Geoscience Reference
In-Depth Information
of these rivers potentially responsive to changes in catchment runoff and sediment supply driven by fluctuations
in the dryland climate, but also illustrates that tectonic, structural, lithological and/or anthropogenic factors exert
influences on river character (Macklin et al. , 2010). Upstream of the gorge, the river flows through the Messara
Plain, a graben that has been infilled with Tertiary sedimentary successions (marls, flysch, sandstone) and more
recent alluvium, but within the gorge, hillslope and river gradients steepen dramatically (Figure 12.5(b)). Abrupt
changes in gorge orientation correspond with regional faults or fractures, with narrower sections and scour pools up
to several metres deep corresponding with resistant limestone outcrop (Figure 12.5(b), inset) and wider sections with
coarse gravel bars and local islands corresponding with weaker flysch outcrop. The gorge margins are characterised
by a succession of coarse-grained (predominantly cobble to boulder) and fine-grained (predominantly silt to sand)
alluvial terraces that locally interfinger with, or are overlain by, coarse colluvial and tributary river deposits.
Coarse-grained sediment is derived locally from the steep, sparsely vegetated hillsides and tributaries surrounding
the gorge, while fine material is derived principally from the Messara Plain. In this dryland climate, the lower
parts of many terraces have become strongly cemented by secondary calcite that has formed in pore spaces
within the gravels or as a replacement for the finer sediment matrix, and the terraces are not easily eroded by
the present-day, short-lived, ephemeral flows. Geochronological investigations demonstrate that these terraces date
to the Mid to Late Holocene, and provide evidence for widespread, coarse-grained aggradational episodes that
have been punctuated by incisional episodes and coarse sediment export (Figure 12.5(c)). Comparison with other
Mediterranean environmental change records, particularly high-resolution marine isotope records, suggests that
these aggradation/incisional episodes were primarily driven by fluctuations between arid and semi-arid conditions,
as reflected in a changing balance between high-energy flood events and hillslope/tributary sediment supply. By
contrast, several phases of widespread fine-grained deposition within the last 2000 years have locally capped the
lower terrace surfaces (Figure 12.5(c)). This provides evidence for decreases in flood competence, possibly coupled
with land cover and land-use changes in the Messara Plain, which represents one of the largest and most fertile
lowlands in Crete. Since the middle of the nineteenth century, several large floods have formed localised boulder
berms (Figure 12.5(c)) and have contributed to stripping of the fine-grained deposits from many parts of the gorge,
but even these floods typically have been unable to cause significant erosion of the cemented coarser-grained units
and lateral channel activity has thus been constrained (Macklin et al. , 2010).
Studies of dryland rivers in southwest Crete have also revealed various Late Quaternary alluvial deposits. In
many small limestone catchments, bedrock channels or coarse-grained, braided channels are widespread. Deposits
include boulder berm deposits that have been related to clusters of flood events occurring over the last 100-150
years or so (Figure 12.5(d)), but finer-grained deposits tend to be less common owing to the restricted extent
of suitable source material (Maas and Macklin, 2002). These findings demonstrate how river process, form and
behaviour in such small, steep, dryland catchments depends on a delicate and dynamic balance between discharge
and the volume and calibre of supplied sediment. In Crete and the Mediterranean more generally, many rivers
have been similarly sensitive to rapid Holocene climate change (Lewin, Macklin and Woodward, 1995; Macklin
and Woodward, 2009), but in most cases these climatic influences on river character and response have also been
strongly conditioned by tectonic, structural, lithological and anthropogenic factors.
The Mediterranean has also been subject to a long his-
tory of human impact, including extensive land-cover and
land-use changes, and various forms of direct flow regula-
tion and channel modification (e.g. diversion, damming,
realignment). The relative roles of climate and human
activity in determining Late Holocene histories of de-
position and erosion in Mediterranean valleys is a fo-
cus of continuing research (e.g. Benito et al. , 2008,
2010; Macklin et al. , 2010), but it is clear that the rate
and extent of human impacts have clearly intensified
over the last 100 years, so that relatively few rivers re-
man impacts, however, climate generally has been the
primary driver of 'very Late Holocene' river activity,
with most activity occurring during long-duration win-
ter flood events or more extreme summer/autumn floods
(e.g. Poesen and Hooke, 1997; Maas and Macklin, 2002;
Greenbaum, Schwartz and Bergman, 2010). Flood records
have been reconstructed from the sedimentary deposits
left by such events, such as coarse-grained (cobble and
boulder gravel) deposits (e.g. Maas and Macklin, 2002;
Laronne and Shlomi, 2007; see also the example in Box
12.1). In some catchments, fine-grained slackwater sed-
Search WWH ::




Custom Search