660 SEDIMENTARY PROCESSES/Fluvial Geomorphology Figure 11 Trends in river catchments (A) in hydraulic factors, based on concepts developed by SA Schumm (in 1977) and M Church (in 2002), and (B) in channel types and activities achieve an end-state ‘peneplain’) More recently, quantitative modelling has allowed exploration of such developments Figure 13 illustrates models of drainage evolution; the results can be compared with patterns achieved over very long time-scales in natural landscapes Such studies may use ‘process laws’ (based on empirical and/or physically justified relationships) applied iteratively to construct evolving forms These may be shown to achieve stable pattern states during experimental runs, even though the prediction of individual forms may not be possible (as in the case of individual river bend form and location) Though unidirectional trajectories in forms have been proposed for higher level landforms, at intermediate scales there has been great concern with perturbations produced by episodic changes in environments (response to a rise or fall in sea-level, tectonic effects, or climate fluctuations that may alter river discharges, sediments, and vegetation cover) Such ‘external’ changes have also been produced by human agency (including the effects of deforestation and agriculture, mineral extraction, urbanization, and river impounding and channelization) Perturbations over decades to centuries have produced effects that have been closely studied, and these may be ameliorated by good management Fluctuations in the rate of catchment erosion and local rates of sediment transfer can result in phases of sediment accumulation and storage, but also of river incision and stored sediment removal Enhanced delivery may follow climatic change, tectonic uplift, or Figure 12 Results of an extreme flood in Big Thompson Canyon, Colorado, USA, in July 1976 Photograph by WC Bradley