12. Channel Stability: Morphodynamics and the Morphology of Rivers

River morphology and morphodynamics are subject to governing conditions that include the volume and timing of water flows, the volume and calibre of sediment introduced into the river, the nature of bed and bank materials and vegetation, and the geologic and topographic setting of the river, including landscape gradient, climate and human interference. Water flows set the scale of the channel and the sediment regime lends distinctive character to river morphology. Consequently, rivers can usefully be classified on the basis of scale, sediment calibre and gradient, leading to the distinction of steep, intermediate and low gradient channels that generally correspond to channels of high, intermediate and low boundary roughness. Channel changes associated with the downstream passage of sediment constitute the morphodynamics of rivers. Primary morphodynamical effects include channel deformation, channel division, and channel gradation. A fundamental lesson is that rivers transporting a significant charge of bed material sediment necessarily have a lateral style of instability: lateral displacement of the channel is a normal part of their equilibrium function. Hence the required channel zone is larger than the presently active channel. Implications of this circumstance are considered in the context of channel management and river restoration.