Geomorphological discontinuities and ecological organisation : a case study of the River Drôme

• Main Author: Toone, Julia
• Published: Loughborough University 2009
• Subjects: 551.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.519944

River 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.