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Figure 10.2. Alluvial terrace exposed on the flanks of a desert dune immediately east
of the Aır Mountains, south-central Sahara.
Hugot, 1977 ; Williams et al., 1987 ) joined by an integrated drainage network (Drake
et al., 2011 ; Coulthard et al., 2013 ). The facies changes displayed by these river
sediments can throw some light on past changes in flow regime. For example, the late
Pleistocene alluvial terraces in narrow valleys draining Tibesti volcano (Hagedorn and
Jakel, 1969 ) and the eastern flanks of the northern Aır Massif ( Figure 10.2 ) (Williams,
1971 ; Williams, 1973b ; Williams et al., 1987 ; Williams, 2008 ) consist of coarse sands
and gravels at the base, fining upwards into finely laminated silts and clays, often
containing unbroken freshwater gastropods (see Chapter 16 ). The gravel facies is
indicative of episodic high-energy flow, while the fine-grained horizontally bedded
silts and clays represent former flood-plain sediments laid down under a low-energy
flow regime (Williams, 2008 ). However, we need to bear in mind that the rivers which
flow into or, more rarely, across deserts are not always good indicators of local climatic
conditions but may instead reflect environmental conditions in their headwaters.
Wasson ( 1996 , p. 6) has pointed out that, although it is useful to compile global
inventories of present-day river discharge and sediment load, the modern records are
too short to provide robust insights into the sensitivity of river systems to climatic
and land-use changes. For this we need the longer-term records provided by a more
comprehensive analysis of river history (Macklin et al., 2012 ; Vita-Finzi, 2012a ; Vita-
Finzi, 2012b ; Williams, 2012a ). The aim of this chapter is therefore to consider the
scope and limitations of using river sediments and fluvial landforms as indicators
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