Discrimination of alluvial and mixed bedrock-alluvial multichannel river networks

Abstract

This paper explores the use of planview morphological metrics to quantitatively describe and distinguish mixed bedrock-alluvial multichannel networks from alluvial multichannel networks. The geometries of the channel planforms of two bedrock-constrained networks (Mekong and Orange rivers) are compared with the classic alluvial anastomosed Upper Columbia River and the wandering Ganga River. Widely recognised indices utilized include: channel link count and channel sinuosity, with additional emphasis being given to the less common metrics: network bifurcation angles and island shape characteristics (i.e. aspect ratio, compactness, roundness and convexity). Link count data, with one notable exception, conform to theoretical expectations. Bifurcation angles for all four multichannel rivers are significantly greater than angles reported for braiding rivers. Island convexity clearly discriminates the two alluvial rivers from the two bedrock-influenced rivers. The width of the macrochannel, in which each network develops, has a positive influence on the number of channel links and is further related to channel slope variations which, in turn, are influenced by terrain structure revealed using trend-surface analysis. The geometry of multichannel networks are often laterally constrained such that the values of channel bifurcation angles and link sinuosity values reduce as the network intensifies and channel links are shortened. These latter observations go some way to explain the oft-noted relatively-‘straight’ links seen within multichannel networks which are a necessary adjustment to space-filling constraints placed on a network

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