Introduction

The retreat of glacier ice exposes land surfaces to processes that progressively modify glacial landforms, landscapes and landsystems. Such modification is often described asparaglacial, a term first defined by Church and Ryder (1972, p. 3059) as referring to 'non-glacial processes that are directly conditioned by glaciation'. Use of 'paraglacial' has, however, subsequently widened to include description of landforms and sediments as well as geomorphic processes. In this chapter the term 'paraglacial' is therefore redefined as describing 'non-glacial earth-surface processes, sediment accumulations, landforms, landsystems and landscapes that are directly conditioned by glacation and deglaciation'. This revised definition retains the essence of the original, but recognizes the more eclectic use of the term now current.

The paraglacial concept is one of adjustment of glacial landsystems to non-glacial conditions, and involves the progressive relaxation of unstable or metastable elements of the glaciated landscape to a new stable state. The concept cannot be defined by process, as all of the geomorphological processes identified as components of paraglacial adjustment operate outside glaciated areas (Eyles and Kocsis, 1989). Nor can relaxation time be considered a unifying factor, as different components of deglaciated landscapes equilibriate to non-glacial conditions over vastly different timescales (Ballantyne, 2002a, b). Paraglacial landform adjustment and the period over which this operates may, however, be conceptualized in terms of glacially conditioned sediment release. All forms of paraglacial adjustment share a common rudiment, namely that deglaciation has resulted in the exposure of unstable or metastable sediment stores that are subsequently released and reworked by a wide variety of processes over a wide range of timescales. The 'paraglacial period', defined by Church and Ryder (1972) as 'the time during which paraglacial processes occur', may thus be redefined as the timescale over which a glacially conditioned sediment source either becomes exhausted or attains stability in relation to particular reworking processes. Once this has occurred, sediment release may be envisaged as having relaxed to an 'equilibrium' or 'non-glacial' state, quantitatively indistinguishable from that which would result from primary denudation of the land surface.

For primary paraglacial systems in which glacigenic sediment sources are not replenished, the rate of sediment reworking (and thus the duration of the paraglacial period) may be approximated by an exhaustion model (Ballantyne, 2002a), in which sediment yield is related to the amount of remaining available sediment by a negative exponential function:

= S e-1t where t is time elapsed since deglaciation, St is the proportion of available sediment remaining for reworking at time t, S0, is the total available sediment at t = 0, and l is the rate of change in the loss of available sediment by release and/or stabilization (Fig. 17.1).

Secondary paraglacial systems (primarily fluvial systems) in which sediment inputs include both in situ glacigenic sediment and reworked paraglacial sediment from upstream sources may behave in an intrinsically more complex fashion. Church and Slaymaker (1989) interpreted a downstream increase in specific sediment yield in rivers draining glaciated terrain as implying a delayed peak in paraglacial sediment yield in large catchments. This concept has been developed by Harbor and Warburton (1993), who suggested that the temporal pattern of fluvial paraglacial sediment transport can be described by a family of curves, with those for the smallest (upland) catchments peaking immediately after deglaciation and those for larger basins peaking progressively later as catchment size increases (Fig. 17.2A). Ballantyne (2002a) has argued that a downstream increase in specific sediment yield is equally consistent with an exhaustion model (Fig. 17.2B), provided that initial sediment availability (S0) is greater in steep tributary basins and that the rate of change in sediment removal (l) declines with increasing catchment size, because whereas glacigenic

Figure 17.1 Exhaustion model of paraglacial sediment release, in which rate of decline in sediment release (X) is related to the proportion of 'available' sediment (St) at time (t) since deglaciation as X= ln (St) / -t. In this example X = 0.002 year-l(i.e. 0.002% of remaining 'available' sediment is released per year), 50% of initial 'available' sediment is removed in the first 345 years and 99% of 'available' sediment has been removed after 2300 years, defining the approximate length of the paraglacial period. (From Ballantyne (2002a).)

Time since déglaciation (t) in years

Figure 17.1 Exhaustion model of paraglacial sediment release, in which rate of decline in sediment release (X) is related to the proportion of 'available' sediment (St) at time (t) since deglaciation as X= ln (St) / -t. In this example X = 0.002 year-l(i.e. 0.002% of remaining 'available' sediment is released per year), 50% of initial 'available' sediment is removed in the first 345 years and 99% of 'available' sediment has been removed after 2300 years, defining the approximate length of the paraglacial period. (From Ballantyne (2002a).)

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