The geometry and complexity of spatial patterns of terrestrial channel networks: Distinctive fingerprints of erosional regimes

Abstract

The morphology of channel networks related to long‐term erosion reflects the mechanisms involved in their formation. This study aims to identify quantitative metrics, drawn from topographic data and satellite imagery, that are diagnostic of the distinctive styles of erosion by rivers, glaciers, subglacial meltwater, and groundwater sapping. From digital elevation models, we identify three geometric metrics: the minimum channel width, channel aspect ratio (longest length to channel width at the outlet), and tributary junction angle. We also characterize channel network complexity in terms of its stream order and fractal dimension. To validate our approach, we perform a principal component analysis (PCA) on measurements of these five metrics on 70 channel networks. We build understanding of these results, in turn using scaling analyses of appropriate physical models. We show that rivers, glaciers, and groundwater sapping erode the landscape in rigorously distinguishable ways. Whereas rivers are characterized by nearly constant minimum width, variable aspect ratio, and high stream orders, glaciers have highly variable minimum widths and aspect ratios and much smaller stream orders. Erosion by subglacial meltwater remains poorly understood, and we argue that we require an additional metric to fully characterize these systems. Our methodology can more generally be applied to identify the contributions of different processes involved in carving a channel network. In particular, we are able to identify transitions from fluvial to glaciated landscapes or vice versa.

Subglacial drainage patterns of Devon Island, Canada: detailed comparison of rivers and subglacial meltwater channels

“…6, we expect the variation of this shape factor to be considerable for subglacial channels as the top width remains constant and the channel deepens, and less important for river valleys, as both cross section and width increase downstream. Another geometrical distinction between both erosional regimes is the width of first-order tributaries (Grau Galofre and Jellinek, 2017). Even at the tip of the channel, subglacial channel widths are up to tens of meters (consistent with arguments in Weertman, 1972), as opposed to widths of first-order river channels which are typically submeter in scale (Grau Galofre and Jellinek, 2017).…”

“…10 for a reference illustration). We measured channel top width following Grau Galofre and Jellinek (2017) as the distance between two points of maximum curvature along a cross-sectional line for consistency, which corresponds with valley width for rivers and the width of the entire cross section in subglacial channels. We present shape factor results in Table 2, column 6, where all measurements correspond to cross sections before tributary junctions.…”

Plants versus streams: Their groundwater‐mediated competition at “El Morro,” a developing catchment in the dry plains of Argentina

“…In order to estimate base water yields for different areas we considered that base flows were generated either by the full watershed as defined by its surface topography (i.e., topographically defined catchment) or, alternatively, by a more restricted zone that was defined considering the uniform spacing among the relatively parallel stream lines observed in the region and recognized as the typical fishbone structures of sapping erosion regimes (Grau Galofre & Jellinek, 2017).…”

“…Groundwater sapping is a hillslope recession process that manifests in the surface but results from the collapse of a less resistant subsurface once it reaches unstable saturated flow velocities. Stronger groundwater hydraulic gradients towards the incisions caused by sapping in the landscape can self‐reinforce this process, which has been proposed as the driver of canyon development in places like the Colorado Plateau and Mars (Grau Galofre & Jellinek, 2017). While increasing rainfall trends, seismic activity and land use changes have been pointed as possible converging causes for this hydrological transformation in the “El Morro” catchment (Contreras et al, 2013), observations in this and other sites have pointed to the onset of groundwater recharge following the conversion of native vegetation to rainfed agriculture as the most determinant factor (Amdan et al, 2013; Contreras et al, 2013; Giménez et al, 2016; Marchesini et al, 2017).…”

Subglacial drainage patterns of Devon Island, Canada: Detailed comparison of river and tunnel valleys

“…4). Another geometrical distinction between both erosional regimes is the width of first order tributaries (Grau Galofre and Jellinek, 2017). Even at the tip of the channel, tunnel valley widths amount up to tens of metres (consistent with arguments in Weertman (1972)), as opposed to widths of first order river channels which are typically sub-metre in scale.…”