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overflowing and are merely enlarged portions of a through-flowing river system, they
will also tend to be highly insensitive to local climatic fluctuations. Finally, is a lake
high because of high rates of precipitation over the lake basin or because of much
lower rates of evaporation related to colder or cloudier conditions?
Interpreting river sediments and landforms is equally fraught with ambiguity. Does
widespread sedimentation reflect a river no longer competent to transport its load
because of aridity in the headwaters and reduced discharge? Or does it reflect an
increase in the supply of sediment from increased erosion in the headwaters, perhaps
related to glacial and periglacial processes? Or might it represent a change from
regular perennial flow to a more seasonal flow regime? To use a river terrace to infer
a particular climate and then use the inferred climate to interpret other river terraces
is to indulge in circular argument. None of these questions is easy to answer. Each
requires accurate dating and careful scrutiny of many independent lines of evidence
for its proper resolution. Throughout this work, we emphasise the different scales at
which evidence of climatic change is to be considered, noting that the evidence is often
fragmentary. The discerning reader needs to be fully aware of the scope and limitations
inherent in the various proxies and archives used to reconstruct past changes in desert
environments. For that reason, this topic seeks to highlight the sometimes labyrinthine
chain of reasoning involved in proceeding from environmental change to climatic
change, noting that it is often more useful to know how the environment has fluctuated
than to be overly concerned about distilling some imprecise climatic signal from
inappropriate data. Tabl e 1 . 2 summarises the types of evidence used to reconstruct
past environmental change and the variable of interest in this type of investigation.
There will be many cases in which a straightforward interpretation of past events
is simply not possible with existing information. For example, it is perfectly feasible
that quite different sets of processes can lead to the formation of a particular land-
form - a concept termed equifinality - so that the landform in question does not
provide a clear signal as to how it formed. Likewise, a small initial perturbation can
often trigger a complex response , one that is often unexpected. A simple example
is strong wind scouring out a hollow in the lee of a small desert hill and eventually
reaching the local groundwater table, so that a shallow lake comes into being without
the need to invoke a wetter climate. This is easy enough to demonstrate experimentally
but harder to show in the real world, because the groundwater table may have risen
some unknown time after the deflation hollow was created. Another example of a
complex response, again demonstrated experimentally, is the creation of a multiple
set of alluvial terraces following the incision of a small channel under flume conditions
(Schumm and Parker, 1973 ). Sounding a cautionary note to those of us involved in
using river sediments to reconstruct Quaternary alluvial history, the authors of this
elegant flume experiment found that 'initial channel incision and terrace formation
were followed by deposition of an alluvial fill, braiding and lateral erosion, and then,
as the drainage system achieved stability, renewed incision followed by a low alluvial
terrace' (Schumm and Parker, 1973 ,p.99).
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