SEDIMENTARY PROCESSES/Fluvial Geomorphology 659 Figure 10 Channel pattern types on a plot of gradient and bankfull discharge in the manner proposed in 1957 by Leopold and Wolman Their original, but smaller, dataset showed better discrimination, especially between meandering and braided channels On this plot, sandy channels occur mainly on larger rivers at lower gradients; the threshold between braiding and meandering is higher (and a little more distinct) for gravel bed channels than for sand bed channels The original Leopold and Wolman threshold is plotted here variation or localized tectonic activity leading to gradient modification But in general, stream gradients decrease and discharge increases with increasing catchment area; stream power (as the product of gradient and discharge) may peak in mid-catchment Bed grain size decreases downchannel, though this is in practice complicated by factors such as tributary sediment inputs and the existence of step changes in grain size with an abrupt gravel/sand transition Morphological trends are also apparent (Figure 11B) In headwaters, eroding channels primarily transport hillslope and bedrock erosion materials, which may be coarse (as large or larger than the depth of the channel) and unsorted by river activity Channels take the form of rapids, pools, and boulder jams Rare flood events can have dramatic and immediate effects (Figure 12) Lower down-valley, the river sorting and abrasion of transported materials and the increasing dominance of storage and alluvial reworking begin to take effect Valleys become wider and hillslopes decouple from the river at their foot In headwaters, channel widths may occupy the whole valley floor, but there is room lower downvalley for rivers to migrate across an increasing body of alluvial storage, with architectural style changing in sympathy with energy, grain size, and other factors Sediment-transporting events occur more frequently with finer sediment and steadier discharge, though channels may become more fixed in position In practice, also, the widest valleys may relate to subsiding tectonically defined sedimentation zones in which very thick alluvial (and other) sediments may accumulate, and rivers occupy or migrate across only a fraction of wide valley floors Here sediment build-up along restricted alluvial ridges allows the periodic relocation of channel belts by avulsion Developments in Time At different scales, it is possible for landforms to achieve quasi-equilibrium with formative factors, exhibit perturbed states in which forms respond and then recover from events/periods of high activity, or evolve unidirectionally towards end-states (in which a highly-organized landscape is reduced to a lowenergy and low-relief one following removal of rock mass) These states have been variously approached – initially via qualitative and theoretical proposals (such as WM Davis’ influential ‘cycle of erosion’ proposed more than a century ago, in which uplift initiated drainage development and relief reduction to