Topological structures of river networks and their regional‐scale controls: A multivariate classification approach

Abstract

Landscape evolution is governed by the interplay of uplift, climate, erosion, and the discontinuous pattern of sediment transfer from the proximal source of erosion to distal sedimentary sinks. The transfer of sediment through the catchment system is often referred to as a cascade, the pattern of which is modulated by the interaction of key network characteristics such as the distribution of transport capacity and resultant zones of sediment storage. An understanding of how sediment production is modulated through river networks with different topological structures at the associated timescales has remained elusive but presents significant implications for the knowledge of river response to disturbance events, and floodplain asset management. A multivariate method of identifying representative topological structures from a range of river networks is presented. Stream networks from 59 catchments in the South Island of New Zealand were extracted from a digital elevation model and their key topological parameters quantified. A principal component analysis was implemented to reduce these to two‐dimensional axes that represent the magnitude of network branching and the topographic structure of each catchment, respectively. An agglomerative hierarchical clustering analysis revealed five network ‘types’, which are examined in terms of their internal structural characteristics and relationships to potential regional‐scale controls. Implications for sediment transfer in these network ‘types’, and their use as representative networks for further analysis, are discussed. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Reanimating the strangled rivers of Aotearoa New Zealand

“…In a New Zealand context, this is supported by impressive long‐term national‐scale datasets and toolkits such as the River Environment Classification (REC) and Freshwater Environments of New Zealand (FRENZ) (Snelder et al, 2004). However, to date, remarkably few studies document systematic catchment‐wide appraisals of river evolution, explaining forms, and rates of adjustment to inform predictions of prospective river futures (Downs & Piégay, 2019; cf., Walley et al, 2020). Carefully selected archetypal histories conducted in different landscape settings for rivers subject to differing forms of anthropogenic disturbance would be very helpful in efforts to address this shortcoming.…”

Topological controls on catchment‐scale sediment dynamics

“…The networks from the Type B, D and E clusters were thus replaced with those that fell closest to the centre of the cluster for which the necessary data was obtainable. The four identified study catchments were evaluated in the same manner as Walley et al (2020), to establish the internal characteristics of the catchment topography and network structure. The Type A network was identified by Walley et al (2020) as the Motueka River, which exhibits a dissected network structure, with wide headwaters narrowing towards the outlet (Fig.…”

“…The network classifications identified by Walley et al (2020) were used to select topologically representative catchments in which modelled spatiotemporal patterns of sediment connectivity could be compared. The five network 'types' are distinguished by catchment topography and network structure (Figure 1), in which Types A, B, D, and E exhibit values along the extremities of each axis.…”

“…It was assumed that the greatest contrast in sediment routing patterns would occur between the outermost network 'types', and Type C was consequently removed from further analysis. The representative networks from the remaining 'types' identified by Walley et al (2020) were evaluated for this study, but the data necessary to parameterize the CAESAR-Lisflood model was only available in the Type A catchment. The networks from the Types B, D, and E clusters were thus replaced with those that fell closest to the centre of the cluster for which the necessary data was obtainable.…”

“…plays a clear role in modulating the spatiotemporal pattern of sediment transfer from source to sink. Building on the theoretical understanding of how sediment is transferred through catchment-scale river systems and the analysis of network topology provided byWalley et al (2020), this study compares patterns of sediment routing across topologically distinct structures, and identifies key differences in the spatiotemporal patterns of sediment transfer.These patterns indicate that dynamic behaviour is structured differently in each of the network 'types', with particular divergencebetween the dissected networks (Type A and B) which exhibit dynamic links throughout the network, and the mainstem-dominant structures (Type D and E) which indicate that change is concentrated within the mainstem reach. Key differences were also observed in the occurrence of hotspots across the networks, with the greatest dissimilarity between the patterns observed between the Type B network (which contained several insignificant hotspots) and the Type D structure (in which a single site significantly influenced the overall pattern of connectivity).…”