GEOMORPHIC BOUNDARIES WITHIN RIVER NETWORKS

Author contributions: MWS and MCT contributed equally to all aspects of this research and manuscript preparation. Key Points 1. The physical character of different functional process zones (FPZs)-river types-is a key driver of the occurrence, strength and distribution of geomorphic boundaries within river networks. 2. Transitions between FPZs are a dominant geomorphic boundary. Only 32 percent of geomorphic boundaries in the river network of the Kimberley region, Australia, occurred at tributary junctions; however some tributary junctions had the greatest boundary strength. 3. The approach can be applied in any watershed with sufficient GIS data, supports quantitative testing of the strength and effect of boundaries on geomorphological and ecological processes in river networks. ABSTRACT River networks have been characterised as a series of links and nodes; the occurrence and spatial organisation of which significantly influence physical, chemical and ecological patterns and processes occurring within them. Nodes, in particular, are boundaries that occur when the structural and or functional properties of adjacent river zones change discontinuously or non-monotonically in space and time. The significance of tributaries as dominant nodes in determining the character of the river discontinuum is a prevailing, yet largely unscrutinised, paradigm of river science. A quantitative approach for characterising riverine landscape is presented, which enables a quantitative assessment of the occurrence, strength and distribution of geomorphic boundaries in river networks. 1410 boundaries were identified in the river network of the Kimberley region, NW Australia, and only 32 percent of these occurred at river confluences. Transitions between different functional process zones or river types, 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 geologies. The approach expands the traditional view that river confluences are the significant geomorphic boundary and it allows boundaries to be observed at any sampled location along a river network. PLAIN ENGLISH SUMMARY Geomorphic boundaries are important transition zones in river networks where significant changes in river process and channel shape and habitat occur. Traditionally, river scientists viewed confluences-where two rivers meet-as the main geomorphic boundaries in river networks. Our approach to quantitatively determine the strength and spatial arrangement of geomorphic boundaries throughout river networks, at the watershed scale, found that of the 1410 boundaries identified in the river network of the Kimberley region, NW Australia, only 32 percent occurred at river confluences. Transitions between different functional process zones or river types throughout the river network were the dominant geomorphic boundary, but some river confluences represented the strongest geomorphic boundaries. The location of geomorphic boundaries was largely related to the boundary between different types of geologies. The approach expands the traditional view that river confluences are the significant geomorphic boundary and it allows boundaries to be observed at any sampled location along a river network. Improved knowledge of these boundaries is important, for example, to identify potential hotspots of riverine diversity or places that may limit species’ movements or invasions through river networks.