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We present laboratory and field evidence that in mountainous catchment-fan systems persistent alluvial fanhead aggradation and trenching may result from infrequent, large sediment inputs. We suggest that the river-fan systems along the fault-bounded range front of the western Southern Alps, New Zealand, are likely to be in a dynamic equilibrium on ≥ ≥ ≥ ≥ ≥10 3 -yr timescales, superimposed on which their fanheads undergo long-term cumulative episodic aggradation. These fanheads are active only in rare events, do not take part in the usual behaviour of the catchment-fan system and require much longer to exhibit dynamic equilibrium than the rest of the fan. These findings (1) increase our knowledge of the effects of extreme events on alluvial fan morphodynamics in humid climates, (2) question the general applicability of inferring past climatic or tectonic regimes from alluvial-fan morphology and stratigraphy and (3) provide a conceptual basis for hazard zonation on alluvial fans. HypothesisAn alluvial fan commonly develops a smooth and sometimes upwardly concave longitudinal profile from head to toe in response to upstream sediment and water inputs, and lateral confinement by topography and fan-toe position. 726 T. R. H. Davies and O. Korup Figure 2. LANDSAT ETM+ subscene (row 075, path 090; band 8; 31 December 2002) showing fault-bounded alluvial fans along the western fringe of the Southern Alps, New Zealand. Image courtesy of Global Land Cover Facility, University of Maryland. The oblique shaded relief image (above; looking east) highlights piedmont morphology with fault-bounded and moraine-constrained alluvial fans fed from steep mountain catchments. For numbers refer to the text. Equilibrium and Non-equilibrium FansUnconfined alluvial fans, e.g. those extending into large inland basins, can aggrade and grow in size indefinitely, because the fan toe is able to prograde over a low-angle surface and the volume of sediment leaving the fan toe per unit time, q so [m 3 yr −1 ], is less than the supply rate at the fanhead, q si , q so < q si .(1)Progradation and aggradation occur episodically, even under steady water and sediment inputs, building the fan up first on one side of the longitudinal axis, and then on the other (Zarn and Davies, 1994). Equilibrium geometry is never reached, although the rates of increase in fan area and elevation become smaller as the fan surface area grows. By contrast, a fan with fixed toe position (due to trimming by coastal or river erosion, for example) achieves an equilibrium geometry when fan surface gradient and morphology are such that 728 T. R. H. Davies and O. KorupFigure 3. Poerua landslide dam shortly after failure; bc = breach channel; t = terraces of reworked landslide debris. Note remnant of landslide-dammed lake upstream. Photo by G. Hancox (October 1999). This figure is available in colour online at Figure 11. Massive sandy deposits from rock-avalanche dambreak flood covering dominantly cobble-bedded active channel bed, Poerua valley, nearly reaching the level of older a...

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Glaciers and slope movements may act simultaneously to erode and modify glaciated slopes. Undercutting by glaciers can destabilize slopes but the extent to which slope failure may progress prior to subsequent glacier withdrawal has not hitherto been considered. The traditional view has been that the buttressing effect of ice prevents slope movement. The problem with this view is that ice is one-third the density of rock and flows under low applied stress. Consequently, failed slopes may move into the glacier if they exert a stress in excess of the resistance provided by the glacier. Slope movement rate depends on ice rheology and other factors influencing driving and resisting stresses. Simple viscous equations are used to investigate these variables. The equations predict that small (<125 000 m 3 ) ice-contact rockslides can deform ice at several mm/year, increasing to several m/year for very large (>10 8 m 3 ) rockslides. To test these estimates, field evidence is presented of slope movements in glaciated valleys of New Zealand; narrowing or squeezing of glaciers adjacent to unstable rock slopes is demonstrated and considered to be the result of slope movement. For one site, geomorphic mapping and slope movement monitoring data show that movement rates are of similar order of magnitude to those predicted by the viscous equations; closer agreement could be achieved with the application of modelling techniques that can more realistically model the complex slope geometries and stability factors encountered, or by obtaining additional empirical data to calibrate the models. This research implies that, while the concept of glacial debuttressing -the reduction of slope support from withdrawal of glaciers -is valid, complete debuttressing is not a prerequisite for the movement of ice-contact rock slopes. These slope movements may contribute to the erosional processes of glaciers and the evolution of glaciated slopes in a previously unrecognized way.

The hypotheses of minimum energy dissipation, minimum stream power, and minimum unit stream power are summarized and compared. Their derivation from analogies with laminar flow and linear thermodynamics is criticized on the grounds that these situations differ fundamentally from river flows, which are usually highly turbulent and strongly nonlinear. The authors' empirical hypothesis of maximum friction factor seems preferable to the minimization hypotheses because it is compatible with the known behavior of turbulent flows and nonlinear processes, it is applicable with a wider range of independent variables, and it is more in keeping with trends shown by experimental data under all constraints. The minimization hypotheses seem likely to give incorrect predictions when flow rate and depth are independent variables. The empirical success of the minimization hypotheses is confined to situations in which they predict similar behavior to the maximum friction factor hypothesis; they may be considered as special cases of this more general hypothesis.