Abstract

Mountainous landscapes often feature alluviated valleys that control both ecosystem diversity and the distribution of human populations. Alluviated, flat valley floors also play a key role in determining flood hazard in these landscapes. Various mechanisms have been proposed to control the spatial distribution and width of valley floors, including climatic, tectonic and lithologic drivers. Attributing one of these drivers to observed valley floor widths has been hindered by a lack of reproducible, automated valley extraction methods that allow continuous measurements of valley floor width at regional scales. Here we present a new method for measuring valley floor width in mountain landscapes from digital elevation models (DEMs). This method first identifies valley floors based on thresholds of slope and elevation compared to the modern channel, and uses these valley floors to extract valley centrelines. It then measures valley floor width orthogonal to the centreline at each pixel along the channel. The result is a continuous measurement of valley floor width at every pixel along the valley, allowing us to constrain how valley floor width changes downstream. We demonstrate the ability of our method to accurately extract valley floor widths by comparing with independent Quaternary fluvial deposit maps from sites in the UK and the USA. We find that our method extracts similar downstream patterns of valley floor width to the independent datasets in each site. The method works best in confined valley settings and will not work in unconfined valleys where the valley walls are not easily distinguished from the valley floor. We then test current models of lateral erosion by exploring the relationship between valley floor width and drainage area in the Appalachian Plateau, USA, selected because of its tectonic quiescence and relatively homogeneous lithology. We find that an exponent relating width and drainage area (cv = 0.3 ± 0.06) is remarkably similar across the region and across spatial scales, suggesting that valley floor width evolution is driven by a combination of both valley wall undercutting and wall erosion in the Appalachian Plateau. Finally, we suggest that, similar to common metrics used to explore vertical incision, our method provides the potential to act as a network-scale metric of lateral fluvial response to external forcing.

Highlights

  • Many readers of this journal will have heard, in a classroom environment, the received wisdom that glacial valleys are shaped like the letter "U" and fluvial valleys are shaped like the letter "V"

  • We demonstrate the ability of our method to accurately extract valley floor widths by comparing with independent Quaternary fluvial deposit maps from sites in the UK and the USA

  • We find that an exponent relating width and drainage area is remarkably similar across the region and across spatial scales, suggesting that valley floor width evolution is driven by a combination of both valley wall undercutting and wall erosion in the Appalachian Plateau

Read more

Summary

Introduction

Many readers of this journal will have heard, in a classroom environment, the received wisdom that glacial valleys are shaped like the letter "U" and fluvial valleys are shaped like the letter "V". In addition to being the flattest parts of mountain environments, are often the most fertile (e.g., Tockner and Stanford, 2002). They tend to be where settlements, vegetation, and farming are concentrated (e.g., Cooper et al, 2003; Thorp et al, 2006; Felipe-Lucia et al, 2014; Tomscha et al, 2017). The width of the valley floor, which we define as the channel plus that of the floodplain and any terrace remnants, controls how confined flood waters are once this channel capacity is exceeded. Despite the importance of valley floors for communities and ecosystems in upland environments, our understanding of what controls the location and width of these valleys is surprisingly poor (May et al, 2013). Some studies have suggested that lateral erosion rates can outpace those of vertical incision (Suzuki, 1982; Cook et al, 2014; Marcotte et al, 2021), yet research into rates and mechanisms of lateral erosion is distinctly lacking compared to vertical bedrock incision

Controls on valley floor widening
River Tweed, Scotland, UK
Russian River, California, USA
Small scale
Scaling up: the southwestern Appalachians, USA
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call