Origin of anastomosis, upper Columbia River, British Columbia, Canada

Abstract

The upper Columbia River is an anastomosing river. Anastomosing rivers are rivers with multiple, interconnected channels that enclose floodbasins. The origin of anastomosis of the upper Columbia River is a matter of debate. It has been hypothesized that excessive sediment supply causes anastomosis. The aim of this research is to investigate this hypothesis. To do this, the geological and lithological composition of the Columbia Valley fill is investigated. This study brings the results of Makaske et al. (2002, 2009) together with new field data. Two floodplain-wide borehole cross-sections are made during this research, one in an upper highly anastomosing reach and one in a lower weakly anastomosing reach; in order to investigate if the long-term floodplain sedimentation rate in the highly anastomosing reach is higher than in the weakly anastomosing reach. Time control is based on AMS radiocarbon dating of macrofossils, wherefrom sediment budgets are derived. These results are compared with bed load transport predictions made with the present-day river characteristics. The results of these cross-sectional sediment budget analysis pointed out that the deposits in the upper reach consist of more bed-material load than in the lower reach and that the long-term floodplain sedimentation rate in the upper reach is higher than in the lower reach. This is confirmed by the long-term floodplain sedimentation rates calculated with the AMS radiocarbon dates. However, bed load transport predictions show erosion in the upper anastomosing reach and deposition in the lower anastomosing reach. The origin of anastomosis of the upper Columbia River is hence interpreted to be a combination of frequent avulsions and slow abandonment of old channels, whereby avulsions are triggered by excessive sediment supply pulses.