Combined effects of mud and riparian vegetation on river morphology

Abstract

Cohesive sediment (mud) and riparian vegetation interact with river morphodynamics and affect the formation of river patterns. Mud and vegetation are both associated with a shift from multichannel braided to single channel meandering patterns, because they (1) increase bank strength, (2) fill in abandoned channels and lows, limiting formation/reoccupation of multiple channels, and because (3) vegetation increases flow resistance in the floodplain, causing focus of flow in the channel. Mud and riparian vegetation also interact with each other. However, it is still unknown how interactions between mud deposition and erosion processes and riparian vegetation development affect the morphological development and patterns of river systems. A better understanding of these interactions and their effects on river morphology would be useful to improve predictive models for river management and renaturalisation. The aim of this study is therefore to investigate the combined effects of mud and vegetation on the morphological development of a river system on centennial timescales. The combined effects of mud and vegetation on river morphology were investigated with a numerical model loosely based on the meandering river Allier. The river Allier is a sandy gravel-bed river of about 800 m wide, located close to the Alps, with a relatively steep gradient and discharge peaks that are about 10 times larger than the low flow. The model includes, for the first time, both mud erosion and deposition processes and settlement, growth and mortality conditions for two riparian vegetation species (Salix and Populus), that affect flow resistance depending on vegetation density and sizes. The main model scenarios are: (1) both vegetation and mud, (2) only vegetation, (3) only mud, (4) no vegetation or mud. Important additional boundary conditions that were varied were the upstream mud supply and the critical shear stress for mud erosion. Spatial mud and vegetation patterns obtained with the model were compared to field observations. For validation, mud deposition patterns were also compared with an established floodplain sedimentation model. In the vegetation-only model, the vegetation pattern is highly dynamic with variations in age and density as a result of the morphodynamics. Vegetation growth along channels focuses flow causing deeper and more confined channels. The vegetation induces formation and migration of meander bends, but also promotes avulsions over a wider part of the floodplain and leads to a relatively high braiding intensity by increasing water levels and diverting flows. The larger braiding intensity opposes many field and experimental studies. One of the possible explanations is that in our study floodplain confinement by the valley walls caused higher water levels and hence avulsions. Mud, as opposed to vegetation, has little impact on river morphology due to the large shear stresses compared to the critical shear stress for mud erosion in this particular system. The morphology of a river system with a combination of vegetation and a moderate mud supply is similar to a system with only vegetation. Moderate mud supplies had limited impact on river morphodynamics even though vegetation promotes mud deposition closer to the river channel and in abandoned channels which are important locations for the morphological development of rivers. Therefore, mud also had no large impact on the vegetation development. Vegetation in combination with an extreme mud supply of relatively cohesive mud leads to a relatively stable river system with a low braiding intensity, where high muddy banks prevent overbank flooding causing a narrow riparian zone. These results indicate that in high-energy gravel-bed river systems such as the Allier, vegetation has a large impact on river morphodynamics, while (interaction with) mud is generally of minor importance because mud is relatively easily eroded. Mud deposition in the floodplain can, however, facilitate other vegetation species by altering the substrate composition. Effects of this interaction on river system ecology and morphology are an interesting topic for future research.