HR: 0830h
AN: H51E-1122    [PDF]
TI: Bedrock Incision Rates Through A Knickpoint Reach
AU: * Kim, J
EM: jkim@geog.gla.ac.uk
AF: Department of Geography & Geomatics, University of Glasgow, Scotland, Glasgow, G12 8QQ United Kingdom
AU: Hoey, T
EM: thoey@geog.gla.ac.uk
AF: Department of Geography & Geomatics, University of Glasgow, Scotland, Glasgow, G12 8QQ United Kingdom
AU: Bishop, P
EM: pbishop@geog.gla.ac.uk
AF: Department of Geography & Geomatics, University of Glasgow, Scotland, Glasgow, G12 8QQ United Kingdom
AU: Fifield, K
EM: keith.fifield@anu.edu.au
AF: Department of Nuclear Physics,The Australian National University, Canberra, ACT, Canberra, 0200 Australia
AU: Levchenko, V
EM: vladmir.levchenko@anu.edu.au
AF: Department of Nuclear Physics,The Australian National University, Canberra, ACT, Canberra, 0200 Australia
AB: Bedrock river incision is a fundamental process in landscape evolution, controlling, for example, the rate at which base-level fall is propagated through the drainage net and hence the rate of sediment flux from source areas to sedimentary basin ­rsinks'. Particular attention has been focused on the processes and erosion rates upstream and downstream of knickpoints. These key bedrock river landforms propagate headwards to transmit the ­rmessage­_ to the drainage net that base-level has fallen. Knickpoints may be sharply defined as waterfalls or cascades, which separate reaches controlled by different environmental settings. They may originate as a result of base-level changes, tectonic uplift, lithologic and structural controls, tributary sediment inputs or environmental changes. Information on changes in bed incision rates through a knickpoint reach provides insight into the role of knickpoints in landscape evolution. The River Etive, Scotland, has alternating alluvial and bedrock reaches and a stepped long profile caused by series of knickpoints. To investigate the controls over bedrock incision in this river, numerical simulations were performed using a process-based incision model. Physical modelling of the impact of bedload on abrasion has been used to verify the numerical simulations and cosmogenic nuclides have been used to estimate incision rates as a final test. To characterise and assess changes in incision rates through the knickpoint reach, five bedrock samples were collected for cosmogenic isotope analysis from above and below a knickpoint in the bed of the River Etive. The samples were taken from as close to the low flow water surface as practically possible and are overtopped even at moderate flows; it is therefore highly unlikely that they have ever been shielded by either peat or alluvial sediment. The sample sites have clearly been fluvially sculpted and are smooth and rounded; they are therefore highly appropriate for the analysis. Quartz was extracted from the samples and fully prepared, following standard procedures. In addition, the overall incision rates in the reach have been estimated using a bedrock incision model. The numerical model, in which incision is dominated by abrasion by saltating sediment particles, shows how interactions between sediment dynamics and hydraulic conditions determine the incision rates. Sensitivity analyses incorporating a wide range of model parameterisations confirmed that erosion rates in bedrock rivers are controlled by stream power, but changes in sediment discharge and size also play important roles in incision. The numerical simulation results are further tested through tumbling mill experiments. Topographic and sedimentological data from the River Etive are used to estimate the erosion rates in the knickpoint reach using the numerical model. The numerical simulations show that the spatial patterns of incision are changed as model parameters are changed. A conventional, diffusion style, model produced declining slopes at knickpoints, while the new model, which incorporates sediment dynamics, produced more complicated pattern of incision. The physical processes operating in knickpoint reaches determined the mode and rate of incision, and these physical processes change with location within the reach. For abrasion, changes in effective sediment flux as transport stage changes plays the critical role in determining incision rates. These conclusions will be evaluated using the cosmogenically derived incision rates.
DE: 1065 Trace elements (3670)
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting