Emergence and pseudo-equilibrium in geomorphology

Abstract

Many geomorphic system states and behaviors often interpreted as tendencies toward establishment and maintenance of steady-state equilibrium are actually emergent outcomes of two simple principles: gradient selection and threshold-mediated modulation. The principle of gradient selection is simply that geomorphic features associated with gradient-driven flows persist and grow relative to other features and pathways. The principle of threshold-mediated modulation reflects the inherent limits on system development along any particular pathway. Thresholds not only define a restricted state space for any given geomorphic system, but may also result in oscillatory behavior around an intermediate condition that resembles fluctuations around a steady-state equilibrium. Together, these principles often produce outcomes that mimic steady-state equilibria. Examples are given involving several aspects of fluvial systems (channel profiles, alluvial channel changes, and drainage density), hillslope gradients, and barchan dunes. Steady-states indeed occur in Earth surface systems, and are a useful simplification in some models. However, the assumption that such states are somehow normative, or the only natural condition, is incorrect, and can lead to problems in geomorphic interpretations, environmental restoration and management, and conceptions of how Earth systems work.