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Impact of Changing Concavity Indices on Channel Steepness and Divide Migration Metrics.

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Title: Impact of Changing Concavity Indices on Channel Steepness and Divide Migration Metrics.
Authors: Gailleton, Boris1,2 boris.gailleton@gfz-potsdam.de, Mudd, Simon M.2, Clubb, Fiona J.3, Grieve, Stuart W. D.4,5, Hurst, Martin D.6
Source: Journal of Geophysical Research. Earth Surface. Oct2021, Vol. 126 Issue 10, p1-33. 33p.
Subject Terms: *Drainage, *Watersheds, *Erosion, River channels, Structural geology
Abstract: The concavity index, θ, describes how quickly river channel gradient declines downstream. It is used in calculations of normalized channel steepness index, ksn, a metric for comparing the relative steepness of channels with different drainage area. It is also used in calculating a transformed longitudinal coordinate, χ, which has been employed to search for migrating drainage divides. A θ value of 0.45 is typically assumed in studies. Here we quantify the variability in θ across multiple landscapes distributed across the globe. We describe the degree to which both the spatial distribution and magnitude of ksn and χ can be distorted if θ is assumed rather than constrained. Differences between constrained and assumed θ of 0.1 or less are unlikely to affect the spatial distribution and relative magnitude of ksn values, but larger differences can change the spatial distribution of ksn and in extreme cases invert differences in relative steepness: relatively steep reaches can appear relatively gentle as quantified by ksn. These inversions are function of the range of drainage area in the considered watersheds. We also demonstrate that the χ coordinate, and therefore the detection of migrating drainage divides, is sensitive to varying values of θ. The median of most likely θ across a wide range of mountainous and upland environments is 0.425. This wide range of variability suggests workers should not assume any value for θ, but should instead calculate a representative θ for the landscape of interest, and exclude basins for which this value is a poor fit. Plain Language Summary: The elevation profiles of rivers are commonly used to interpret their tectonic and erosion history. The slope of river channels tends to decline downstream, and this decline can be described by a river's concavity. Estimating the concavity is important when comparing river profiles across a region, and using an assumed value for concavity may result in spurious interpretations. Key Points: We develop metrics to explore the variability of the concavity index in a given landscape, and assess its suitability for a given landscapeWe compute the values of concavity index (θ) in basins across the globe (N = 5,033). The central tendency is 0.425, corroborating previous studies, but there is a large range in values, with interquartile range of 0.225–0.575We find that the channel steepness index (ksn), the χ coordinate and knickpoint extraction are all sensitive to the value of θref, with implications for river profile analysis and the detection of migrating drainage divides in landscapes with variable concavities [ABSTRACT FROM AUTHOR]
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Abstract:The concavity index, θ, describes how quickly river channel gradient declines downstream. It is used in calculations of normalized channel steepness index, ksn, a metric for comparing the relative steepness of channels with different drainage area. It is also used in calculating a transformed longitudinal coordinate, χ, which has been employed to search for migrating drainage divides. A θ value of 0.45 is typically assumed in studies. Here we quantify the variability in θ across multiple landscapes distributed across the globe. We describe the degree to which both the spatial distribution and magnitude of ksn and χ can be distorted if θ is assumed rather than constrained. Differences between constrained and assumed θ of 0.1 or less are unlikely to affect the spatial distribution and relative magnitude of ksn values, but larger differences can change the spatial distribution of ksn and in extreme cases invert differences in relative steepness: relatively steep reaches can appear relatively gentle as quantified by ksn. These inversions are function of the range of drainage area in the considered watersheds. We also demonstrate that the χ coordinate, and therefore the detection of migrating drainage divides, is sensitive to varying values of θ. The median of most likely θ across a wide range of mountainous and upland environments is 0.425. This wide range of variability suggests workers should not assume any value for θ, but should instead calculate a representative θ for the landscape of interest, and exclude basins for which this value is a poor fit. Plain Language Summary: The elevation profiles of rivers are commonly used to interpret their tectonic and erosion history. The slope of river channels tends to decline downstream, and this decline can be described by a river's concavity. Estimating the concavity is important when comparing river profiles across a region, and using an assumed value for concavity may result in spurious interpretations. Key Points: We develop metrics to explore the variability of the concavity index in a given landscape, and assess its suitability for a given landscapeWe compute the values of concavity index (θ) in basins across the globe (N = 5,033). The central tendency is 0.425, corroborating previous studies, but there is a large range in values, with interquartile range of 0.225–0.575We find that the channel steepness index (ksn), the χ coordinate and knickpoint extraction are all sensitive to the value of θref, with implications for river profile analysis and the detection of migrating drainage divides in landscapes with variable concavities [ABSTRACT FROM AUTHOR]
ISSN:21699003
DOI:10.1029/2020JF006060