Climate and Tectonics: Why Self-organized River Channels Just Don't Care (that much)

Abstract:

Many studies seeking to link climatic and tectonic drivers to fluvial records start with the assumption that such links exist, and that our failure to identify these links is due to either noisy data or confounding factors. This study begins from a different perspective: the robust hydraulic-geometry scaling relations of both alluvial and bedrock rivers suggests an insensitivity of these systems to climatic and tectonic drivers. The pattern stability of river channels, in the face of an onslaught of stochastic driving over a wide range of scales, becomes the feature in need of explanation. To explore this question in a constrained manner, we examine the case of gravel-bedded (d > 10 mm) rivers where one may reasonably suggest that bed-load transport is the dominant means of channel adjustment and erosion. We examined data from 188 rivers across the USA, covering a wide range of climatic, tectonic and bedrock controls. Almost without exception, the rivers exhibit two remarkable features: (1) they are organized such that the bankfull fluid stress is slightly in excess of the critical value; and (2) the distribution of fluid stresses in excess of critical follows a common exponential-decay function. Although discharge regimes and distributions for these rivers vary widely as a function of climate (as expected), the fluid-stress distribution is universal. We surmise that the generic self-organization of a river channel to a near-critical condition acts to decouple important aspects of sediment transport from its regional climate. This "critical filter" suggests that finding strong signatures of climate and tectonics in river morphology and erosion should be the exception, rather than the rule. We anticipate and invite criticism of this view. Results imply that landscape evolution models can get by with a simple treatment of climate, and may use a constant Shields stress condition as a channel closure scheme even for bedrock-influenced rivers.