The complexity of the real world in the context of the field tradition in geomorphology

Abstract

Among the dominant twentieth century conceptual models of geomorphology that rely on insights resulting from field-based research are Stanley A. Schumm's formulations of complex response, intrinsic thresholds, river metamorphosis, and spatial zonation of drainage basins. Schumm's research focused primarily on finer grained alluvial channels in lower relief environments. As a result of his work, most investigators now approach river process and form within a framework based on three fundamental assumptions. First, channel changes are abrupt and driven by crossing external and internal thresholds. Second, channel change is likely to be asynchronous, resulting in different portions of a river or a river network behaving in very different manners at a given point in time. Third, different portions of a river network are dominated by distinct disturbance regimes and resulting suites of geomorphic processes and forms. More recent research on resistant-boundary mountain channels illustrates how field evidence demonstrates that river process and form are inherently nonlinear, with spatial and temporal thresholds. Multithread channels can form within unconfined valley segments in mountainous river networks of the Colorado Front Range, but only in the presence of biotic drivers in the form of (i) old-growth forest that facilitates the formation of closely spaced, channel-spanning logjams or (ii) beavers that build dams. Thresholds of channel and valley geometry govern the occurrence and persistence of jams and dams, and these channel obstructions initiate specific nonlinear responses in valley and channel form. When the biotic drivers are removed, river metamorphosis occurs. Alluvial channels, which are typically regarded as being relatively responsive to changes in water and sediment yield and substrate composition, and channels with more resistant boundaries that typically respond to external changes over longer timespans exhibit nonlinear complex behavior. In both cases, the nonlinear behavior of rivers with numerous interdependent variables, multiple internal and external thresholds, and complex responses would be difficult to conceptualize and quantify in the absence of extensive field data. One of the management implications of complex, nonlinear behavior is that a one-size-fits-all approach to managing rivers is inadequate. Field research, initially focused on understanding specific examples of river process and form, revealed underlying patterns that give rise to conceptual models broadly applicable within fluvial geomorphology.