Avulsion: The relative roles of autogenic and allogenic processes

Avulsion, the abandonment of all or part of a channel belt in favor of a new course, is controlled by both autogenic (intrabasinal) and allogenic (extrabasinal) processes. However, their relative roles are not known well. 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) allogenic and autogenic processes influencing different aspects of avulsion, (2) the relative importance of these processes in time and space, and (3) observations that can be used to differentiate autogenic and allogenic causes of avulsion. The following avulsion parameters are quantified, and their allogenic and/or autogenic nature are discussed: period of activity of channel belts (period between beginning and end of overbank sedimentation by the river channel), interavulsion period (time period between successive avulsions of a channel belt), avulsion frequency (number of avulsions per time interval in a given area), and avulsion location. In the RhineMeuse delta, the period of activity of the Holocene channel belts varies considerably, but shows no significant trend over time, whereas extrabasinal factors (climate, base-level, and tectonics) changed. This suggests that this parameter is mainly autogenic 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. Seven avulsion sequences (as predicted by the Mackey and Bridge [Mackey, S.D., Bridge, J.S., 1995. Three-dimensional model of alluvial stratigraphy: theory and application. Journal of Sedimentary Research B65 (1), pp. 7-31.] model) may be recorded in the Holocene dataset of the Rhine-Meuse, suggesting a periodicity of similar to 500-600 yr. Each avulsion sequence coincides with a peak in the avulsion frequency. The periodicity of similar to 500 yr in the avulsion frequency is proposed to be autogenic. However, there is also 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 not randomly distributed over the delta, and are related to the allogenic factors: relative sea level rise, local tectonics, and changes in discharge and sediment load. (c) 2007 Elsevier B.V. All rights reserved.