Controls on submarine channel-modifying ... - GeoScienceWorld

Research Paper

GEOSPHERE GEOSPHERE; v. 14, no. 5

Controls on submarine channel-modifying processes identified through morphometric scaling relationships Lauren E. Shumaker1, Zane R. Jobe1, Samuel A. Johnstone2 , Luke A. Pettinga1, Dingxin Cai1, and Jeremiah D. Moody3

https://doi.org/10.1130/GES01674.1

Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, Colorado 80225, USA Chevron Energy Technology Company, Houston, Texas 77002, USA

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CORRESPONDENCE: shumaker@mines.edu CITATION: Shumaker, L.E., Jobe, Z.R., Johnstone, S.A., Pettinga, L.A., Cai, D.X., and Moody, J.D., 2018, Controls on submarine channel-modifying processes identified through morphometric scaling relationships: Geosphere, v. 14, no. 5, p. 1–17, https:// doi.org/10.1130/GES01674.1. Science Editor: Shanaka de Silva Associate Editor: Andrea Fildani Received 26 January 2018 Revision received 31 May 2018 Accepted 20 July 2018

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ABSTRACT Submarine channels share morphological similarities with rivers, but observations from modern and ancient systems indicate they are formed under processes and controls unique to submarine settings. Morphologic characteristics of channels—e.g., width, depth, slope, and the relationships among them—can constrain interpretations of channel-forming processes. This work uses morphometric scaling relationships extracted from high-resolution seafloor bathymetry to infer connections between morphology and process in submarine channels. Analysis of 36 modern channels in five geographic regions shows that channel widths vary regionally (from 10 km wide) but occupy the same range of aspect ratios (~10:1–100:1). This suggests an autogenic control on aspect ratio, perhaps resulting from feedback processes in levee growth and/or bank erosion, and allogenic (e.g., sediment supply, grain size) controls on channel width. Submarine channel aspect ratios tend to decrease with increasing dimensions, while the opposite relationship has been observed for fluvial channels, likely due to opposing relationships between flow discharge and channel distance. Additionally, observation of an apparent lag between channel thalweg and levee responses to gradient changes suggests that thalweg and levee deposition and erosion may be partially decoupled due to the vertical structure of turbidity currents, with thalweg evolution driven by the basal, higher-shear-stress portion of the flow and levee evolution by the dilute upper portion. The data presented here provide a basis for predicting channel metrics in exploration scenarios, in which data coverage may be sparse. This documentation of a diverse suite of channels also captures the range of scales and variability exhibited globally by sub marine channel systems, providing context for local studies.

OPEN ACCESS

INTRODUCTION

This paper is published under the terms of the CC‑BY-NC license.

Through decades of study employing ever-improving technologies, researchers have demonstrated the important role submarine canyons and channels play in shaping the seafloor, building the sedimentary rock record, and transporting sediment and nutrients from coastal to deep-marine envi-

ronments. However, fundamental questions remain regarding how these channels form, and what controls the diversity of scales and morphologies of submarine channels around the world. Despite advances in direct observation of active flow events in submarine canyons and channels (Paull et al., 2002; Cooper et al., 2013; Sumner and Paull, 2014; Xu et al., 2014; Azpiroz-Zabala et al., 2017), these efforts are hindered by difficulty of access, high expense, and infrequency of events (e.g., Clare et al., 2016). Due to the difficulties of studying submarine channels directly, numerous authors have attempted to understand submarine processes and dynamics by drawing on fluvial channels and networks as analogs (e.g., Straub et al., 2007; Brothers et al., 2013). Although submarine channels can be an order of magnitude larger than rivers (Konsoer et al., 2013), their cross-sectional and planform morphologies bear striking similarities (Fig. 1) with many submarine channels exhibiting levees (Hansen et al., 2015), erosional terraces (Babonneau et al., 2004), and/or migrating meander bends and cutoffs (Kolla et al., 2012; Maier et al., 2012). Given the morphological similarities between submarine channels and rivers, can techniques developed in fluvial geomorphology shed light on the processes that control submarine channel morphologies? A century of study in fluvial geomorphology has led to the discovery of numerous scaling relationships among morphological parameters of rivers. Channel properties such as width, depth, and catchment area scale with river length, reflecting the importance of precipitation and cumulative discharge along tributary networks in fluvial systems (Leopold and Maddock, 1953). Fluvial channel width and depth follow a power-law relationship, such that an increase in width is associated with a predictable increase in depth (Wilkerson and Parker, 2011). Leopold and Wolman (1960) show that meander dimensions scale systematically with channel width not only for rivers, but also for glacial meltwater streams and ocean currents. Flood and Damuth (1987) and Clark et al. (1992) demonstrate that similar width-wavelength scaling relationships exist in submarine channels, supporting the possibility of a universal behavior or property of channelized flows that dictates their geometries. Although the subaqueous turbidity currents that shape submarine channels share some physical similarities with fluvial flows, in many respects they are fundamentally different, and direct comparisons of fluvial and submarine channels may not be appropriate (e.g., Keevil et al., 2006). Both fluvial flows and tur-

© 2018 The Authors

GEOSPHERE | Volume 14 | Number 5

Shumaker et al. | Submarine channel morphometric scaling relationships

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Figure 1. (A) Satellite photo of a meandering reach of a tributary to the Amazon River in Bolivia (Image from Google Earth, © 2017 DigitalGlobe and [Centre National d’Études Spatiales] CNES/Airbus). (B) Digi tal elevation model and hillshade of a portion of the submarine Joshua channel (GoM 12 channel in this study) in the Gulf of Mexico, off the southern coast of the USA (bathymetry data from the Bureau of Ocean Energy Management). Flow is left to right in both images. The channels show remarkable similarities in their planform characteristics, despite physical differences in flow properties (e.g., schematic velocity profiles of a fluvial flow (C) and turbidity current (D); no scale is implied for flow height or velocity).

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5 km bidity currents are vertically stratified in terms of their sediment concentration and flow velocity, but the shapes of their respective velocity and concentration profiles differ (Fig. 1; Choux et al., 2005; Le Roux, 2005; Xu, 2010). Turbidity currents typically contain a thin, high-velocity, coarse-grained basal portion of the flow where the maximum bed shear stress occurs (Azpiroz-Zabala et al., 2017; Symons et al., 2017), with a fine-grained upper portion that may not be fully confined by the channel along which the flow travels (e.g., Piper and Normark, 1983; Abd El-Gawad et al., 2012). Turbidity currents may also reach extra ordinary thicknesses of tens of meters to over 100 m (Talling et al., 2007; Xu, 2010) due to the small density contrast between the sediment-laden flow and the ambient seawater. Turbidity currents are driven by gravity acting on this excess density, and contrary to fluvial flows, cannot travel downslope without entrained sediment. Turbidity currents are thought to typically last hours or days, although week-long flows have been documented (Cooper et al., 2013; Azpiroz-Zabala et al., 2017), and occur relatively infrequently in deep-water settings (a few times per year to every few hundred years; Paull et al., 2014; Stevens et al., 2014; Jobe et al., 2018). This contrasts the perennial flows of many fluvial channels, although highly variable discharge in some rivers may result in relatively few flows exhibiting disproportionate control on channel morphology (Wolman and Miller, 1960; Plink-Björklund, 2015). In this study, we analyze morphometric scaling relationships in submarine channels to investigate the extent to which morphological similarities between


fluvial and submarine channels reflect similarities in formative processes. We hypothesize that fluvial scaling relationships that arise from terrestrial processes, such as downstream increases in discharge driven by precipitation and overland flow, will be different in submarine systems where such processes do not occur. This study presents a rich data set of submarine channel morpho metrics that can be further employed in numerical modeling of turbidity currents (e.g., Sequeiros, 2012; Traer et al., 2015, 2018a, 2018b), offshore energy resource exploration, seafloor infrastructure hazards assessment (e.g., Piper et al., 1999), and marine habitat and ecosystem research (e.g., Vetter and Dayton, 1998). By directly testing whether certain fluvial scaling relationships are also present in submarine channels, we address whether morphological similarities reflect analogous processes, and assess the relevance of fluvial analogs in deep-water systems (Mitchell, 2004; Keevil et al., 2006; Hughes Clarke, 2016).

METHODS Data Sources and Descriptions This study utilizes publicly available and industry-provided high-resolution bathymetric data sets to map 36 channels from five regions around the world (Fig. 2A). Lateral resolution of the digital elevation models (DEMs) ranges from


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Figure 2. (A) Map showing locations of data collection. 1—Gulf of Alaska, offshore the southern coasts of Alaska, USA and Canada in the Pacific Ocean; 2—Gulf of Mexico, off the southern coast of the USA; 3—Amazon fan, offshore northeast Brazil in the Atlantic Ocean; 4—Niger slope, offshore Nigeria in the Atlantic Ocean; 5—Bengal fan, in the Bay of Bengal in the northeastern Indian Ocean. (B) Bathymetry from the Bengal fan showing the channel mouth zone of the Bengal 1 channel. Note relatively straight and simple planform geometry near the channel mouth relative to the partially buried, sinuous channel to the west. Flow direction is to the southwest. (C) Bathymetry from the eastern Gulf of Mexico (GoM) showing the compound and recurved meanders of the GoM 12 channel. Flow direction is to the southeast. (D) Bathymetry from the Amazon fan showing the downstream end of the Amazon 1 channel, including a meander loop cutoff. The actual channel mouth is likely downstream of the last mappable point. Flow direction is to the north. White lines in B–D are the manually mapped channel thalwegs and margins. CI—contour interval.



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We mapped 36 channel thalwegs and their associated margins in the geographic information system ArcGIS and extracted morphometric measurements in Matlab. Following the convention of “bankfull” width and depth measurements (e.g., Konsoer et al., 2013), channel margins are here defined as the highest point of the external levee crest (Kane and Hodgson, 2011; Hansen et al., 2015) for leveed channels, and the rollover point of the eroded edge for non-leveed or poorly leveed channels (Figs. 4A and 4C). Although alternative methods for measuring channel width and depth hold promise (e.g., “GLORIA width” of Pirmez and Imran, 2003), the bankfull width method is beneficial for areas with relatively poor-quality imagery, where levee crests may be resolvable while internal channel characteristics are not. Channel widths were measured perpendicular to the thalweg as the distance between the two channel margins (Fig. 4). Channel depths were measured as the maximum vertical distance between the channel margins and the thalweg (i.e., for asymmetrical channel cross-sections, the larger of the two depths was recorded; Fig. 4). Width and depth measurements were extracted from thalweg-perpendicular elevation profiles at two times the DEM resolution to avoid duplicating data from a single pixel. To filter out erroneous measurements (e.g., anomalously large width measurements arising from bend geometries; Fig. 4E), any cross-section whose width was more than 150% or 170% greater than the

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12.5 m per pixel (from the seafloor reflection of 3-D seismic data) to 380 m per pixel (from multibeam bathymetry). In all cases, the resolution of the DEM imparts a bias toward detection and mapping of channels larger than ~3 pixel widths across, but the Amazon fan bathymetry is the only data set used here that approached this limitation. Some of the channels mapped in this study have previously been analyzed; references and correlations to previously published channel names are noted where appropriate. We highlight four well-imaged channels ranging in streamwise length from 56 km to over 700 km for detailed analysis (Figs. 2 and 3): (1) channel 1 of the Amazon fan, offshore northeast Brazil in the Atlantic Ocean (Pirmez and Imran, 2003), (2) channel 1 of the Bengal fan, in the Bay of Bengal in the northeastern Indian Ocean (Schwenk et al., 2003), (3) channel 12 of the eastern Gulf of Mexico, off the southern coast of the USA (Posamentier, 2003), and (4) channel 9 of the Niger Delta continental slope, offshore Nigeria in the Atlantic Ocean (hereafter called the “Niger slope”; Pirmez et al., 2000; Jobe et al., 2015). We also briefly discuss glacially influenced channels in the Gulf of Alaska, offshore the southern coasts of Alaska, USA and Canada in the Pacific Ocean, which are exceptionally large (up to 18 km wide and 300 m deep; Swartz et al., 2015). We also document dozens of smaller, isolated channel segments captured in the multibeam bathymetry from a single ship track (Amazon and Bengal channels) and relatively short, slope-confined channels lacking any visible connection to a submarine canyon or shallow marine sediment source (western Gulf of Mexico, northwestern Niger slope channels).

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Figure 3. (A) Bathymetry from the Niger slope, offshore Nigeria in the Atlantic Ocean, showing dramatic slope variability due to shale diapirism and localized width changes in the Niger 9 channel. Note widening at channel mouth at the downstream (west) end of the channel, where the upstream end of a lobe deposit can be observed. (B) Bathymetry from the Gulf of Alaska (GoA), offshore the southern coasts of Alaska, USA and Canada in the Pacific Ocean, showing the confluence of the west (GoA 6) and east (GoA 5) legs of the Chirikov channel. Flow direction is to the south and west. White lines in A and B are the manually mapped channel thalwegs and margins. CI—contour interval.

minimum width of the three previous measured cross-sections (the precise threshold value was determined manually for each channel) was excluded. Channel measurements are referenced to the downstream distance along the thalweg, calculated from the first mappable point on the channel. We began mapping channel thalwegs at either the farthest upstream point at which


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Figure 4. Example topographic cross-sections through (A) Amazon 1 channel, offshore northeast Brazil in the Atlantic Ocean, (B) Bengal 1 channel, in the Bay of Bengal in the northeastern Indian Ocean, (C) Niger 9 channel, offshore Nigeria in the Atlantic Ocean, and (D) Gulf of Mexico 12 channel, off the southern coast of the USA, show the variety in channel dimensions and aspect ratios. Cross-sectional area was calculated between bankfull height and the channel profile (e.g., gray shading in A). All cross-sections shown at the same scale. VE—vertical exaggeration; w—width; d—depth. (E) Planform view of part of the Amazon 1 channel shows how erroneous measurements arising from exceptionally tight meanders were excluded from the data set. XS—cross-section.

the channel is detectable in the data set or the farthest upstream point at which the channel margins are easily defined (e.g., without significant tributary networks). While sinuous submarine channels are the focus of this work, the transition from erosional submarine canyon to leveed channel is typically gradual and difficult to pinpoint (e.g., Pirmez and Imran, 2003). Therefore, we do not attempt to robustly distinguish between canyons and channels. Some data sets presented here, particularly those from the Gulf of Alaska, may include measurements from features that could be classified as canyons. We calculate cross-sectional area as the area between bankfull height and the channel base for each channel profile (gray shading in Fig. 4A). We calculate an average whole-channel sinuosity for each channel, where sinuosity is the ratio between the along-channel (streamwise) distance and straight-line distance between the first and last measured points. Aspect ratio is defined as the ratio of width to depth (i.e., large aspect ratios are wide and shallow).

RESULTS We extracted 28,921 width and depth measurements from 36 submarine channels in five geographic regions (Fig. 2A). We summarize the distribution of observations by reporting the tenth, fiftieth, and ninetieth percentile of measurements for each channel (P10, P50, and P90, respectively; Table 1) and plotting distributions of width, depth, and aspect ratio for each geographic region


(Fig. 5). The P50 (median) channel width values range from 195 m to 6.8 km; median depth values range from 4 m to 132 m; and median aspect ratios range from 8:1 to 146:1 (Table 1). Average whole-channel sinuosities range from 1.0 to 4.2. Although there is significant overlap among aspect ratios throughout the data set, the channels from each geographic region appear to cluster together by width (Fig. 6E). The largest aspect ratios (>100:1) were primarily recorded from the Amazon fan, the Bengal fan, and the Gulf of Alaska (Fig. 5C) and the smallest aspect ratios (1, while in submarine data bP90) aspect ratios up to ~80:1 occurring along exceptionally wide reaches of the channel, including the channel mouth (Fig. 10E).

Although the Surveyor fan in the Gulf of Alaska, offshore the southern coasts of Alaska, USA and Canada in the Pacific Ocean, is not connected to any major fluvial inputs, it has received large volumes of sediment associated with glacial erosion in the St. Elias Mountains since late Miocene time (Reece et al., 2011). The Surveyor channel is unique among deep-water channel systems in that it terminates into the Aleutian subduction trench (Reece et al., 2011). We mapped the Surveyor and Chirikov channels and their associated tributary legs (Fig. 3B), with 131–271 km mapped for each channel (Table 1). With median widths of 3.3–6.8 km and depths of 56–132 m, the Gulf of Alaska channels are exceptionally large relative to other channels mapped in this study (Table 1; Figs. 5A, 5B, and 6E). These glacially influenced channels likely represent upper end-members of the range of submarine channel morphologies. Due in part to their large scale, determining the transition point between canyon and channel is difficult for the Gulf of Alaska channels. Additionally, modification of the levees by sediment waves obscures the location of the true levee crest, so channel margins were mapped using the rollover point (as for incised channels; Fig. 4C). Given these uncertainties, we consider the Gulf of Alaska channels as a group rather than performing a detailed analysis on any single channel from the region. The channels exhibit broad bends rather than classic meander loops, and their average whole-channel sinuosities (1.1–1.3) are among the lowest recorded in this study (Table 1). The P10 –P90 aspect ratios occupy a similar range of values as those of the Amazon and Bengal fans, from a low of 22:1 to a high of 145:1 (Table 1; Figs. 5C and 6E).

Western Gulf of Mexico Channels Channels mapped in the western Gulf of Mexico, off the southern coast of the USA, range from 8.2 to 51.8 km in length, mapped at 12.5 m lateral resolution (Table 1). Channels 1–8 and 10 are associated with the Rio Grande delta, off-


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Figure 8. (A) Longitudinal profile of maximum levee crest (black) and thalweg elevations of channel 1, Bengal fan, in the Bay of Bengal in the northeastern Indian Ocean. Thalweg map is inset at lower left, and selected cross-sections are shown along the top. Color corresponds to streamwise distance. Red marks bracket notable changes in thalweg profile slope and their corresponding locations on thalweg map. VE—vertical exaggeration. (B) Width—W, (C) depth—D, (D) cross-sectional area—XS, and (E) aspect ratio—W/D, of the channel plotted against streamwise distance. Although the channel width is relatively stable for its entire mapped length, the depth increases gradually then decreases more rapidly, resulting in variable cross-sectional areas and aspect ratios along the channel, with the most rapid changes occurring near the channel mouth. Dashed gray lines highlight changes in downstream depth and slope trends.

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Hydraulic Geometry of Submarine Channels Numerous properties of fluvial channels (e.g., width, depth) exhibit power- law scaling with channel discharge (and by analogy, length; Leopold and Maddock, 1953; Wohl and David, 2008). A fundamental process that links fluvial channel width, depth, and drainage area to channel length is the accumulation of discharge through tributary input along a trunk stream (Leopold and Maddock, 1953). The processes of precipitation and overland flow, which are important factors in the formation of tributary geometries and cumulative downstream discharge, do not have obvious analogs in deep-water environments. Indeed, tributary geometries are common at submarine canyon heads but are rarely observed for submarine channels on continental slopes and basin plains. Instead, our understanding of turbidity current dynamics predicts that discharge will decrease with downstream distance, as the current gradually loses sediment load to overspill and levee construction. This process should result in a progressively smaller channel with increasing downstream distance, as has been observed in some instances (Fig. 7; Babonneau et al., 2002; Pirmez and Imran, 2003). Thus, submarine channels should exhibit scaling relationships between width and length, and depth and length, opposite those

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shore southernmost Texas, USA (Banfield and Anderson, 2004), and channels 9 and 11 are part of the Brazos-Trinity slope minibasin system (Mallarino et al., 2006). Many of these channels appear to initiate and terminate on the slope, but they exhibit weakly to well-developed levees and sinuous planform geometries, distinguishing them from submarine slope gullies (e.g., Field et al., 1999; Shumaker et al., 2017). Some of these channels, most notably GoM 5, GoM 6, and GoM 9, traverse variable slope gradients caused by salt tectonics, and as a result exhibit dramatic changes in width and depth with downstream distance, similar to the Niger 9 channel (Fig. 10). Some channels crossing steep seafloor gradients become deeply incised, resulting in low aspect ratios. Given that the western Gulf of Mexico channels (GoM 1–11) are more significantly impacted by salt tectonics than the GoM 12 channel in the eastern Gulf of Mexico, we consider them separately in data compilations (Figs. 5 and 6).

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observed in fluvial channels. Konsoer et al. (2013) documented submarine channel width-depth and depth-discharge relationships that follow a power law similar to that calculated for fluvial channels. However, the width-depth correlation is poorly constrained due to limited data and the derived depth-discharge relationship is dependent on flow sediment concentration, a value that has never been measured directly in natural systems (Talling et al., 2015). Addi


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Each geographical region documented in this study exhibits a similar range of channel aspect ratios, despite variability in channel dimensions (Figs. 5 and 6). The median aspect ratios for each region range from 17:1 (Niger) to 63:1 (Amazon); excluding the two regions with strongly incisional channels (Niger slope and western GoM), the range is 30:1 (Bengal) to 63:1 (Amazon; Fig. 5C). The similarity of channel aspect ratios despite variations in width among systems suggests that channels of all scales may have a preferred geometry, but this geometry may vary slightly depending on channel dimensions. The upper and lower bounds on the range of observed aspect ratios could be caused by self-correcting feedbacks in channel-forming processes (Fig. 11). For example, very low aspect ratio channels (approaching 1:1) are not likely to be maintained or preserved because they require steep channel walls that would be prone to mass failure (e.g., Hansen et al., 2015), which promotes channel widening. Very high aspect ratio channels (approaching 1000:1) have a diminished ability to confine turbidity currents, driving either overspill and levee aggradation (which would serve to decrease channel aspect ratio), or full loss of confinement (i.e., channel-lobe transition; Wynn et al., 2002). The autocyclic process of levee aggradation driven by low channel relief is consistent with interpretations of basinward channel propagation (Babonneau et al., 2010; Hodgson et al., 2016; Bengal 1 channel in this study). Kane et al. (2007) noted possible autocyclic feedbacks between thalweg aggradation and upward increases in levee event bed thickness. Similar methodologies could be used to test the ideas discussed here, particularly given paired core and shallow seismic reflection data sets through submarine levees. The observed trend of decreasing aspect ratios with increasing channel scale (Fig. 6) aligns with expectations for submarine channel behavior—for instance, we anticipate shallowing and widening at channel mouths as confinement is lost and flows can spread laterally (e.g., Bengal 1 and Niger 9 channels; Figs. 8 and 10). Additionally, channels are typically largest at their upstream


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tionally, Konsoer et al. (2013) note a bias toward larger channels in their data set (widths ranging from 300 m to 30 km) due to limitations of data resolution. Below, the width-depth relationships exhibited by channels in this study are examined from the perspective of regional and global trends in aspect ratios, and changes in width and depth in individual channels downstream.

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Figure 9. (A) Longitudinal profile of maximum levee crest (black) and thalweg elevations of channel 12, eastern Gulf of Mexico, off the southern coast of the USA. Thalweg map is inset at lower left, and selected cross-sections are shown along the top. Colors correspond to downstream distance. VE—vertical exaggeration. (B) Width—W, (C) depth—D, (D) cross-sectional area—XS, and (E) aspect ratio—W/D, of the channel against streamwise distance. Width remains relatively stable along the length of the channel while depth gradually increases, resulting in a subtle increase in cross-sectional area and decrease in aspect ratio downstream. The gradual deepening of the channel relative to its width can also be seen in the cross-sections in A.

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Streamwise Distance [km] end, where due to steeper slopes they may be partially confined by erosion, and therefore have lower aspect ratios (e.g., Amazon 1 channel; Fig. 7). The scale-dependency of aspect ratios among fluvial channels is opposite that observed for submarine channels in this study—that is, fluvial channels exhibit increasing aspect ratios with increasing scale (b>1), while the submarine channels studied exhibit decreasing aspect ratios with increasing scale (b