Network dynamics and origin of anastomosis, upper Columbia River, British Columbia, Canada.
Haas, T. de
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The upper Columbia River is an anastomosing river. Anastomosing rivers consist of a network of channels enclosing floodbasins. The factors that determine the configuration of river networks such as the upper Columbia River are unknown. Additionally the origin of anastomosing rivers is unclear. Two main theories exist on the origin of anastomosis; Nanson and Knighton (1996) state that it is hydraulically more efficient to transport flow and sediment in multiple channels instead of a single channel, for rivers that are unable to adjust their slope. Makaske (2001) states that frequent avulsion and/or slow abandonment of old channels leads to continuing coexistence of multiple channels. The Nanson and Knighton (1996) hypothesis on the origin of anastomosis implicates that anastomosis is an equilibrium channel pattern, whereas Makaske (2001) states that it is a disequilibrium channel pattern. The aim of this research was to explain the network dynamics and origin of anastomosis in the upper Columbia River. Field measurements were performed in the upper Columbia River to collect model input and to test model concepts. A river network model was developed to address the factors that determine the configuration of river networks, and to test the current hypotheses on the origin of anastomosis. The discharge distribution through a river network is mainly determined by the network topology. Additionally, the bifurcation (morpho)dynamics affect the discharge distribution through a river network. The bifurcation dynamics are determined by downstream slope differences between bifurcates, upstream bend effect and amount of downstream confluences and bifurcations. When a constant Chézy roughness is assumed to represent bed roughness, it is more efficient to transport flow and sediment in multiple channels than in a single channel. When a constant Nikuradse roughness length is assumed to represent bed roughness, it is more efficient to transport flow and sediment in a single channel. The effect on network configuration is largest for the latter roughness assumption. Additionally, in natural rivers the constant Nikuradse roughness length assumption is more plausible since the wall friction is relatively large in small channels compared to large channels. Hence the Nanson and Knighton (1996) hypothesis on the origin of anastomosis does not hold for the upper Columbia River. A network model is applied to a 21km reach of the upper Columbia River that is characterized by well developed anastomosis. The steepest part of this reach is characterized by the best developed anastomosis. Model results indicate that a sediment pulse of 150yr of on average 3 times the transport capacity is able to reproduce the bed slope of this steep reach. It follows that the Makaske (2001) hypothesis on the origin of anastomosis explains anastomosis in the upper Columbia River. Sediment pulses induce a high avulsion rate, and therefore cause the coexistence of multiple channels on the floodplain. In the absence of a new sediment pulse the river network will most likely evolve towards a single main channel again. Consequently anastomosis in the upper Columbia River is a disequilibrium channel pattern. The cause of anastomosis in other anastomosing river systems might possibly also be a constant overload of sediment or a temporary sediment pulse.