The Western Cordillera
George P. Malanson
David R. Butler
A significant portion of the research on the physical geography of North America has been conducted within the Western Cordillera (see the classics in Ives and Barry, 1974; Ives, 1980). The reasons for this richness are varied, but we can identify three. First, many physical geographers, and colleagues in other disciplines contributing to physical geography, have been drawn to the mountains and to the science simultaneously or have found that the science suited their predilection to spend time in the mountains. Second, physical geography is often revealed on environmental gradients, conceptually distinct from spatial transects, but gradients are relatively evident in mountain areas because they are compressed on and correlated with spatial transects. Also, in the mountains, processes can be accelerated because of the steep slopes. Third, the Western Cordillera is a rich source of data for the study of physical geography (Price, 1978). The area contains many distinct slopes, valleys, ridges, and streams; spatially and temporally distinct evidence of glaciation, vegetation, and climatic changes exists; human impacts have been limited or at least relatively discrete in time and space; and agencies with funding (U.S. Forest Service, National Park Service, U.S. Geological Survey, Environment-Canada) have been interested in research in the Western Cordillera.
Given the abundance of scientific information about the Western Cordillera, the following discussion highlights selected areas of information and research through the lens of hierarchy theory as conceptualized in ecology (Allen and Hoekstra, 1992). Briefly, hierarchy theory holds that processes and patterns at different scales have typical relations: those at a given scale are created by the combinations of processes at the next lower scale while constrained by the patterns of the next higher scale. The physical geography of the Western Cordillera is thus reviewed by examining process and pattern at four scales: continental, focusing on the whole system as the spatial context for processes operating at smaller scales; across ranges, but actually spanning the continent; within ranges, with a focus on valley heterogeneity; and within valleys, with a focus on both elevational and transverse heterogeneity (fig. 18.1). In this chapter, we emphasize the last scale because this is the scale at which most research activity occurs; we also recognize the constraints that coarser scale phenomena place on finer scale phenomena.
The existence of the Western Cordillera can be ascribed to a single process: plate tectonics. The Western Cordillera is the result of two major tectonic mechanisms that have led to mountain building, namely, collision and subduc-