The job of the river

Abstract

Earth scientists have traditionally conceptualized rivers and streams as geomorphic machines, whose role is to transfer sediment and to sculpt the landscape. Steady-state relationships between sediment supply and transport capacity have traditionally been considered normative in fluvial systems. Rivers are hydrological entities, however, whose function is to redis- tribute excess moisture on land. The geomorphic work of the river - erosion, transport, deposition, etc. - is a byproduct of the hydrological job of the river. There is therefore no reason to expect any particular relationship between sediment supply and transport capacity to develop as a normative condition in fluvial systems. The apparent steady-state equilibrium slope adjustments of rivers are a byproduct of four basic phenomena: (1) hydraulic selection, which favors channels and branching networks over other flux patterns; (2) water flows along the available path of least resistance; (3) energy dissipation; and (4) finite relaxation times. Recognizing converging trends of stream power or slope and sediment supply as common (but far from inevitable) side effects rather than self-regulation has important implications for interpreting and predicting fluvial systems, and for river manage- ment and restoration. Such trends are variable, transient, contingent, and far from universal. Where they occur, they are an emergent byproduct of fundamental physical mechanisms, not a goal function or attractor state. © 2009 John Wiley & Sons, Ltd.