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Alluvial sedimentation styles


Modern studies have shown that many different styles of alluvial sedimentation may be distinguished dependent on such factors as stream energy, the calibre and load of transported sediment and the effects of streamside vegetation.  Alluvial streams comprise a spatial complex of environments involving different combinations of in-channel and floodplain erosional and depositional activities and rates.   For present purposes, it is important to note that the relative depths, widths and number of active channels vary between pattern types, as do the extents to which laterally shifting channels remove and replace prior alluvial deposits on a routine basis.  The figure 1 represents in section five types of alluvial system.  The relative dimensions and number of channels are shown schematically, as are the lateral channel shift (and thus replacement) zones in different sedimentation styles.

 

Preservation potentials contrast under different alluvial styles.  With braiding, frequent channel shifts and lateral accretion / erosion activity (including bar and meander bend sedimentation and channel erosion) may replace near-surface sediments across valley floors within decades.  With active meandering, replacement of floodplain and channel facies by lateral channel movement may involve reworking on a scale of centuries, but also the slow build-up of overbank facies.  In both inactive meandering and anastomosing channels there may be very little lateral reworking, though avulsion may relocate channel / levée zones within floodplains, especially as channel aggradation leads to a raising of channel levels.

When one style of sedimentation is succeeded by another (generally in response to changes in river discharge, sediment load or bankside vegetation), suites of sedimentation niches and of work-over rates are also transformed – but at variable rates.  A change to braiding is highly destructive of prior alluvial bodies given intrinisic workover rates and the laterally-extensive pattern of channel shift.  By contrast, a change from braiding to inactive meandering may have quite different implications.  Prior braiding phenomena may be relatively unaffected for significant proportions of valley floors, where braiding topography will remain and condition over-bank sedimentation.  The transforming effects of a change in style are also likely to take longer to achieve by combined processes of over-bank sedimentation and relatively minor channel relocation.

Given these kinds of consideration, it is possible to re-examine the Quaternary alluvial record from a broadly sedimentological perspective to see whether the presence or absence of alluvial units in particular time periods and locations can be meaningfully interpreted in terms of sequences of sedimentation styles.  It should be stressed that this involves no assumptions about tectonic activity nor overall aggradation which may lead to elevation, incision and the stacking of alluvial sequences.  Rather the focus is on ‘working depth’ (the deepest pool to flood level) and lateral and vertical dimensions of accreting and sedimenting units.


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