Multi-thread river channels: A perspective on changing European alpine river systems

Rivers are natural systems whose planform pattern in alluvial reaches reflects a balance between three fundamental ingredients: flow energy, sediment calibre and supply, and vegetation. Whilst early research on river channel classification emphasised flow (stream power) and sediment controls, the impact of vegetation is now recognised in increasingly detailed classification schemes. Different planform patterns are more or less sensitive to changes in these three fundamental ingredients, which in the absence of human interventions all respond to changes in climate, allowing different morphological configurations to evolve and in some cases shift from one planform style to another. Multi-thread, braided and transitional river channel styles are common in European regions where conditions for the development of these planform styles, notably high bed material supply and steep channel gradients, exist. However, widespread, intense human impacts on European river systems, particularly over recent centuries, have caused major changes in river styles. Human activities impact on all three major controls on channel pattern: flow regime, sediment regime, vegetation (both riparian and catchment-wide). Whilst the mix of human activities may vary greatly between catchments, research from across Europe on the historical evolution of river systems has identified consistent trends in channel pattern change, particularly within rivers draining the Alps. These trends involve periods of narrowing and widening, and also switching between multi-thread and single-thread styles. Although flow regulation is often the key focus of explanations for human-induced channel change, our review suggests that human manipulation of sediment supply is a major, possibly the dominant, causal factor. We also suggest that “engineering” by riparian trees can accelerate transitions in pattern induced by flow and sediment change and can also shift transition thresholds, offering a new perspective for interpretation of channel change in addition to the focus on flow and sediment regime within existing models. Whilst the development of planform classifications of increasing complexity have been crucial in developing terminology and highlighting the main factors that control channel styles, additional approaches are needed to understand, predict and manage European Alpine river systems. A combination of field, laboratory and numerical modeling approaches are needed to advance the process understanding that is necessary to anticipate river landscape, particularly planform, changes and thus to make ecologically sound management choices.