A global atlas of river morphologies
Summary
Assessments of planform river morphology have historically been limited in scope. Today, remote sensing technology is powerful enough to enable global coverage of the world’s rivers.
Existing river morphology classification schemes have historically tended to be somewhat idiosyncratic, being shaped by the experience and functional goals of the classifier. Here, I explore the output of a simple classification algorithm on a globally spanning dataset of the world’s rivers in order to lay the groundwork for extending previously localized analyses to a worldwide scale.
Allen & Pavelsky have mapped the global extent of rivers and streams as well as their widths, Frasson et al. examined the statistical relationships between width, slope, catchment area, meander wavelength, sinuosity, and discharge, and Altenau et al. expanded and refined the global map of rivers. Attempts to classify rivers based on shared characteristics are as varied and numerous as rivers themselves.
In geomorphological practice, rivers are generally differentiated by substrate (bedrock vs. sediment), sediment caliber, and channel pattern (straight/meandering, braided, anastomosing). These definitions are partially quantified, partially qualitative. Our goal was to parametrize the criteria for differentiating rivers by their planform morphology and examine whether strict application of these criteria by an algorithm would give the same results as assessment by a human expert. Additionally, I examined whether the statistical relationships and patterns established by spatially localized studies hold true on a global scale. Based on river centerlines and channel numbers from the NASA Surface Water and Ocean Topography mission river database, I assess planform morphology of the world’s rivers wider than 30 m. I use a simple algorithm to sort individual river reaches into the broad morphological categories braided, anastomosing, and meandering, and examine spatial distribution and relative prevalence of individual morphological groups. I find that single-threaded rivers are overwhelmingly meandering, with only very short sections fulfilling the criteria for planform morphology defined as straight. Channel number and reach sinuosity are insufficient to accurately categorize planform morphology for multi-threaded rivers. Further, principal component analysis of physical parameters included in the Surface Water and Ocean Topography river database shows that width, slope, water surface elevation, and flow accumulation do not appear to follow any trends related to expected morphological categories. I thus conclude that one or more parameters necessary for accurate empirical classification of planform river morphologies are not contained in the databases analyzed. I suggest adding channel island color as an indicator of riparian vegetation as proxy for island stability and expanding the analysis to incorporate a range of temporal and spatial scales to examine whether these factors allow for definite empirical identification of planform river morphology.