Basin watersheds to increase our understanding of the effects of past and present climate change on ecosystem processes.
2. Elucidate the underlying geomorphic and hydrologic processes that characterize the watersheds and riparian areas, and evaluate the effects of both past and present climate change and anthropogenic disturbance on these processes.
3. Determine the sensitivity of the study watersheds to both natural and anthropogenic disturbance, and develop a model of watershed sensitivity for managing riparian ecosystems.
4. Evaluate the effects of watershed geology and natural and anthropogenic disturbance on flow regimes and water quality, and relate this information to watershed sensitivity.
5. Determine the relationships among riparian vegetation dynamics and hydrogeomorphic processes, and evaluate the effects of past and present climate change and anthropogenic disturbance on those relationships.
6. Examine relationships among faunal distributions and the physical environment, and evaluate approaches for studying and managing native fauna, including documentation of historical changes.
7. Use our understanding of past and present ecosystem processes to develop guidelines and techniques for restoring and maintaining sustainable riparian areas.
Project Area
The EM Project is located in the central Great Basin (fig. 1.1). The Great Basin is the largest section of the Basin and Range Physiographic Province and contains almost all of the state of Nevada, the western half of Utah, and adjoining portions of Oregon, California, and Idaho. Although its name implies that the region consists of a single large depression, it is actually composed of a series of north-northwest-trending mountain ranges separated by alleviated, intermountain basins that were formed by high angle extensional faulting. In the central Great Basin, the ranges comprise more than 40 percent of the total landscape and may reach elevations in excess of 3,500 meters, more than 1,500 to 2,000 meters above the surrounding basin floors (Dohrenwend 1987). With the exception of a few areas along the north, northwestern, and southeastern margins of the Great Basin, drainage is completely internal. More than two hundred separate hydrologic systems can be delineated, about half of which are characterized by closed drainage (Dohrenwend 1987). Many of these closed basins contained pluvial lakes during the late Pleistocene (Mifflin and Wheat 1979). The focus of the EM Project is on small (less than 100 square kilometer) upland watersheds within the Shoshone, Toiyabe, Toquima, and Monitor Ranges. These catchments are underlain by a wide variety of rock types that reflect the complex structural geology of the area (Kleinhampl and Ziony 1985). Most of the upland watersheds terminate in large, well-developed alluvial fans along the range fronts.
Because of the large elevational gradients, precipitation and temperature varies significantly from the upper elevations to the mouths of the watersheds. Precipitation approaches 55 centimeters at upper elevations but decreases to as little as 20 centimeters in the central valleys. At Austin, Nevada (2,168 meters), where one of the few long-term weather stations in the area is located, annual average precipitation is 34 centimeters. Most precipitation falls during the winter as snow, and peak runoff and most flood events occur during snowmelt in late May to early June. Convective summer storms occasionally result in flash floods. The upland watersheds are typically characterized by narrow valleys, and the stream systems are high gradient, coarse-grained, and often highly incised (fig. 1.2). Although low flows in these stream systems range from about 0.015 to 0.063 cubic meters per second, high flows can range from 0.214 to 0.683 cubic meters per second (Hess and Bohman 1996). Precipitation and stream flows are highly variable both within and among years (fig. 1.3). Most of the change in stream channel pattern and form occurs during high-magnitude, low-frequency flows (Miller et al. 2001). Although many of the stream systems are ephemeral, the EM Project has focused primarily on perennial streams. Most of these streams can be categorized as second or third order at the point where they flow into the central valleys (as measured on 1:24,000 topographic maps).
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Excerpted from Great Basin Riparian Areas by Jeanne C. Chambers, Jerry R. Miller. Copyright © 2004 Island Press. Excerpted by permission of ISLAND PRESS.
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