The non-fluvial nature of Western Norwegian rivers and the implications for channel patterns and sediment composition

Abstract

Fluvial processes are considered decisive for the formation of river channel patterns. The concepts of fluvial morphology contain a functional relationship between catchment area, local bed slope and grain size that leads to characteristic channel patterns. In the postglacial landscapes of Western Norway, however, the channel patterns are highly diverse, do not follow the traditional concepts of fluvial models and are distributed seemingly randomly over the river environments. The objective of this study was to investigate whether Western Norwegian rivers have a characteristic morphology determined by non-fluvial features and whether existing fluvial morphology concepts must be supplemented to represent these characteristics. River channel patterns in Western Norway were classified for 53 rivers on the basis of (i) aerial photos, (ii) LiDAR bathymetry data, (iii) sediment sampling, (iv) validation of pre-classified patterns in the field and (v) maps of geologic deposits. Sediment composition was sampled using a novel, modified Wolman–Count method, focusing on the distribution of the largest grains (Dmax). The results show that Western Norwegian rivers are dominated by glacial and colluvial deposits and partly bed rock. These non-fluvial features determine the longitudinal profile shape, bed slope and channel patterns. The collected data allow the definition of two novel types of channel forms: (i) the ‘diamictic plane bed’, with a large variation in sediment size, and (ii) the ‘mixed riffle-pool type’, with pool riffle morphology but boulder elements related to post-glacial or colluvial processes. It is presumed that the characteristic morphology evolved during the Holocene due to riverbed incision and low fluvial sediment yields, which are typical for the region and caused by the lack of dynamic tectonics and by rock resistance to weathering. The results are in contrast to the fluvial channel formation processes observed in many rivers of the world. The study thus contributes to an improved understanding of river morphology and channel pattern formation in a post-glacial landscape. It was concluded that the non-fluvial nature of the rivers studied has significant implications for flood protection measures, hydropower utilization, river ecology and ecological restoration since it determines sediment characteristics and channel stability.