Faulty Rivers: The effect of faulting on river morphodynamics and morphology

Rivers tend to maintain a gradient that is more or less constant, and that fits the discharge and sediment supply of that river. The altered gradient of the river leads to adaption processes in the river channel in order to restore the faulting induced change in gradient. One of the commonly observed responses is a change in sinuosity of the river channel. Via changes in the amount and size of the individual meander loops, the river channel is presumed to dynamically alter its channel length so that the channel gradient remains unchanged despite the tectonic tilting. However, sinuosity changes and erosion and sedimentation patterns in meandering rivers depend on many more factors than faulting alone. These different forcing factors can operate simultaneously, making disentanglement of the effects of the different forcing factors difficult. The proposed relationships between tectonics and meandering remain to a large extent hypothetical due to a lack of physical modeling, and because of the small number of detailed case studies that are able to disentangle the different forcing factors. In addition, the hypothetical meandering river responses are based on block-wise tectonic motions (e.g. tilting of fault-bounded blocks, doming) and do not include local tectonic movements, at and near the fault trace. The aim of this research was to unravel the morphodynamic and morphological response(s) of alluvial rivers to faulting induced vertical motions. Both case study’s and numerical modelling were used to achieve this aim. Because both the Meuse and the Roer river flow through the tectonically active Roer Valley Rift System, whose tectonic activity is exceptionally well known, this is an ideal study area to investigate how river dynamics are influenced by faulting. The objectives of this study were to; i) Reconstruct the palaeogeographic evolution of the downstream part of the Meuse and Roer rivers in order to disentangle the influence of climatic, tectonic and anthropogenic forcings on river morphodynamics and morphology, ii)Determine the relation(s) between morphodynamic river response and tectonic movements at different spatial scales and on long and short (earthquake) time-scales, using the Meuse-Roer river system and Roer Valley Rift System (RVRS) data on river morphology and fault activity, iii) Use a numerical meander simulation model to study the effects of various tectonic deformation rates and styles along normal fault zones on alluvial meandering river morphodynamics and morphology, and test the conceptual models based on the reconstructions. Palaeogeographic reconstructions of the Meuse and Roer rivers show that the tectonic structure of the subsurface on the scale of fault-bounded blocks determines variations in river pattern and preservation of river terraces along the longitudinal profile of the river. When the results of the reconstructions and numerical modelling are combined, it appears that the effects of faulting on river dynamics, and sinuosity in particular, are not unambiguous. In short, a sinuosity anomaly can only be used as an indicator for faulting when the influence of other, non-tectonic, factors at that location are also known. The absence of a sinuosity change does not imply inactivity of the fault at geological time-scales, nor is a deviating sinuosity value proof of fault activity. It was shown that a sinuosity response to faulting may be only a part of the total morphodynamic adaption, indicating that it is crucial to consider the morphodynamic response of an alluvial river to faulting as transient when interpreting the sedimentary record. Finally, it can be concluded that tectonics should not only be seen as a constant, continuous and large-scale forcing on river dynamics, but that vertical motions along fault zones are also an important forcing factor on river dynamics on local and short time scales.