The discontinuum of river networks: the importance of geomorphic boundaries

Abstract

Abstract Context Rivers are heterogeneous landscapes characterised by distinct patches separated by boundaries. The significance of tributaries as dominant geomorphic boundaries in determining the character of the river discontinuum is a prevailing, yet largely unscrutinised, paradigm of river science. Objectives This study examines the spatial organisation and strength of geomorphic boundaries within the river network of 10 drainage basins in the Kimberley region of NW Australia. The possible drivers of the spatial organisation of boundaries throughout the river networks are also identified. Methods Using a suite of GIS tools and statistical analyses, distinct rivers zones or functional process zones (FPZs) and the strength of geomorphic boundaries between these FPZs were empirically determined for > 35,700 km of river network. The spatial distribution of boundary strengths throughout the river network was analysed against a set of environmental variables hypothesised to influence the location of boundaries, specifically: lithology, slope, elevation, and tributary confluences. Results 1410 boundaries were identified in the river network of the Kimberley region, an average of one boundary every 25 km of river. Only 32% of these occurred at river confluences. Transitions between different FPZs – large scale river patches, present in the river network were the dominant geomorphic boundary. Although a range of boundary strengths occurred, some river confluences represented the strongest geomorphic boundaries. The location of geomorphic boundaries was significantly associated with the boundary between different types of lithologies. Conclusion Our analysis shows that the river network of the Kimberley region is naturally highly fragmented, and that tributary confluences are not the dominant control on discontinuities in the river network. We suggest that the character of river network fragmentation depends not only on dams, waterfalls, and confluences, but also on the strength and spatial organisation of geomorphic boundaries between FPZs.