Medium basin

Large basin

Time since déglaciation

Figure 17.2 Two explanations for the downstream increase in specific sediment yield characteristic of glaciated terrain. A) The model proposed by Harbor and Warburton (1993), in which sediment yield in the smallest catchments peaks immediately after deglaciation, but the peak of paraglacial sediment transport is progressively lagged as catchment size increases. This model predicates limited sediment release in large basins immediately after deglaciation, which appears unlikely. B) The exhaustion model proposed by Ballantyne (2002a), which assumes (1) that initial sediment availability is greatest in small upland catchments and decreases with catchment size and (2) that the rate of decline of sediment reworking is highest in small upland basins but decreases as catchment size increases. Both models predict that sediment yield is initially greatest in small basins during period t0-tl, then in medium-sized basins during t-t2, and then in large basins during t2-t3, thus accounting for a present-day increase in specific sediment yield with increasing catchment area.

Large basin

Time since déglaciation

Figure 17.2 Two explanations for the downstream increase in specific sediment yield characteristic of glaciated terrain. A) The model proposed by Harbor and Warburton (1993), in which sediment yield in the smallest catchments peaks immediately after deglaciation, but the peak of paraglacial sediment transport is progressively lagged as catchment size increases. This model predicates limited sediment release in large basins immediately after deglaciation, which appears unlikely. B) The exhaustion model proposed by Ballantyne (2002a), which assumes (1) that initial sediment availability is greatest in small upland catchments and decreases with catchment size and (2) that the rate of decline of sediment reworking is highest in small upland basins but decreases as catchment size increases. Both models predict that sediment yield is initially greatest in small basins during period t0-tl, then in medium-sized basins during t-t2, and then in large basins during t2-t3, thus accounting for a present-day increase in specific sediment yield with increasing catchment area.

sediments removed from upland tributaries are not replenished, sediment recruitment in larger catchments is sustained not only by release of in situ glacigenic deposits but also by reworked sediment supplied by tributary streams (Church and Ryder, 1972; Brooks, 1994). It seems likely that these two competing models (Figs 17.2A and 17.2B) represent end-members of a continuum of possibilities determined by initial catchment relief and particularly the initial abundance and distribution of glacigenic sediment sources.

The models of paraglacial sediment release illustrated in Figs 17.1 and 17.2 assume steady-state conditions in which there is no systematic change in process-generating mechanisms (e.g. in the magnitude and frequency of extreme rainstorm events) or other boundary conditions. This is rarely the case over millennial timescales, and consequently the actual course of paraglacial landscape response often departs from the monotonic decline in sediment release predicted by the exhaustion model (Fig. 17.3). Climatic change may affect rates of sediment release, for example through the regeneration or readvance of glaciers in mountain catchments (Brooks, 1994), accelerated retreat of mountain glaciers (Leonard, 1997) or changes in runoff regime, all of which may produce secondary peaks in sediment yield. Extreme climatic events may also trigger renewed reworking of glacigenic sediment, sometimes millennia after termination of the initial period of paraglacial adjustment (e.g. Ballantyne and Benn, 1996; Ballantyne and Whittington, 1999). In paraglacial fluvial systems, glacio-isostatic uplift may cause a fall in base levels, rejuvenating sediment release as rivers cut down into glacigenic valley-fills and paraglacial sediment stores such as debris cones and alluvial fans (Church and Ryder, 1972). Similar effects are evident on paraglacial barrier coasts where rising seas permit coastal erosion to tap pristine sources of glacigenic sediment, causing renewed pulses of reworked sediment to enter the nearshore sediment budget (Boyd et al., 1987; Forbes and Taylor, 1987; Forbes et al., 1995a, b). At all scales, therefore, the exhaustion model of paraglacial sediment release is vulnerable to extrinsic perturbation, which may produce secondary peaks of sediment transfer as sources of glacigenic sediment are mobilized or remobilized. Such effects tend to prolong or rejuvenate paraglacial sediment release. Thus, although it is possible in many contexts to identify a discrete duration for the operation of paraglacial sediment release and reworking (the paraglacial period), external perturbation may remobilize glacigenic sediment long after the period of initial adjustment has ended.

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