HR: 16:30h
AN: H54A-03    [Abstracts]
TI: Avulsion and Bifurcation Stability; the Relative Roles of Internal and External Processes in a Holocene Deltaic Distributary Network
AU: * Stouthamer, E
EM: E.Stouthamer@geo.uu.nl
AF: Utrecht University, Faculty of Geosciences, Department of Physical Geography, PO Box 80.115, Utrecht, 3508 TC, Netherlands
AU: Berendsen, H J
AF: [deceased]
AB: Avulsion, the abandonment of all or part of a channel belt in favor of a new course, and hence bifurcation stability, is controlled by both internal and external processes. External controls comprise the boundary conditions that are unaffected by local evolution (climate, base level, tectonics). Internal controls evolve due to internal system dynamics (e.g. bar and meander dynamics, delta lobe switching). The Rhine-Meuse delta is without doubt the best studied delta regarding Holocene avulsion processes. Based on studies of this delta, an overview is given of 1) external and internal processes influencing different aspects of avulsion and bifurcation stability, 2) the relative importance of these processes in time and space, and 3) observations that can be used to differentiate internal and external causes of avulsion. The following avulsion parameters are quantified, and their external and/or internal nature are discussed: period of activity of channel belts (period between beginning and ending sedimentation of the river channel), interavulsion period (time between successive avulsions of a channel belt), avulsion duration (time between initiation of a new channel and complete abandonment of the previous channel=bifurcation stability), avulsion frequency (number of avulsions per time interval in a given area), and avulsion location. In the Rhine-Meuse delta, the period of activity of the Holocene channel belts varies considerably, but shows no significant trend over time, whereas external factors changed. This suggests that this parameter is mainly internally controlled. The average interavulsion period increased from 8000 cal yr BP to 2800 cal yr BP, and decreased since then. Significant fluctuations occurred on a shorter time scale. A maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr BP. This was a time, when large channel belts gradually came into existence. During this period the avulsion frequency also reached a maximum. The long-term increasing trend in interavulsion period is related to the decreasing rate of sea level rise. The decreasing interavulsion period since 2800 cal yr BP can be explained by increased discharge and sedimentation, resulting in an increase in the number of avulsions. The avulsion duration fluctuates between less than 200 and 1250 cal years and averages 335 cal years. The avulsion duration shows no significant trend over time and remained constant until at least 1900 cal yr BP. During an avulsion sequence, avulsion sites shift progressively upstream with a simultaneous decrease in interavulsion period. The sequences can be explained as a result of continued growth of alluvial ridges and increasing cross-valley slopes upstream of avulsion locations. New channel belt segments down-valley from avulsion locations have low natural levees and a low probability of avulsion; therefore avulsion sites tend to shift upstream until the apex of the delta is reached. The next avulsion can then occur far downstream again (Mackey & Bridge, 1995). Seven avulsion sequences may be present in the dataset of the Holocene Rhine-Meuse, suggesting a periodicity of ~500-600 yr. Each avulsion sequence coincides with a peak in the avulsion frequency. The periodicity of ~500 years in the avulsion frequency is proposed to be internally controlled. However, there also is a long-term trend in the avulsion frequency that can be related to sea level rise, and (after 3000 C-14 yr BP) to climate change and human influence. Avulsion locations are related to the external factors relative sea level rise, local tectonics, and changes in discharge and sediment load.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting