Death of an anabranch: Reconstruction of a Holocene avulsion

Anabranching rivers are a globally prevalent planform occurring in a range of climates, valley gradients, and floodplain grain size distributions. The multichannel planform is maintained by the formation of new channels through the avulsion process. There is little information regarding the avulsion process and how long a river takes to develop and complete an avulsion. The avulsion cycle can be described using three terms: avulsion frequency, avulsion duration, and the inter avulsion period. The avulsion frequency describes how often an avulsion occurs on a particular reach of river, the avulsion duration describes how long it takes for an avulsion to develop and become the dominant channel, and the inter avulsion period describes how long it takes for the river to set up for the next avulsion to occur. Schumm et al. (1996) uses a five stage evolutionary model to conceptually describe how the planform (i.e. sinuosity) changes from the initial development of an avulsion through to the point where a new avulsion is once again forming on the floodplain. Their five stage model effectively defines the first two stages as the avulsion duration, and the last three as the inter avulsion period. The combination of all five stages defines the avulsion frequency. However, their conceptual model does not provide stratigraphic or geomorphic evidence of transitions from one stage to the next, nor do they discuss the length of each stage. We hypothesize that there is stratigraphic evidence of the avulsion cycle stored in the paleochannels and cutoffs in the abandoned alluvial ridge. We use a series of strategically collected cores from the abandoned alluvial ridge to reconstruct the length of each evolutionary stage and to provide stratigraphic evidence of the transition from one stage to the next. We used Optical Stimulated Luminescence (OSL) dates on specific stratigraphic units that related to each evolutionary stage in the conceptual model. Our results provide an empirical data point regarding the length of the avulsion duration, inter avulsion period, and avulsion frequency that can potentially aid in validation of basin infill models. Additionally the stratigraphic evidence from the cores and the dates provides researchers and managers with a better understanding in the lifecycle of an avulsion.