FLUVIAL MORPHOLOGY AND SEDIMENTS: ARCHIVES OF PAST FLUVIAL SYSTEM RESPONSE TO GLOBAL CHANGE

For many decades it has been known that rivers respond to a number of stimuli or forcing functions which act over a variety of temporal and spatial scales (e.g. Schumm and Lichty, 1965). Over the longer term (i.e. 10³-10⁶ years) geology and climate may play the dominant role in fluvial system behaviour, generating basin-wide sediment-landform assemblages. Changes in these variables exert control over sediment and water supply and thus often promote system adjustment. Runoff and sediment supply, however, vary also over the shorter-term (i.e. 10¹ - 10² years) as a result of sediment exhaustion and starvation, and changes in flood frequency and magnitude. In the context of immediate future global change it is perhaps these latter mechanisms that appear to be of most obvious concern however, in many areas, particularly the mid-latitudes, basins sculptured by Pleistocene glaciation may still contain rivers adjusting to longer-term processes.

One approach to understanding how rivers might respond to any given future change scenario is to look at the record of past long-term behaviour. These changes are archived within the sediment-landform assemblages of the river basin and their investigation requires consideration of both the morphological and sedimentological attributes of this archive. Unfortunately establishing the relative importance of forcing functions acting at different time and spatial scales within the fluvial sedimentary record can be extremely difficult, especially as the changes observed within the sedimentary record are often very similar. This paper will illustrate how recent advances in methodology, and in particular the increasing availability of geochronology, may help to decipher these records with higher temporal resolution, allowing more accurate models of fluvial system behaviour to be erected, especially with respect to establishing cause-effect relationships. Such models can provide valuable insight into the future consequences of global change on individual river system behaviour.