Drainage rearrangement, involving stream piracy (capture), drainage diversion and/or beheading, may be significant for sediment budgets (including sediment provenance) and biotic distributions, as well as for its more usually considered role in landscape evolution. The processes involved in drainage rearrangement are not as self-evident as its abundant literature indicates. This is especially the case with the commonly invoked stream capture. The key process in stream capture, namely, drainage head retreat, is difficult to envisage as a normal part of drainage net evolution, especially in the light of recent findings on drainage hollow evolution. Stream capture may therefore be a relatively rare event in drainage net evolution. This, and uncertainties with interpretations of supposed elbows of capture, mean that stream capture should not be routinely invoked in interpretations of long-term drainage evolution. Further uncertainties associated with the maintenance of drainage lines during the erosion of significant crustal sections, especially in faulted and folded terrains, diminish the likelihood of many supposed examples of stream capture. It is more likely that examples of drainage rearrangement attributed to stream capture were generated by drainage diversion, but even this may involve special conditions.
Knickpoint behaviour is a key to understanding both the landscape responses to a base-level fall and the corresponding sediment fluxes from rejuvenated catchments, and must be accommodated in numerical models of large-scale landscape evolution. Knickpoint recession in streams draining to glacio-isostatically uplifted shorelines in eastern Scotland is used to assess whether knickpoint recession is a function of discharge (here represented by its surrogate, catchment area). Knickpoints are identified using DS plots (log slope versus log downstream distance). A statistically significant power relationship is found between distance of headward recession and catchment area. Such knickpoint recession data may be used to determine the values of m and n in the stream power law, E = = = = = KA m S n . The data have too many uncertainties, however, to judge definitively whether they are consistent with m = = = = = n = = = = = 1 (bedrock erosion is proportional to stream power and KPs should be maintained and propagate headwards) or m = = = = = 0·3, n = = = = = 0·7 (bedrock incision is proportional to shear stress and KPs do not propagate but degrade in place by rotation or replacement). Nonetheless, the E Scotland m and n values point to the dominance of catchment area (discharge) in determining knickpoint retreat rates and are therefore more consistent with the stream power law formulation in which bedrock erosion is proportional to stream power.
Research in landscape evolution over millions to tens of millions of years slowed considerably in the mid-20th century, when Davisian and other approaches to geomorphology were replaced by functional, morphometric and ultimately process-based approaches. Hack's scheme of dynamic equilibrium in landscape evolution was perhaps the major theoretical contribution to long-term landscape evolution between the 1950s and about 1990, but it essentially 'looked back' to Davis for its springboard to a viewpoint contrary to that of Davis, as did less widely known schemes, such as Crickmay's hypothesis of unequal activity. Since about 1990, the field of long-term landscape evolution has blossomed again, stimulated by the plate tectonics revolution and its re-forging of the link between tectonics and topography, and by the development of numerical models that explore the links between tectonic processes and surface processes. This numerical modelling of landscape evolution has been built around formulation of bedrock river processes and slope processes, and has mostly focused on high-elevation passive continental margins and convergent zones; these models now routinely include flexural and denudational isostasy.Major breakthroughs in analytical and geochronological techniques have been of profound relevance to all of the above. Low-temperature thermochronology, and in particular apatite fission track analysis and (U-Th)/He analysis in apatite, have enabled rates of rock uplift and denudational exhumation from relatively shallow crustal depths (up to about 4 km) to be determined directly from, in effect, rock hand specimens. In a few situations, (U-Th)/He analysis has been used to determine the antiquity of major, long-wavelength topography. Cosmogenic isotope analysis has enabled the determination of the 'ages' of bedrock and sedimentary surfaces, and/or the rates of denudation of these surfaces. These latter advances represent in some ways a 'holy grail' in geomorphology in that they enable determination of 'dates and rates' of geomorphological processes directly from rock surfaces. The increasing availability of analytical techniques such as cosmogenic isotope analysis should mean that much larger data sets become possible and lead to more sophisticated analyses, such as probability density functions (PDFs) of cosmogenic ages and even of cosmogenic isotope concentrations (CICs). PDFs of isotope concentrations must be a function of catchment area geomorphology (including tectonics) and it is at least theoretically possible to infer aspects of source area geomorphology and geomorphological processes from PDFs of CICs in sediments ('detrital CICs'). Thus it may be possible to use PDFs of detrital CICs in basin sediments as a tool to infer aspects of the sediments' source area geomorphology and tectonics, complementing the standard sedimentological textural and compositional approaches to such issues.One of the most stimulating of recent conceptual advances has followed the considerations of the relationships between te...
[1] We have used early Miocene valley-filling basalts to reconstruct fluvial long profiles in the Upper Lachlan catchment, SE Australia, in order to use these as well-constrained initial conditions in a forward model of fluvial incision. Many different fluvial incision algorithms have been proposed, and it is not clear at present which one of these best captures the behavior of bedrock rivers. We test five different formulations; the ability of these models to reproduce the observed present-day stream profiles and amounts of incision is assessed using a weighted-mean misfit criterion as well as the structure of the misfit function. The results show that for all models, parameter combinations can be found that reproduce the amounts of incision reasonably well. However, for some models, these best fit parameter combinations do not seem to have a physical significance, whereas for some others, best fit parameter combinations are such that the models tend to mimic the behavior of other models. Overall, best fit model predictions are obtained for a detachment-limited stream power model or an ''undercapacity'' model that includes a river width term that varies as a function of drainage area. The uncertainty in initial conditions does not have a strong impact on model outcomes. The model results suggest, however, that lithological variation may be responsible for variations in parameter values of a factor of 3-5.INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 1815 Hydrology: Erosion and sedimentation; 3210 Mathematical Geophysics: Modeling; 9330 Information Related to Geographic Region: Australia; KEYWORDS: landscape evolution, bedrock rivers, fluvial incision models, river long-profile development, SE Australia Citation: van der Beek, P., and P. Bishop, Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models,
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