Thesis-2009-Toone.pdf (12.55 MB)
Geomorphological discontinuities and ecological organisation: a case study of the River Drôme
thesis
posted on 2010-09-09, 07:33 authored by Julia TooneRiver channel changes, and patterns of in-stream macroinvertebrate community
organisation, are both well explored in alluvial channels. Less is known about the behaviour of
mixed bedrock-alluvial rivers, and their patterns of macroinvertebrate community structure. In
response to relatively recent, widespread acknowledgment that sustainable and holistic river
channel management is best achieved by a strategy that accounts for large-scale controls on
long-term channel behaviour, there is a clear need to identify how spatial and temporal
variation in rates and patterns of channel adjustment constrain the long-term evolution of
mixed bedrock-alluvial channels, and their in-stream macroinvertebrate ecology.
The River Drôme in south-eastern France is a fascinating example of river channel
change in response to a complex history of natural and anthropogenic disturbances. Previous
work has documented a long-term trajectory of channel degradation, but a reach linking the
upper and lower sections of the river has not been explored and is of particular interest
because of its striking mixed bedrock-alluvial morphology. Over five kilometres the channel is
characterised by abrupt changes in style and substrate that naturally divide the reach into six
geomorphic zones. These indicate that long-term degradation is not a simple, linear evolution
but a complex non-linear process. The distinct pattern of geomorphological zonation along this
reach forms the central focus of this thesis, specifically in terms of its historical development,
present-day macroinvertebrate community structure, and future evolution.
The first part of this thesis documents decadal changes in channel morphology
between 1948 and 2006 by using GIS to analyse eight series of high-altitude aerial
photography. Field work in 2005 and 2006 was used to ground truth observations. Changes in
longitudinal channel profile are derived from topographic surveys from 1928, 2003 and 2005.
There have been modest overall amounts of channel constriction, narrowing and incision, but
there is substantial spatial and temporal variability in these trends. The present-day
configuration of wide alluvial, and narrow bedrock zones has developed in response to a
particular sequence of natural and anthropogenic events, emphasising the importance of
contingency and channel sensitivity to disturbance. The reach is located 200 m downstream
from le Béoux, a tributary to the Drôme, and historical changes in this sub-catchment have
been manifest in the long-term behaviour of the reach. A significant flood in 1978 is the likely
catalyst of present-day zonation, which locked the reach into a geometry that has constrained
subsequent channel adjustments. While zonation remained distinct as recently as 2006, there
is evidence that the channel is now operating under different boundary conditions to those
that produced and maintained zonation before 2001. It is clear that predictions of future
changes in channel morphology require a long-term perspective of channel behaviour,
patterns and connectivities through time.
The second part of this thesis characterises the present-day macroinvertebrate
community of the study reach. Previous work in alluvial channels has considered how
discontinuity in channel morphology influences spatial variation in patterns of
macroinvertebrate organisation, but this has not been extended to mixed bedrock-alluvial
channels. Relatively few studies have considered how spatial differences in channel behaviour
(rather than simply channel character) may drive macroinvertebrate community organisation.
Associations between channel morphology and macroinvertebrate community structure were
tested by fieldwork carried out under low-flow conditions in 2006. Fifteen quantitative surber
samples were collected at each of 10 sites along the reach. Local habitat characteristics,
including particulate organic matter, bed sediment grain size (surface and subsurface), water
quality (conductivity, temperature, pH) and flow velocity (benthic and mean) were also
measured. Results indicate that the mass of particulate organic matter and the diversity of
both surface and sub-surface grain size influence patterns of macroinvertebrate community
structure along the reach. It is also apparent that zone-scale differences in flow and sediment
regime, and differences in lateral and vertical channel dynamics are important. Variation in the
frequency and intensity of bedload dispersal, which is a result of zone-scale variation in
channel bed structure, is a dominant control on spatial patterns of macroinvertebrate
community structure. These dynamics are the product of, and maintained by, the particular
geography and history of the study reach, emphasising the primacy of place and the
importance of understanding how larger-scale morphological processes constrain smallerscale
patterns of macroinvertebrate biodiversity.
At present, there are plans to manage channel degradation, along the study reach and
in downstream parts of the river Drôme, by restoring sediment supply to the channel. This will
be achieved by the remobilisation of bedload in the Béoux sub-catchment and so it is likely
that there will be future changes to the present-day morphology and macroinvertebrate
community organisation in the study reach. Combining the findings of parts one and two, the
third part of this thesis evaluates the likely impacts of this management strategy. It is
suggested that if the channel is connected to its active floodplain, and receives a sufficient
external supply of bedload, then the reach-scale dynamic of zonation is self-regulating.
Successful management intervention relies upon an understanding of this behaviour.
Compared to predominantly bedrock and predominantly alluvial sections of the reach, the
mixed bedrock-alluvial mid-reach supports significantly more diverse macroinvertebrate
communities and higher proportions of EPT taxa. These characteristics are a function of the
morphological diversity of the mid-reach, which results from the particular configuration and
the connectivity of the present-day zones. It is considered that if future bedload supply to the
channel is not deficient or excessive to present-day flow and sediment regimes, then the
present-day morphological diversity of the reach could be maintained. This would also
maintain present-day patterns of macroinvertebrate community structure. It is, therefore,
hypothesised that if the reintroduction of bedload from the Béoux secures the reach with a
moderate volume and frequent supply of sediment, then this drip feed would be a desirable
management strategy.
History
School
- Social Sciences
Department
- Geography and Environment
Publisher
© Julia ToonePublication date
2009Notes
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.EThOS Persistent ID
uk.bl.ethos.519944Language
- en