SEDIMENTARY PROCESSES/Fluvial Geomorphology 661 Figure 13 Models of river network development by (A) seepage erosion and (B) a numerical simulation of headward growth (A) Reproduced with permission from Dunne T (1980) Formation and controls of channel networks Progress in Physical Geography 4: 211 239 (B) Reproduced with permission from Willgoose GR, Bras RL, and Rodriguez Iturbe I (1991) Results from a new model of river basin evolution Earth Surface Processes and Landforms 16: 237 254 human activity over a range of time-scales Channel incision into alluvial deposits or planar bedrock valley floors may produce terraces and inset channels and floodplains Incision episodes have been related to periods of decreased sediment supply from upstream, to tectonic river steepening, and to sea-level fall Characteristically, channel steepening tends to propagate up-valley, with a knickpoint or zone moving headward at the expense of the former low-gradient valley floor or floodplain Downcutting streams can produce terraces at a variety of levels as they migrate (‘unpaired terraces’), whereas episodes associated with the occurrence of forcing events may give ‘paired’ or regionally developed terraces that can be matched over long distances Major research efforts have been devoted to relating Quaternary climatic fluctuations (as in glacial–interglacial cycles) to the flights of terraces associated with many mid-latitude rivers, and to the relationships with cyclical sea-level change over rather longer time-scales (see Figure 14) Explanatory models have gradually become more complex as the interrelationships between climatic and vegetation change, sea-level fluctuation and tectonics, and river channel activity have been explored Studying the effects of environmental change, whether human or natural, has been greatly aided by improving dating techniques for alluvial sediments and by the availability of historical maps and remote-sensing imagery Episodes of activity can be pinpointed over appropriate time-scales so that, for example, episodes of Holocene and to some extent Pleistocene river activity can be reasonably closely identified However, alluvial sedimentation is seldom continuous; rivers also rework their own materials to the extent that what is left is only a ‘partial archive of remnants’ that leaves much activity unrecorded Nevertheless, evidence is becoming available to allow change/evolutionary models to be compared not simply with static patterns (for example, drainage networks or meander trains) but