The fluvial record of climate change
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Geography, School of Environment and Development, University of Manchester
,
Oxford Road, Manchester M13 9PL
,
UK
1
Institute of Geography and Earth Sciences, Aberystwyth University
,
Penglais, Ceredigion SY23 3DB
,
UK
Fluvial landforms and sediments can be used to reconstruct past hydrological conditions over different time scales once allowance has been made for tectonic, base-level and human complications. Field stratigraphic evidence is explored here at three time scales: the later Pleistocene, the Holocene, and the historical and instrumental period. New data from a range of field studies demonstrate that Croll-Milankovitch forcing, DansgaardOeschger and Heinrich events, enhanced monsoon circulation, millennial- to centennialscale climate variability within the Holocene (probably associated with solar forcing and deep ocean circulation) and flood-event variability in recent centuries can all be discerned in the fluvial record. Although very significant advances have been made in river system and climate change research in recent years, the potential of fluvial palaeohydrology has yet to be fully realized, to the detriment of climatology, public health, resource management and river engineering.
1. Introduction
The impacts of long-term and large-scale Quaternary climatic change on river
system dynamics have been scientifically studied for more than 50 years. Over
the last decade in particular, there have been significant empirical, methodological
and technological advances. Some studies have pointed to matches between
climate and the record afforded by fluvial sediments [13], and others to the
complicated behaviour of river systems in processing climatic events [4,5],
while investigations based on numerical and physical modelling have suggested
that fluvial sediment transport processes can completely destroy environmental
signals [6,7]. Such new and contrasted perspectives, and field evidence, make it
timely to review critically the ways in which alluvial systems may in practice
respond to and record climatic changes. Section 2 of this paper considers
key concepts, opportunities and challenges underlying climatefluvial system
relationships. In 3; field evidence for climatic change impacts from fluvial
sediments and landforms is reviewed through focusing on three time scales: the
later Pleistocene, the Holocene prior to documentary records; and the historical
period, during which better dating resolution and particular sedimentary contexts
in some cases allow individual flood events to be identified and related to
instrumental climate records. We conclude with a consideration of future climate
change impacts and why a longer-term perspective on river responses to climatic
instability should be attempted in order to aid catchment management in the
twenty-first century.
2. Concepts, challenges and opportunities
A change in the frequency and magnitude of floods is the main direct driver that
determines the response of river systems to climatic change. This results from
altered precipitation and runoff regimes, and it is coupled with variation in the
input of sediments from channel-way and slope domains (figure 1). Factors that
alter erodibility and sediment supply, such as riparian and catchment vegetation,
frost action and permafrost melt, as well as the short- and long-term build-up
and decay of glaciers and ice sheets, also play a key role in river system sediment
dynamics. There is also the nature of river catchments affected by climate change
to consider, because their individual hydraulic systems and morphologies may
process flood-producing events differently [4]. Alluvial systems can locally both
record the history of climate change in the form of sedimentary signals and
sequences (together with biological indicators of climate) and respond to climatic
change by adjusting gross sediment throughput and geomorphological character,
including channel dimensions, change rates and patterns. This may involve net
changes in the form and quantity of valley sediment storage through aggradation
or lateral dispersal, or alluvial incision and bedrock erosion. In time, change may
be propagated through entire fluvial systems, transforming floodplain fluvial
style or alluvial architecture ensemble, river long profiles and the extent of
channel networks [3].
In practice, however, there are four things that make the effects of climate
more difficult to determine in fluvial sedimentary and landform records. First,
there are non-climatic factors that may also be simultaneously changing. These
include human land use and river management (which can modify both sediment
inputs and river flow regime), tectonic activity (which can prompt river incision
or accelerated sediment input) and base-level effects related to falling or rising
sea or lake levels. It is not at all easy to separate out these effects in the many
situations where there is interplay between these factors but, as we argue later,
considerable progres (...truncated)