Global alluvial channel patterns

Domain: earth_science
Published: 2026-01-21
Editorial outcome: major_revision

Abstract
Quantitative analysis of alluvial channel patterns is essential in fluvial science, as their morphology both reflects and influences river behavior through interactions with water and sediment regimes, landscape processes, and human interventions. However, a comprehensive global study to identify predominant channel patterns and their underlying mechanisms remains lacking. Here, we map global alluvial channel patterns using high-resolution satellite observations. Our results reveal a hidden dominance of anabranching channels that surprisingly constitute half (51%) of total reach length globally, exceeding meandering (24%), straight (18%), and braided (7%) channels. In non-mountainous settings, anabranching channels dominate most continents except Oceania. They are not confined to typical alluvial lowland systems, also comprising 50% of alluvial tracts in mountains. While anabranching is typically associated with gentle slopes and expansive floodplains, these conditions are not exclusive determinants. The prevalence of anabranching channels transforms fundamental perspectives on global river systems, posing challenges for the fluvial geomorphology community.

Introduction
Fluvial geomorphology is a cornerstone of Earth system science that reveals insights into the complex processes that govern river dynamics1,2, landscape evolution3, and the multifaceted interactions between the hydrophysical functioning of rivers and human activities, ecosystems, and natural disasters4,5. Since the discipline’s early days, researchers have classified the morphological diversity of river systems by identifying distinct styles of alluvial channel patterns6–8. Traditionally, four primary alluvial channel patterns have been recognized in fluvial studies worldwide: straight, meandering, braided, and anastomosing9,10. This four end-member classification has served as a baseline for shaping our understanding of river processes since the late 1970s, and is commonly applied to interpret the fluvial geological record and the role of fluvial systems through Earth’s history8,10,11.

Anabranching channels, another multichannel pattern, were proposed through pioneering studies of inland Australian rivers12–14. Anabranching rivers are characterized by a network of interconnected channels separated by stable vegetated alluvial islands that preserve their hydraulic and morphological identity even at bankfull stages1,12,15. Some studies have further restricted the term “anastomosing” to a specific subset of relatively distinctive low-energy anabranching systems, typically associated with fine-grained or organic vertical accretion deposition12,16–19. Thus, anastomosing channels are part of the anabranching family, and the full extent of anabranching channel systems is much broader. Until recently, anabranching rivers were regarded as relative rarities and, consequently, received limited attention in the development of fluvial geomorphology theories20,21.

However, this perception began to change with the discovery that the top ten largest rivers in the world, with mean discharges exceeding 17,000 m3 s−1 (megarivers), are dominated by anabranching patterns22. A further study revealed that nineteen of the twenty largest rivers by water discharge exhibit anabranching patterns, with only the Mississippi River showing a stronger tendency for meandering, and rivers such as Ob, Ucayali, and Ganges with reaches displaying anabranching sinuous patterns23. Thus, it may be inferred that anabranching patterns represent the ultimate planform adjustment for megarivers and can be considered major and fundamental components of the fluvial system.

Though advancements have been made in anabranching research, the perception that anabranching is less common than single-channel meandering or braiding persists14, possibly due to the limited scope of previous studies, which often focused on specific case study sites, such as inland Australian rivers13,24. Recent advancements in remote sensing technology have enabled the observation of river systems at a global scale2,25, allowing us to classify river channel patterns, understand their geographic distribution, examine their change through time, and finally re-evaluate the classification of river channel patterns26–28. A recent study explored the key variables influencing anabranching, but its focus was on the twenty largest river basins rather than offering a global perspective across different scales29. It did not provide quantitative or statistical insights into the distribution of meandering, braided, and straight channels, nor did it assess geomorphological predictors to discriminate anabranching from meandering and braided patterns. Another recent effort characterized river channel belts for 2020 using Landsat-8 images30, finding that the likelihood of river channels being single- versus multi-threaded by surface area was 48 vs 52%, respectively. However, they grouped braided and anabranching channels under the same multi-threaded category, which represents a conceptual error in geomorphology. Despite being valuable, due to the absence of a geomorphologically based framework, their study did not provide a proper assessment of channel patterns but rather employed a simplistic two-category classification that conflated distinct channel types, and they did not differentiate between alluvial and rocky channels, resulting in the flawed grouping and counting of them together with alluvial channels. In addition, they relied on pixel-based predictions instead of reach-based estimates, resulting in the simultaneous representation of single- and multi-threaded types within the same cross-section. These methodologic limitations underscore the pressing need for a thorough and explicit study on global alluvial channel patterns. Despite this recognized need, a systematic global study was yet to be conducted to determine the dominant fluvial patterns on Earth.

Here, to address these knowledge gaps, we map the entire population of alluvial channels and present a comprehensive global analysis, detailing the geographic distribution of various channel patterns and assessing their quantitative representativeness across major climatic-physiographic zones and continents. We uncover the distribution and dominance of anabranching rivers, challenging the long-standing view that anabranching rivers are anomalies. Instead, we propose that they are not only globally widespread but also the dominant alluvial channel pattern worldwide. We further assess potential factors driving the prevalence of different alluvial channel patterns in non-mountainous regions. Based on our findings, we advocate for reshaping foundational concepts in fluvial geomorphology to support the development of future river conservation, restoration, and management strategies essential for the sustainable management of river systems on a global scale.

Results
Establishment of a global alluvial channel pattern (GACP) dataset
Here, we present the global alluvial channel pattern (GACP) dataset, a queryable, global-scale dataset that delineates the spatial distribution of alluvial river channel types. The GACP distinguishes four major channel types—anabranching, braided, meandering, and straight—by integrating two state-of-the-art datasets: the Global Surface Water Occurrence (GSWO)31 dataset and the Surface Water and Ocean Topography River Database (SWORD)32. The GSWO dataset, derived from 30 m resolution Landsat satellite observations spanning 1984 to 2020, provides long-term estimates of water occurrence probabilities. SWORD serves as the primary global hydrography reference, with each river reach averaging approximately 10 km in length. Channel pattern classification was performed using a hierarchical algorithm with quadruple control, based on key morphology and hydrological attributes, including channel branching degree (Islands and Bars Index, IBI), the ratio of islands/bars area to total channel area, sinuosity, and the vegetation and inundation characteristics of in-channel features (“Methods” and Supplementary Figs. 1–4). Validation against high-resolution images from Google Earth confirms the reliability of our classification approach, with an overall accuracy exceeding 88% (Supplementary Fig. 5 and Supplementary Table 1).

We exclude reaches flowing over bedrock (see “Methods”) or narrower than 150 m (Supplementary Note 1). In addition, very dry rivers with extremely low water occurrence over the past four decades —those with water occurrence rarely exceeding 25%, equivalent to less than one season of inundation per year based on the available historical series31— were removed from the analysis. Following these exclusions, we categorized the remaining alluvial reaches into three physiographical regions: non-mountainous reaches, mountainous landscapes, and coastal reaches. Coastal reaches are defined as areas within 25 km of the coastline33 or within deltas34, with an elevation below 100 m; mountainous alluvial reaches are identified as regions that fall within a 1 km buffer of mountainous areas35,36 and overlap with the drainage network and unconsolidated sediment zones37; and the remaining areas are classified as non-mountainous alluvial reaches (see “Methods”). Rivers flowing through mountain valleys or bedrock are often geologically confined and store a small proportion of fluvial deposits, which dilute the importance of the alluvial channel-floodplain fractions, while coastal reaches affected by waves or tides are subject to particular hydro-geomorphological processes, and non-mountainous alluvial river reaches are those unaffected by coastal processes (i.e., tides and waves) and ideally not confined by geology (i.e., geologic lateral constraints or bedrock). Notably, our study focuses on non-mountainous alluvial rivers, including those in lowlands, low dissected terrains, plateaus, and highlands. For example, into this category were included alluvial reaches of rivers draining old terrains (shields and plateaus), such as the cratonic areas of South America and large parts of Central and Southern Africa.

With the classification algorithm applied to the selected reaches from SWORD, we analyzed alluvial channel pattern predominance and distribution at multiple spatial scales, including the global level, physiographical regions, physio-climate zones, continents, and river basins.

Global channel patterns and the dominance of anabranching
The total length of alluvial channel reaches investigated in our GACP datasets is 0.69 million km, with individual reaches ranging from 0.3 to 38 km, covering 38% of alluvial reaches in the SWORD dataset (Supplementary Fig. 1). Our GACP results at a global scale suggest that multi-threaded channels (anabranching) account for 51% (0.35 million km) of the total reach length, while single-threaded channels (braided, meandering, and straight) constitute the remaining 49% (Fig. 1). Specifically, anabranching channels emerge as the predominant pattern, braided channels account for 7%, meandering channels represent an additional 24%, and straight channels constitute 18%. Notably, 5% of global channels occur in axial river reaches, where straight channels are connected to two or more anabranching or meandering morphological structures (Supplementary Fig. 6).

Non-mountains host 71% of our observed global reach length, predominantly featuring anabranching channels (50%), while braided, meandering, and straight channels constitute 6%, 26%, and 17%, respectively (Figs. 2a, c–j). Within the equatorial belt (~10°S–5°N) and cold high-latitude (~50°N–80°N) non-mountainous regions, anabranching and meandering channels are the most prevalent. Approximately 40% of non-mountainous channels are located in cold physio-climatic conditions where multi-threaded channels are more prevalent (56% anabranching in cold non-mountainous channels; Fig. 2b) compared to single-threaded channels (22% meandering, 18% straight, and 4% braided). When examining the distribution of non-mountainous channels by continent, anabranching channels dominate across all continents, except for Oceania (Fig. 2a and Supplementary Tables 1 and 2). However, dry ephemeral anabranching channels in central Australia’s desert were not included in the analysis, leading to an underestimation of anabranching in these areas. Anabranching channels are particularly prominent in Africa, accounting for 54% of the river length in African non-mountainous channels—the highest proportion among the six continents—due to their widespread presence across most parts of the Niger, Congo, and Nile River basins (65%, 64%, and 61% of the total length of respective basins in non-mountains) (Fig. 3). Regarding braided channels, Asia holds the highest fraction of non-mountainous braided channels (10% of reach length in Asian non-mountains), predominantly found in upland regions (Supplementary Fig. 13).

Mountain rivers are characteristically confined to rocky, steep valleys with in-channel dominant processes and landforms, and commonly have bedrock channels with non-existent to poorly developed floodplains and conspicuous hillslope sediment delivery from mass wasting processes38,39. Those kinds of mountain rivers are not included in our analysis, and we concentrate our study on alluvial patterns in mountain areas only (see “Methods”).

In mountainous environments, which contribute nearly a tenth of our observed global river length, anabranching channels comprise 50%, with braided, meandering, and straight channels representing 16%, 14%, and 21%, respectively (Supplementary Fig. 7a). Anabranching channels are prevalent in alluvial tracts of mountainous regions, and notable examples include the Huallaga and Ene Rivers in the Andes Mountains, Indigirka River in the East Siberian Mountains, as well as Keele and South Nahanni Rivers within the Mackenzie Mountains (Supplementary Fig. 14). Mountainous braided channels are extensively distributed across major ranges, such as the High Mountain Asia, East Siberian Mountains, Alaska Range, and New Zealand’s mountainous regions. Meandering channels are also found in alluvial mountain valleys and the influence of peat bogs in periglacial zones. For example, in Tierra del Fuego, Argentina, rivers predominantly show meandering patterns in the post-glacial period, driven by extensive peat bog deposits within large former glacial zones, dictating the fluvial styles (Supplementary Fig. 15e, f)40. In coastal regions, which represent 17% of our observed global river length, anabranching channels dominate with 58%, followed by meandering at 21%, straight at 18%, and braided at 4% (Supplementary Fig. 7b). Notably, European coastal channels are distinctively marked by straight patterns, constituting nearly one-third of their lengths, largely due to extensive human channelization41–43.

In large river basins where rivers exceed a mean annual discharge of 1000 m3 s−1, fifteen of the twenty largest basins exhibit anabranching channel patterns that span at least half of their non-mountainous basin areas. The proportion of anabranching reaches in these basins ranges from a minimum of ~40% (St. Lawrence) to as much as ~74% (Amur), with an average of 57±10% across all observable non-mountainous reaches at their basin scale (Fig. 3). Our findings corroborate previous studies, reinforcing that anabranching channels represent the end-member pattern within large river basins (Supplementary Fig. 5b)22,29. Even the Mississippi basin, traditionally regarded as a classic example of meandering river, approximately 44% of its non-mountainous tracts are characterized by anabranching patterns. We highlight the widespread prevalence of anabranching channels not only in large river basins worldwide with tens of thousands of cumecs (m3 s−1) of mean annual discharge, but also across smaller-sized river basins with a few hundred cumecs (m3 s−1). Beyond the 20 large river basins mentioned, we observed that in half of the remaining smaller-sized river basins, anabranching channels constitute over one-third of reach length within their non-mountainous basin areas. The highest fraction of anabranching channels relative to total non-mountainous reach length is found in the Ili River basin, Kazakhstan, where they account for 86% of reach length within its non-mountainous basin areas. This pattern is similarly prominent in other smaller basins, such as the São Francisco and Magdalena basins, where anabranching channels comprise over three-quarters of reach length within their non-mountainous basin areas.

Interestingly, global non-mountainous anabranching channels demonstrate diverse behaviors, with some showcasing tendencies towards braiding, notably in Asia (i.e., the Brahmaputra, Indus, Irrawaddy Rivers), as well as the northwestern South American foreland Andean rivers (i.e., Meta, Napo, Aguarico Rivers). Conversely, other large rivers show anabranching reaches with sinuous-meandering tendencies, such as the Ucayali, Ob, and Ganges Rivers (Supplementary Fig. 17).

Mechanisms underlying anabranching dominance and alluvial channel pattern formation in non-mountainous terrains
To unravel the intricate interplay of hydro-geomorphic factors driving alluvial channel patterns in non-mountains worldwide, we synthetically delved into four classical key variables: water surface slope, unit stream power, average floodplain width to average channel width ratio (FPW/W), and suspended sediment concentrations (Supplementary Note 3).

Rivers can possess identical slopes yet exhibit different channel patterns (Fig. 4a), with these disparities often influenced by the associated discharge (mean annual discharge or bankfull discharge)22. Conventionally, slope-discharge plots show a decrease in slope as discharge increases, but they are often inadequate in distinguishing between channel styles, particularly in the context of large rivers22. When bankfull discharge is considered, anabranching channels tend to feature shallower slopes than braided channels at similar discharges (Fig. 4e). Specifically, some high-slope (i.e., >200 cm km−1) anabranching rivers, with bankfull discharges between 100 and 500 m3 s−1, are often situated in periglacial regions, such as Siberia, Alaska, Scandinavian Peninsula, and Baffin Island where permafrost exerts a substantial interaction on fluvial processes (Supplementary Fig. 16). However, meandering channels, associated with bankfull discharges from 100 to 5000 m3 s−1, typically occur in lower slope ranges and exhibit even shallower slopes than anabranching channels, until discharges reach 5000 m3 s−1, at which point slopes across different channel types converge.

The unit stream power across different channel types overlaps substantially (Fig. 4b), with half of the braided channels exceeding 25 W m−2. In contrast, 84% of the anabranching channels and 93% of the meandering channels have unit stream power below 25 W m−2. Anabranching channels with high energy are mainly located in periglacial zones mentioned earlier (i.e., Siberia, Alaska, Scandinavian Peninsula, and Baffin Island), where unique seasonal processes contribute to maintaining high energy and enabling the development of anabranching patterns on steeper slopes than those typically observed.

When examining the adimensional parameter FPW/W (Fig. 4c), half of the straight and braided channels have values below 2.1 and 1.8, respectively, suggesting floodplain widths similar to channel widths for these channel types. In contrast, anabranching channels exhibit a wide range of FPW/W values, reflecting considerable variability in floodplain width from narrow to expansive. Meandering channels, on the other hand, show higher average FPW/W values, suggesting that their floodplains may be wider relative to their channel widths due to higher migration rates.

Our global-scale quantification confirms the requirements of high slope and high unit stream power for braided formation, yet it does not directly correlate with high sediment transport rates reported by previous studies44,45 (Fig. 4a, b, d, e). Suspended sediment concentrations in braided channels are not notably high (~200 mg l−1, Fig. 4d), primarily due to the dominance and morphogenetic role of bedload transport44. Thus, despite short-lived peaks during floods, suspended sediment may not be noticeably relevant to braided channels. In contrast, meandering channels exhibit a wide range of suspended fine sediment concentrations, from ~10 mg l−1 to exceptionally high levels in mountainous rivers, where concentrations can exceed ~15,000 mg l−1. Anabranching channels display a broad range of suspended sediment concentrations as well (see Supplementary Table 4), but overall, suspended sediment concentrations in anabranching rivers are slightly lower than meandering rivers (Fig. 4d).

Employing principal component analysis (PCA) as a trial (Supplementary Note 3), we quantified the combined influence of these variables on non-mountainous alluvial channel patterns (Fig. 4f). Our global results demonstrate that the channel types are not strictly segregated within the PCA space, although braided channels tend to differ substantially from the other three types, showing higher values of slopes, power, and lower FPW/W (Fig. 4a–c). We recognize that categorizing alluvial channel patterns using simple discriminators remains challenging, as rivers with different patterns can exhibit subtle changes in variables, such as discharge or slope (e.g., differences might be as minimal as a few centimeters per kilometer in slope for the same discharge). This complexity is further compounded by regional hydro-climatic and geomorphological contexts, which induce overlap in these variables across diverse channel types, and also because anabranching is a geomorphologically diverse population that spans through a broad range of morphodynamic conditions, from low to high suspended sediment concentrations, low to relatively high unit stream power, and low to high slopes (Fig. 4a–c).

Widespread implications
Our database of global alluvial channel patterns provides a platform for re-evaluating the long-standing view that anabranching rivers are less common than meandering or braided rivers. We imply that the widespread physioclimatic and physiographic occurrence, the diversity of anabranching patterns, the scale of the rivers, and their dominance among large rivers position anabranching as the global dominant channel pattern. While fluvial theory has focused for decades on meandering systems and 28,46–50, to a lesser extent, on braided systems, theoretical developments for anabranching have been largely confined to maximum flow efficiency and the least action principle20,51. Considering that their distribution is pervasive, with proportions of 50%, 50%, and 58% observed in non-mountainous, mountainous, and coastal regions, respectively, the widespread prevalence of anabranching channels underscores their pivotal role in advancing fluvial geomorphological theories.

As anabranching spans a broad range of morphodynamic conditions, it has been postulated that anabranching systems may develop under both equilibrium and disequilibrium state21,51,52. Adding to that, by considering the complex morphologies found in nature, we claim that anabranching channels are also the most geomorphologically diverse among all the alluvial channel pattern types. Given this diversity, further conceptual research and the development of classification systems covering the entire spectrum of anabranching diversity remain to be explored.

Perhaps some critics can contend that the continuous spectrum of fluvial patterns challenges the effectiveness of a framed four-end-member classification and threshold-based approach, as certain transitional cases may not be adequately categorized. Such concerns often reflect differences between visually based interpretations of channel planforms and quantitative frameworks that rely on systematic metric measurements or comprehensive analyses of processes, landforms, and channel–floodplain relationships. Nonetheless, categorizing river reaches according to prevailing morphologies—anabranching, braided, meandering, or straight—is a scientifically validated methodology. To mitigate uncertainty and avoid classification artefacts, we employed a hierarchical algorithm with quadruple control, grounded in established statistical techniques and methods prominent in previous fluvial geomorphology and large-scale geomorphological research. Our results demonstrate a statistically robust classification; for instance, 85% of total anabranching reaches have an IBI ≥ 1.1, indicating that low thresholds have not skewed outcomes and thereby affirming the validity of the reported global patterns.

A key application of discriminating channel types is their relations with sediment analyses and their interpretation and representativity in the geological record53–56. However, reconstructing channel types from the geological records remains challenging due to the limited understanding of the sedimentary structure of contemporary anabranching analogs. Consequently, the recognition of anabranching rivers in the geological record is likely to be significantly underestimated, given the scarce knowledge of the sedimentary signatures of the geomorphological diversity across the spectrum of anabranching channel styles. Another implication of the current dominance of anabranching channels relates to the interaction between vegetation and anabranching patterns throughout the geological record55, as our results align with the hypothesis of increasing diversification and its dominance of anabranching through the Cenozoic23.

The diverse interactions between vegetation and channel morphology (sediment trapping, hydraulic effect, banks resistance) in anabranching rivers have been recognized13,23,57,58. While some results have explored the hydro-sedimentary and hydro-ecological connectivity in anabranching rivers, our understanding remains limited. The role of floodplains in hydrochemical and biochemical cycles, sediment transit time, and fluxes of particulate organic carbon oxidation59,60 is central. Given that the widest and most extensive floodplains (in terms of total global length) are associated with anabranching rivers, the dominance and diversity of these systems have significant implications for biodiversity conservation and the resilience of aquatic ecosystems to climate variability.

Global climate change, driven by warming, is likely to accelerate erosion and sediment supply in cold regions61, which may result in shifts in river channel patterns. For example, glacier retreat and permafrost melt in mountainous regions could lead to an increase of braided rivers in periglacial zones, while accelerated surface erosion and island migration in large periglacial anabranching rivers, such as Siberia and northern North America26,62,63, may also occur. Monitoring these shifts and understanding the evolving dynamics of river patterns in response to climate change in those areas are necessary for future river management28. As such, it is imperative to integrate perspectives on anabranching systems into ongoing research and management strategies.

Our findings provide substantial insights into fluvial channel patterns, particularly the prevalence of anabranching channels, enhancing our understanding of alluvial channel patterns’ spatial characteristics and underlying formation processes. We anticipate that these insights will foster advancements that integrate the unveiled role of anabranching patterns in river science, driving innovations in context-specific fluvial engineering, management, and restoration strategies.

Methods
Data sources
The GSWO dataset31 served as our primary data source, providing comprehensive and extensive information regarding the location and probability (quantified on a range from 0 to 100%) of historical water occurrences over the past four decades (from 1984 to 2020). It was established using high-resolution 30 m Landsat satellite observations at a global scale spanning a substantial geographical expanse from 60°S to 80°N. By harnessing the GSWO dataset, readily accessible through Google Earth Engine (GEE), we embarked on a comprehensive investigation that sheds light on the extent of global rivers and the classification of different alluvial channel types.

The SWORD32 was employed as the primary reference for our study of global river networks and their hydrologic attributes. SWORD represents a congruent product amalgamating multiple global hydrography databases to assemble an extensive river network. Its integration includes a wide array of resources such as the Global River Widths from Landsat64, MERIT Hydro33, HydroBASINS65, Global River Obstruction Database66, Global Delta Maps67, and HydroFALLS. SWORD hosts a vast collection of 213,485 reaches, each approximately 10 km in length, replet with valuable hydrologic and morphological attributes, including reach average water surface elevation, reach average slope, and reach maximum width, among others.

The GLAKES68 is a global compilation of lakes derived from the GSWO dataset using a deep-learning model. It delineates the explicit maximum extent of 3.4 million lakes with a surface area greater than 0.03 km2. Its validation has been corroborated by exceptional accuracy levels, with overall accuracy exceeding 98.7%. In our study, we utilized the GLAKES dataset to precisely ascertain and mask the presence of lakes.

We used the Global Unconsolidated Sediments Map database (GUM)37 and Global Mountain Biodiversity Assessment (GMBA) Mountain Inventory v235,36 to distinguish mountainous and non-mountainous reaches. The GUM provided valuable insights into the distribution of sediments in low relief terrains, an endeavor that incorporated information from 126 individual map sources. It encompassed approximately 50% (68 × 106 km2) of the global land area (i.e., excluding ice and water bodies), with alluvial sediments covering about 23%, followed by aeolian sediments (21%), glacial sediments (20%), and colluvial sediments (16%). The GMBA Mountains Inventory v2 offered a standardized resource of global mountains with a clear definition based on ruggedness (quantified as highest minus lowest elevation) within eight circular neighborhood analysis windows (NAWs) of different sizes (from 1 pixel to 20 around each point) combined with empirically derived thresholds for each NAW. The GMBA also allowed to complement the identification of non-mountainous regions that are not overlapped by unconsolidated sediments map, such as shields, plateaus, and other hilly areas that are not cataloged as mountains. In addition, we also integrated the deltaic distribution data as identified by Caldwell et al.34, along with the OpenStreetMap Water Layer (OSMWL) dataset33, to pinpoint coastal reaches.

The Global Artificial Impervious Areas (GAIA)69 annual maps provide an extensive record spanning over three decades from 1985 to 2018 at a 30 m resolution, with the mean overall accuracy surpassing 90%. The transition year, from pervious to impervious, can be identified through the pixel values, ranging from 34 (corresponding to 1985) to 1 (indicative of 2018). We leveraged the GAIA to examine whether the islands were overlapped by artificial impervious areas, which comprises any material that impedes or prevents natural water infiltration into the soil.

The hydro-geomorphic factors used to study the complex interactions driving alluvial channel patterns include water surface slope, unit stream power, average floodplain width to average channel width ratio (FPW/W), and suspended sediment concentrations. The global reach-scale ICESat-2 River Surface Slope (IRIS) dataset70 provides water surface slope derived from ICESat-2 observations. Unit stream power is calculated using bankfull discharge and bankfull width provided by the reach-level bankfull river width dataset71. Average floodplain width to average channel width ratio is determined by GSWO dataset using morphological operations and RivWidthCloud algorithm. Global fluvial sediment dataset72 supplies suspended sediment concentrations for global river reaches. For a detailed calculation of these variables please see Supplementary Note 3 and Supplementary Fig. 1.

Development of an alluvial channel pattern classification algorithm
We generated global maps of alluvial channel patterns using GSWO imagery, with all processing conducted within GEE. The classification workflow is outlined in Supplementary Fig. 1 and involves two key steps: (1) extraction of channel extent from GSWO data; and (2) classification of alluvial channel patterns. The classification step proceeds by (a) distinguishing channels with islands (bars) from those without; (b) differentiating braided and anabranching patterns among islands-bars channels; and (c) identifying straight versus meandering patterns in channels without islands (bars). It is important to note that terms such as braided, meandering, and anabranching apply specifically to alluvial channels and are not appropriate for bedrock-confined rivers, which lack the morphological freedom to develop such planforms. For instance, the Colorado River in Canyonlands National Park incises resistant sedimentary formations, while the Tongtian and Yalu Rivers are constrained by mountainous bedrock valleys (Supplementary Fig. 15a–d). Accordingly, our analysis is limited to alluvial channels, where planform morphology reflects fluvial sediment dynamics rather than structural control.

Our initial step involved delineating the extent of global river channel using the RivWidthCloud algorithm73,74 to differentiate river pixels from non-river pixels. This distinction was based on binary water extent derived using a threshold of GWSO and river centerlines supplied by the SWORD dataset. Specifically, the GSWO was masked by the OSMWL to exclude ocean pixels and masked by the GLAKES to exclude instances where reaches encountered the pixels of lakes or reservoirs. To ensure comprehensive coverage of expansive multi-threaded reaches under high-flow and within-bank conditions, we applied a 25% threshold to the GSWO image to generate the binary water extent and exclude intermittent arid channels inundated for less than one season (Supplementary Note 2). Subsequently, buffer zones were created around each SWORD centerline, extending to triple the maximum reach width available via the SWORD dataset to ensure comprehensive coverage2. These buffers were used to clip the binary water extent image, following which the RivWidthCloud algorithm was applied to extract the channel extent and channel width. All binary water extent pixels connected to the SWORD centerlines were considered channel extent pixels, effectively excluding isolated water bodies lacking connection to the SWORD centerlines.

To reduce uncertainties in planform classification, we applied a minimum channel width threshold of 150 m for all analyses. Islands and bars are difficult to reliably detect in narrower channels (30–<150 m; corresponding to 1–<5 pixels at 30 m resolution), increasing the risk of misclassification (Supplementary Note 1)29. A comparison with manual digitization showed that only 10% of channels with widths of 90–120 m had detectable islands or bars using our automated extraction method. This detection rate improved to 45% for widths of 120–150 m and reached 93% for widths wider than 150 m. Accordingly, we excluded all channels narrower than 150 m from pattern frequency analysis. We also removed reaches associated with lakes, reservoirs, dams, waterfalls, artificial canals, and bedrock-confined valleys. Artificial canals were excluded due to their engineered nature and high human intervention, especially in western Europe and lowland deltas of China. The final analysis encompassed 58,790 alluvial river reaches globally, representing 38% of alluvial reaches in the SWORD dataset, or 58% when excluding intermittent arid reaches (i.e., inundated for less than one season per year or not detected at all by GSWO).

Global alluvial channels exhibit a broad array of planform patterns traditionally divided into straight, meandering, braided, and anabranching (multichannel)1,6,10,75. Straight and meandering rivers can typically be distinguished by sinuosity, while anabranching rivers are defined as a system of multiple channels separated by semi-permanent alluvial vegetated islands that divide flows even at bankfull discharge12,14,22, and braided rivers are characterized by unconsolidated channel separated by ephemeral bars with limited riparian vegetation that can be overtopped at less than bankfull flow44 (Supplementary Fig. 3).

The primary distinction between straight/meandering and anabranching/braided channels lies in the presence of mid-channel islands or bars. Accordingly, we first identified islands and bars fully enclosed within the channel, excluding those in contact with the channel margin (i.e., lateral and point bars). To minimize noise and ensure robust detection, we applied an area threshold of 0.01 km2 (equivalent to 11 pixels), discarding any islands or bars below this size (Supplementary Fig. 18).

While using the absolute area of islands and bars offers a practical means of identifying single-threaded channels, it alone proved insufficient to reliably differentiate between straight, meandering, anabranching, and braided channels at the ~10 km reach scale. Transitional or ambiguous morphologies could lead to misclassification. To improve classification accuracy, we introduced an IBI, adapted from the approach of Wang et al.29. For each reach containing islands or bars, we generated a bounding circle with a diameter equal to the corresponding SWORD reach length. IBI was defined as the ratio between the total active channel length within this circle and the original SWORD reach length:

IBI = LT / LR (1)

where LT is the total channel length of extracted centerlines—derived from the 25% water occurrence maps using the RivWidthCloud algorithm—and LR is the SWORD reach length. We excluded islands or bars with a surface area <0.3 km2 to reduce centerline complexity, smoothed the extracted centerlines using the SmoothLine function to eliminate sharp angles, and removed short spurious branches (<300 m), aligning with Wang et al.29.

In theory, reaches with IBI > 1 are considered anabranching or braided channels, whereas those with IBI ≤ 1 are considered straight or meandering. However, when short reaches are located solely on islands, the boundary circle may not encompass the full river extent, leading to inaccurate IBI values. To address this, we additionally calculated the relative area (RA) metric for reaches with IBI ≤ 1 to evaluate whether the presence of islands or bars was morphologically meaningful. RA was defined as the ratio of the total area of extracted islands and bars to the total channel area:

RA = A islands and bars / A channel (2)

This additional filter helped differentiate reaches where minor features might otherwise lead to misclassification. Specifically, reaches with IBI ≤ 1 and RA < 1% were deemed to lack geomorphologically meaningful islands or bars and were subsequently reclassified as straight or meandering, rather than anabranching or braided types.

Some may argue that reaches with IBI values ranging between 1 and 1.1 could be, for an unaware observer that does not measure systematic metrics, prone to be classified as single-channel reaches because of the relatively short length of secondary branches. But we consider that the threshold of IBI > 1, as proposed by Wang et al., together with the complementary use of the RA metric, are useful to identify and classify anabranching reaches even within this lower IBI range (1–1.1). Notably, 85% of the global anabranching reaches have IBI ≥ 1.1 while only 15% of the anabranching population fall within the 1.0–1.1 range, and from those, just 36% are sinuous (i.e., sinuosity ≥ 1.3). Statistics on branching indexes are presented in Supplementary Figs. 8 and 9.

For anabranching and braided channels, the primary diagnostic criterion lies in the dominance of either stable vegetated islands or dynamic sediment bars within the reach. Braided bars are typically unstable, exhibiting substantial shifts in size, position, and morphology between flood events, whereas islands are geomorphologically stable features that persist over years to millennia. These islands often support perennial vegetation, agricultural use, or even permanent human settlement14,23,76. The most elementary model of island formation views them as later stages in the geomorphic evolution of fluvial sandbars, wherein vegetation colonization progressively increases surface roughness and reduces inundation frequency, even under bankfull conditions6,77. However, the development of islands and branches is not solely the result of avulsion or amalgamation and vegetation colonization of sandbars. Rather, it involves complex erosional and depositional processes within multi-channel systems interacting with floodplains shaped by a variety of inherited landforms with diverse morphosedimentary structures23,78–87. Vegetated islands allow branches to maintain hydraulic and hydrologic identity, preventing them from acting as a single unified cross-section during floods—even when partially inundated at times14,23. The resilience of these features during seasonal floods, often due to cohesive sediment substrates, allows their identification at multiannual scales (Supplementary Fig. 3d, e). In contrast, braided systems are characterized by active beds, with multiple sediment bars exposed at low stages and shallow channels surrounding these bars. These bars do not act as individual branches, and the cross section tends to morpho-hydraulically homogenize during high discharge events. As a result, braided bars lacking stabilizing vegetation are frequently submerged during floods and undergo rapid morphological change (Supplementary Fig. 3j–l)21.

Overall, a fundamental distinction between anabranching and braided channels lies in their in-channel features: islands in anabranching systems are typically vegetated and morphologically stable, whereas bars in braided systems tend to be barren and more dynamic12,44. A second key difference is that vegetated islands are more resistant to inundation, while sediment bars are more frequently and easily flooded. To quantitatively differentiate these two channel types, we employed two diagnostic metrics: the normalized difference vegetation index (NDVI) and the inundation ratio (IR). NDVI is a widely used proxy for vegetation cover and greenness, computed as:

NDVI = (SR(near infrared) − SR(red)) / (SR(near infrared) + SR(red)) (3)

where SR(λ) is the atmospherically corrected surface reflectance (dimensionless) at a specific Landsat band. We derived NDVI using the full archive of Landsat surface reflectance products from 1984 to 2020, including Landsat-4 TM, Landsat-5 TM, Landsat-7 ETM+, and Landsat-8 OLI, all accessed through the GEE platform. Low-quality observations—such as those affected by clouds, cloud shadows, snow, ice, or radiometric saturation—were excluded using the CFMask quality assurance (QA) flags88,89. To characterize long-term vegetation presence, we applied a maximum value composite approach, selecting the highest NDVI value per pixel across all valid observations. The median NDVI value across all extracted island or bar pixels within each reach was then computed to represent the overall vegetation status of in-channel features. To assess the inundation behavior of in-channel features, we defined the IR as:

IR = 1 − (TC0 / TC) (4)

where TC0 is the total number of island and bar pixels with zero water occurrence (i.e., never inundated), and TC is the total number of pixels classified as islands or bars. The term, TC0/TC thus represents the fraction of island and bar area that remain exposed even during the flood season (i.e., where GSWO = 0), relative to the total area of these features exposed during the broader wet season (i.e., GSWO < 25). Consequently, IR serves as a proxy for the degree of inundation, with higher IR values indicating more frequent submergence and lower values suggesting greater geomorphic stability and flood resistance.

To differentiate anabranching from braided channels, we employed a support vector machine (SVM) classifier, a widely used supervised machine learning technique effective for binary classification tasks90. The SVM seeks an optimal decision boundary that maximizes the margin between classes while minimizing hinge loss associated with misclassified samples91. We used the median NDVI (derived from the maximum value composite) and the IR as classification features, calculated for 2529 representative reaches distributed across diverse environmental and physiographic regions (Supplementary Fig. 2). Of these, 70% were used to train the model and 30% were reserved for testing its performance. Model optimization was conducted using a grid search approach to identify the best-performing hyperparameters. The final model used a radial basis function kernel with a gamma value of 0.8, which controls the influence of individual data points, and a regularization parameter of 0.5 to balance model complexity and accuracy. The classifier was implemented using the Scikit-learn library92 and achieved a test accuracy of 0.93. We then applied the trained SVM model to all global river reaches containing mid-channel islands or bars, enabling reliable and consistent classification of anabranching versus braided systems based on their vegetation cover and inundation behavior.

It is important to note that some reaches exhibited a “mixed pattern”, presenting challenges in classification. However, these reaches were categorized as either anabranching-dominant or braided-dominant based on our systematic classification approach, rather than arbitrary criteria. Representative examples of these mixed patterns, observed in Landsat imagery from various reaches, are provided in Supplementary Fig. 4. While these reaches may resemble braided channels due to the presence of extensive channel bars, the identification of multiple stable vegetated islands—not covered by floods—led to their classification as anabranching-dominant. This pattern was evident even in regions with steep slopes, such as Siberia and Alaska, where one particular reach exhibited a much steeper slope of 200 cm km−1, contrasting with other anabranching systems (Supplementary Fig. 4a–f).

For straight and meandering channels, sinuosity—an established morphological index—was used to differentiate these two types, given their distinct morphological differences. Sinuosity was calculated as the ratio of the reach length (i.e., the SWORD reach length) to the valley length (i.e., the straight-line distance between the reach endpoints):

Sinuosity = LR / LV (5)

where LR is the SWORD reach length and LV is the valley length. We utilized the sinuosity threshold of 1.3, in line with previous studies93–95, to distinguish between straight (low sinuosity) and meandering (high sinuosity) channels. Consequently, all the reaches with sinuosity ≥ 1.3, IBI ≤ 1, and RA < 1% were classified as meandering, and those with sinuosity < 1.3, IBI ≤ 1, and RA < 1% were classified as straight. These criteria were incorporated into our classification algorithm used to assign channel types globally. To reduce potential misclassification, we examined well-known active meandering hotspots and found that most temporary islands—often forming at meander necks—did not persist as multi-threaded channels in the 40-year GSWO record. Such short-lived cutoffs therefore, did not result in misclassifying strongly meandering reaches as anabranching.

It is essential to clarify that the term “wandering channels”, which refer to a limited number of channels on Earth, was not used in our classification framework. These channels, typically dominated by bedload transport with fluvial bars exposed at low water levels, resemble braided channels but exhibit some sinuosity and occasionally contain islands. Traditionally considered a transitional pattern between braided and meandering channels96,97, wandering channels were classified in our study based on their specific characteristics. Specifically, sinuous wandering channels without islands (sinuosity ≥ 1.3) were classified as meandering, while bedload-dominated reaches with exposed bars but lacking islands were categorized as braided channels. Reaches containing multiple channels and stable vegetated islands were classified as anabranching channels. Furthermore, we included anastomosing channels within the broader anabranching category. Anastomosing channels, which are characterized by alluvial multi-channel systems dominated by vertical accretion and low energy, often surrounded by flood basins, have been described in early sedimentological and geomorphological studies of rivers such as those in Canada17, the lower Magdalena River16, and later in Niger and Rhine Rivers98. Following these definitions, anastomosing channels are considered a subset of anabranching family in our classification system.

Assessments of the algorithm performance and comparisons with previous channel pattern classifications
We selected 4413 reference reaches over high-resolution (sub-meter) Google Earth images to evaluate the accuracy of our classified channel types. The selection was carried out through a stratified random sampling strategy to ensure representation across different physiographical regions and channel types. The total number of validation samples for anabranching, braided, meandering, and straight reaches were 2153, 225, 1060, and 975, respectively, spanning various regions globally, including non-mountainous (3091 reaches), mountainous (491 reaches) and coastal (831 reaches) regions (see Supplementary Fig. 5a). For each selected reference reaches, we thoroughly examined all available high-resolution historical images during wet season on Google Earth. Comparing these referenced reaches with our results, we achieved an overall accuracy of 88.4% (Supplementary Fig. 5a and Supplementary Table 1).

We further compared the fraction of anabranching rivers with the corresponding values delineated in previous study29 (Supplementary Fig. 5b). They utilized an anabranching index defined as the total active channel length (i.e., sum of extracted channel centerlines) divided by the SWORD reach length to identify anabranching. Nevertheless, it was constrained to only the top 20 largest river basins, instead of the whole population of alluvial channels in the world. Their calculation of the anabranching fraction, which included bedrock mountainous and coastal reaches, may be less well justified. Rivers flowing through mountain valleys or bedrocks are often geomorphologically constrained and do not develop any channel patterns. Additionally, coastal reaches influenced by waves or tides are shaped by distinct hydro-geomorphological processes. However, to facilitate a more direct comparison with the previous findings, we included all reaches from mountainous, non-mountainous, and coastal regions for the same 18 largest river basins, although we removed certain reaches crossing bedrock, lakes, reservoirs, dams, waterfalls, artificial canals, etc.

When only comparing channel widths ≥ 180 m in the same 18 basins, as studied by Wang et al., the fractions of anabranching reaches in our GACP dataset showed a strong correlation with the previous study. However, our results revealed higher fractions of anabranching channels provided by GACP compared to Wang et al.’s findings in certain basins, such as Irrawaddy, Parana, and Congo (Supplementary Fig. 5b). These discrepancies arose from methodological differences and inherent limitations within either the GACP or otherwise Wang et al. For instance, our analysis excluded reaches crossing bedrock, reservoirs, artificial canals, etc which usually presents single-threaded channels. In contrast, the constraints imposed by their anabranching index could potentially cause an overestimation of the anabranching fraction in certain reaches. Because if the total length of active channel (i.e., the sum of extracted channel centerlines) slightly surpasses the length of a SWORD reach but the reach is in fact single-threaded, it will be erroneously classified as anabranching according to their criteria. These factors underline the complexity of accurately characterizing and quantifying channel patterns on a global scale. Overall, both GACP and Wang et al.’s results offer valuable insights into channel patterns, each with its own strengths and limitations. An important difference that distinguishes GACP is its global coverage (60°S–80°N instead of only a few large basins), and a more thorough categorization of channel types (not only single- or multi-threaded, but also subtypes, i.e., anabranching, braided, meandering, straight), long-term channel pattern (four decades), overall accuracy (>88%), and the distinction of different environments (non-mountain, mountain, and coastal).

Analyzing the patterns of global channels
We analyzed the dominance and distribution of four channel patterns at different scales (i.e., global, physiographical regions, physio-climate zones, continents, and basin levels), and quantified the degree of anabranching coupled with braiding or sinuous-meandering tendencies in these non-mountainous regions.

To characterize the global distribution of four reach types in different physiographical regions, it is key to establish distinct boundaries to separate non-mountainous from mountainous and coastal reaches. Our study concentrates on fluvial reaches with identifiable floodplains from non-mountainous terrains (simply named “non-mountains”), including those in lowlands, low dissected terrains, cratons, plateaus, and hilly regions that are not classified as mountains. Initially, we excluded the bedrock reaches that are only overlapped by the 1 km buffer boundaries of the GMBA Mountains Inventory but not intersected by the GUM dataset. Subsequently, we differentiated coastal reaches (including deltaic reaches) from inland reaches. Coastal reaches exhibit distinct mechanisms of channel pattern formation, as they are influenced not only by fluvial processes but also by the dynamic interactions of waves and tides99,100. Most deltas have formed through the action of distributary channels, which may exhibit anabranching characteristics alongside meandering patterns. Therefore, we identified coastal reaches as those spatially intersected with a 25 km buffer zone of global ocean boundaries obtained from the OSMWL dataset and the deltaic polygons delineated by Caldwell et al.34. Reaches with an average elevation exceeding 100 m were also excluded from the coastal reaches, following the same method established by previous research101. It is worth noting that we retained inland deltas or fans (e.g., the Peace-Athabasca delta) within the category of inland reaches, as these regions were not influenced by coastal processes. Lastly, we divided inland reaches into mountainous and non-mountainous categories. Likewise, a spatial intersection approach was applied to identify mountainous reaches, defined as those intersected with both the GUM dataset and the 1 km buffer boundaries of the GMBA Mountains Inventory. Therefore, the remainder were divided into non-mountainous reaches that were not overlapped by the GMBA Mountains Inventory. It’s important to note, however, that not all alluvial reaches we define are located on sedimentary plains due to the unavailability of high-quality datasets, such as the cratonic areas of South America and large parts of Central and Southern Africa, some relatively smooth terrains that do not fall within the boundaries of mountainous areas and unconsolidated sediment. Besides, although some reaches fall within the boundaries of the unconsolidated sediment defined by GUM dataset, they still remain geologically constrained, resulting that rivers do not develop any channel patterns but rather undergo incision (down-cutting) (e.g., lower reach of the Baram River). Overall, the quantities of non-mountainous, mountainous, and coastal reaches were 40753, 6478, and 11559, respectively.

Physio-climatic conditions were defined by the present (1980–2016) Köppen-Geiger climate classification map102 (see Supplementary Fig. 7e), and we used it to separated reaches into five climate zones globally. Furthermore, we performed basin-level statistics for non-mountainous anabranching fractions (see Fig. 3) using level-3 boundaries provided by HydroBASIN dataset103, with particular emphasis on the 20 largest river basins (with rivers exceeding a mean annual discharge of 1000 m3 s−1, as defined for large rivers by Latrubesse22). Notably, our study only accounted for exorheic river basins that eventually flow into the sea. For example, rivers such as the Negro, Madeira, or Japurá are usually considered independent rivers and rank among the top 10 largest rivers globally, but they are treated as part of the Amazon basin in our context.

For all non-mountainous anabranching reaches, we calculated the Degree of Anabranching with Braiding or Sinuous-Meandering Tendencies (DABSMT) as follows:

DABSMT = WO / (TC * 25) (6)

where TC represents the total pixel counts of islands and bars, WO represents the water occurrence on islands and bars, capped at 25 for normalization when the water occurrence exceeds this limit. In essence, this metric serves as a quantifier of channel morphology, where higher DABSMT values (i.e., a greater likelihood of seasonal flooding) signify a pronounced tendency with braiding or sinuous-meandering for anabranching channels, contrasting with lower values (i.e., a less likelihood of seasonal flooding) that indicate completely anabranching (Supplementary Fig. 17).

At first glance, our analysis seems to indicate a potential overestimation of straight reaches, considering that natural processes in alluvial regions rarely result in single straight channel. But, straight alluvial reaches typically occur under certain specific conditions: (a) in steep mountainous rivers constrained by geological factors (e.g., Lancang River, China), or short reaches in piedmont areas influenced by neotectonic movements (e.g., piedmont areas of Jaldhaka and Daina rivers, India104), (b) in coastal regions where straight reaches often coincide with single channel systems in remarkably cohesive materials, such as deltas and tidal areas (e.g., Mekong Delta and Ganges-Brahmaputra Delta); (c) in transitional straight reaches connected to two anabranching zones; and (d) in transitional reaches within highly deformed tortuous meandering patterns incised on older alluvium or bedrock. Our data showed that a substantial number of straight channels were connected to anabranching or meandering morphological structures (i.e., scenarios of c and d, see Supplementary Fig. 6). Specifically, globally, we identified 1457 straight reaches (13% of all straight reaches in GACP) connected to anabranching, and 1523 (also 13%) connected to meandering. Mean values, with standard deviations, for scenarios c and d were 10,976 ± 4067 m and 11,472 ± 3853 m, respectively.

Statistical analyses to investigate underlying mechanisms in non-mountainous terrains
The use of empirical and mechanistic discriminators has been instrumental in distinguishing alluvial channel patterns in river systems1,6,75,105,106. These discriminators (i.e., slope-discharge approach6,105,107, sediment load approach1,8, stream power and grain size75,108, channel stability106, etc.) have remarkably contributed to our comprehension of local channel controls, yet understanding the global determinants of alluvial channel patterns remains a formidable challenge due to the complex interplay of various factors and the diversity of river systems109.

We implemented statistical analyses to identify the major variables that played an important role in influencing the formation of various alluvial channel patterns in non-mountainous regions after removing the straight reaches that were connected to anabranching or meandering morphological structures. Among the selected explanatory variables, namely water surface slope, unit stream power, average floodplain width to average channel width ratio (FPW/W), and suspended sediment concentrations (see Supplementary Note 3), we provided histograms illustrating their distributions in Fig. 4, with additional boxplots displaying the characteristics of slope and bankfull discharge. Note that, a portion of reaches lacked values of certain variables, constituting 36% of non-mountainous reaches. These reaches were excluded from our statistical analysis, ensuring that we were still able to preserve and extract meaningful global patterns, even in the face of missing information. Principal component analysis (PCA) was performed as a trial to disclose differences among the four channel types in compound manner (Supplementary Note 3), which helped us assess the relevance of these variables in controlling alluvial channel patterns (Fig. 4f).

Methodological uncertainty and limitations
Uncertainties and limitations regarding the classification and analysis of fluvial channel patterns are acknowledged. For instance, while the majority of reaches can be classified into distinct categories, certain reaches may exhibit transitional patterns that defy easy classification. This introduces uncertainty in the channel pattern classification, particularly regarding the parameters used and the size thresholds for identifying islands or bars that distinguish straight, meandering, braided, and anabranching channels. To minimize these uncertainties and enhance classification reliability, we implemented a quadruple-control framework comprising the adapted IBI, the ratio of islands/bars area to total channel area, channel sinuosity, and a SVM classifier trained on vegetation (NDVI) and inundation (IR) characteristics. This integrative strategy substantially improves the robustness and accuracy of the global classification of alluvial channel patterns.

For channel pattern analysis, the quality of auxiliary datasets—such as the water occurrence map, mountain map, and unconstrained sediments map—can also introduce potential uncertainties. For example, we excluded rivers with extremely low water occurrence over the past four decades (e.g., water occurrence rarely exceeding 25%, indicating less than one season of inundation) when extracting river extent from the GSWO. This was particularly relevant for ephemeral anabranching channels found in central Australia’s desert, leading to an underestimation of anabranching in these regions. However, because the channel pattern analysis was performed only for the reaches defined by our GACP dataset, the impact of this underestimation should be limited. Additionally, extracting smaller bars and islands in narrow reaches, where the width is too small to detect such features from the 30 m GSWO map that led us to study only 38% of alluvial reaches in the SWORD dataset or 58% when excluding intermittent arid reaches, presents another challenge. Nevertheless, higher-resolution images (e.g., Sentinel images) can potentially overcome this limitation.

Despite these challenges, our study offers valuable and original insights into the global distribution of fluvial alluvial channel patterns and deepens our understanding of their spatial characteristics and formation processes. Fundamentally, we mapped the entire population of rivers on Earth, encompassing thousands of different channels. Recognizing the inherent complexity of rivers, we acknowledge that there will be outlier cases; however, these are not quantitatively significant enough to affect the overall results and conclusions.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper consists of an innovative and globally extensive analysis, technically a very full and meticulous desk study using remote sensing imaging to provide reach-scale data for global rivers, categorise their types, and expose their spatial distribution & frequencies. It covers a wide scale of river sizes and physiographic regions world-wide.\n\nThe authors begin their Abstract with a declaration about the essentialness of such quantification. They then move to focus on, and in effect promote, the case for anabranching as a separable pattern – seen here as one of four only, and co-equal with straight, meandering and braiding. Anabranching itself refers to situations where two or more channels branch and rejoin ‘back-to-back’. By contrast, the other three types relate to variously defined geometries within individual channel reaches. Moving on from the data analysis, this paper concludes that anabranching is way dominant globally over all the others. This viewpoint builds from, and echoes, the (supervising?) senior authors’ previous research, especially in using data from remote sensing sources, and as in South America and the Amazon in particular.\n\nThe authors’ well-laid out procedures amount to a valuable advance into global perspectives, but I have strong reservations. These concern the four alternative ontological categories recognised, and then the labelling of anabranching as ‘dominant’. Categorisation is indeed carefully based on criteria using Landsat imagery: size (over 0.01 km2 ) and vegetation presence is used to distinguish ‘Islands’ from exposed river sediment ‘bars’ (although their exposure depends on the timing of the imagery used in terms of river discharge). This presence/absence of vegetated islands then dominates categorisation (e.g., line 585): all island-present rivers are solely categorised as ‘anabranching’. Their branches were not otherwise characterised by their characteristics such as meandering, braiding or straight (lines 532-536) as were single channels. Furthermore, meandering is defined as sinuosity  1.3; some would restrict such a term as applicable if involving an evolution-based process of spatially repeated geometry, and something rather more than mere sinuosity. If it’s sinuosity that’s here measured, why not label their pattern as such? (‘wandering’ is another declared form of channel pattern not used here; others of course also write of ‘patterning continua’).\n\nNanson & Knighton (1996) proposed what many others would now generally accept as a decent binary classification system: straight, meandering and braided on the one hand, and on the other whether these are present as single-channels or come in a range of multi-channel combinations. Individual component channels around islands can be each of straight, meandering and braided, or plurally of several kinds alongside each other within valley floors (e.g., one branch being relatively straight, another actively meandering. Dominating main channels may be any of all three). Lower-level processes such as bar-bend evolution, avulsion, in-channel aggradation and extra-channel drainage are differently involved in each.\n[Relevant but unmentioned recent literature includes Ashworth & Lewin (2012) Earth-Science Reviews 114, 84-107, and Lewin & Ashworth (1914) Geomorphology 215, 83-98, wherein it is channel pattern rather than island presence that is used as the key, and processing, discriminator.]\n\nAnabranching, as a visual geometric style, actually dates back in the literature as far as 1841. Here this is used to mean ‘interconnected channels separated by islands’, mentioning (line 57), but later ignoring (lines 581, 586), the more recent use of ‘anastomosing’ as a named separate and well-accepted multi-channel style in low-energy rivers (with straight to meandering channels). These enclose floodbasins rather than upstanding islands [see Makaske Earth-Science Reviews 53 (2001) 149-196]. Floodbasins are negative rather than being positive relief features on floodplains; they may or may not be vegetated, crossed by levee’d channels, or lacustrine.\n\nOthers researchers have happily accepted braiding or meandering as having occasional islands (as enclosed following cutoffs and avulsions or emerging through aggradation), but without changing their categorical naming. Many geomorphologists would probably agree that a four-fold classification (line 44) is just not adequate; it really hasn’t been used ‘traditionally’ since the late 1970s, nor is braiding manifestly that scarce in large rivers (Brahmaputra/Jamuna, Indus?). And who has claimed that anabranching is anomalous anyway (line 89)? To take a specific example (lines 193ff.), seventeen of the twenty largest basins ‘exhibit anabranching patterns’: but equally, and here ‘under cover’, these are variably also straight, meandering or braiding. Is the ‘classically’ meandering Mississippi really ‘characterised by anabranching patterns’ (line 200) – because it has some ‘islands’?\n\nUnfortunately (given the careful and extensive data collection), there seems in the paper to be a purposefully preferred, & subjective, categorisation choice in favour of designating rivers solely as anabranching if islands are present. This greatly expands Latrubesse’s admirable pioneering advocacy on behalf of multi-channel systems [in Geomorphology 101 (2008) 130-145]. But to my mind a frame of mind has now, in this new paper, come to bias the way in which very useful data-derivation project has come to be used, classified, interpreted and invoked – for an over-egged promotion of ‘anabranching’ (lines 293-299; 335-337). Brief attention only was given to ‘mixed patterns’ (line 651 ff.). In claiming that about half of global rivers as being ‘anabranching’ (and ignoring their other characteristics, however dominant they and their processes may be), the authors downgrade the straightness, meandering and braiding that clearly enough co-exist alongside channel multiplicity. For example, the authors prefer to call the illustrated images of the Huallaga and Ene Rivers in the Andes (supplementary Figure 4) ‘anabranching’ rather than ‘braiding’ with islands. Braided channel processes may produce vegetated islands quite commonly; meandering bifurcations and rejoinings also create binary courses through crevassing and avulsions in anastomosing systems, too. When it comes to explaining pattern mechanisms, failing to distinguish low-power anastomosis from island-branching (or even meander-branching from braiding-branching) as separable ontologies seems regrettable.\n\nParticularly considering the praiseworthy effort and labour taken in obtaining this large data set, I am so sorry to find that it has been (in my opinion) misdirected. It could have been (and adjusted, still could be) so useful without its methodological pre-determination in favour of the conclusions drawn.", "reviewer_2": "This manuscript develops the largest accounting of river channel forms using large remote sensing datasets and analyzes the patterns in these forms using physiographic, climatic, and geomorphic variables. The study is well conceived and well designed, with the exception of one concern below. My main concern with this manuscript is the significance given the recent publications with similar findings on the prevalence of multi-channel rivers. The authors do cite these studies, but further explanation of why the other studies’ results were incomplete would bolster the significance of this manuscript. Besides this concern, the framing of the importance of this study is well argued.\n\nI really enjoyed reading the introduction and review of channel form classifications throughout literature. Similarly, the ‘Widespread Implications’ section is interesting and provides good perspective on what could be seen as simply classifying river form and instead focuses on the connection between river form and river function.\n\nMajor comments:\n¬Lines 78-81: The characterization of Wang et al. 2022 as not globally representative and only focused on the largest rivers is slightly misleading as they collect data and study the entire basins (down to 180m width which is admittedly large) which are globally distributed and include a range of river sizes. Similarly, the methodological differences in Nyberg et al., 2023 are real and important, but it is notable that they have very similar final percentage of multi-threaded rivers. I don’t have any personal stake in these publications, but I think more credit is owed to these papers for identifying the global abundance of multi-channel rivers and the misconceptions around channel form occurrence. Further contrast with their findings would help explain the novelty of your work.\n\nLines 111-116. I think the combination of mountainous and unconsolidated sediment areas is confounding, as mountains have high slope and uplift in addition to bedrock constraints. Further, I’m not sure the GUM dataset captures what you are looking for in terms of substrate control on morphology. For example, the Canadian Shield region is heavily controlled by bedrock joints and fractures. GUM classifies this as ‘glaciated’, and I didn’t read further to look into this, but it may just require tweaking which sediment types in GUM you consider unconsolidated sediment. Similarly with parts of the lower Congo that are influenced by bedrock. I would like to see a map of just the reach classifications in the supplemental.\nJust saw that you recognized this in methods section (lines 753-755), but I still think this needs to be either recognized in the main text as a limitation of the classifications or addressed by revising the method. Given the importance of the physiographic classification importance to the results, I would really encourage revisiting this classification to see if you can improve it.\n\nMinor comments:\nI don’t often see “cumecs” written, mostly as a spoken shorthand. I think m3/s is more interpretable and succinct, but you don’t need to address this you prefer cumecs.\nLine 76: “who” should likely be “which”\nFigure 1: Should the composited channels be their own inner grouping? (at the AN, BR, ME, ST level)", "reviewer_3": "The manuscript makes a massive effort to quantify prevalence of four channel patterns in global rivers. The methods are sound, and the results are encouraging for advancing river studies. I only have some minor comments:\n\nLines 102-104: Please double check the SWORD. It is incorrect to state that it “excludes widths narrower than 150m”. Many channels in SWORD are actually less than 100-m wide.\n\nFig.2: The percentage numbers in the histograms appear a bit crowded. It might be better to adjust the font position or reduce the size slightly for better readability.\n\nLines 202-205: Do you have a figure and statistical data to support this argument about small and medium-sized river basins?\n\nExtended Data Fig. 5: Please change Wang to Wang et al., 2022 and include the citation in the caption.\n\nLine 485-Data sources: Could you clarify the data sources used for floodplain width and discharge?\n\nLines 784-786: This section is a bit unclear to me. The numbers presented here (i.e. 1846 and 1935) seem quite low on a global scale. Given SWORD includes over 210,000 reaches, are you suggesting only 1846 and 1935 reaches are fully anabranching and meandering, respectively?"}, "author_response": "ANSWER TO REVIEWERS:\nREVIEWER 1\nReviewer #1 (Remarks to the Author):\nThis paper consists of an innovative and globally extensive analysis, technically a very\nfull and meticulous desk study using remote sensing imaging to provide reach-scale\ndata for global rivers, categorise their types, and expose their spatial distribution &\nfrequencies. It covers a wide scale of river sizes and physiographic regions world-\nwide.\nANSWER: We appreciate the reviewer considers our article innovative and supported\nby a meticulous methodology and that our study produces original results on rivers\non a global scale.\nThe major criticisms by the reviewer are:\n- Critics of the use of four major categories of channel patterns. The reviewer\nclaims that it has not been in use since the 1970s.\n- The reviewer argues that because channel pattern planforms are continuous,\nit would be difficult to classify/frame some transitional channel patterns into\nsome of the four categories.\n- To sustain the above on transitional patterns or difficulties faced to\ndiscriminate some cases, the reviewer uses some examples\na) two large rivers (Indus and Brahmaputra) and our examples on Huallaga\nand Ene rivers, which under the reviewer's perspectives, could be difficult\nto classify as anabranching and probably would fall into the braided\ncategory by some authors. The reviewer sustains its criticisms on channel\npatterns by using some papers such as Ashworth and Lewin 2012\n(Ashworth and Lewin 2012) among a few others.\nb) The reviewer criticizes our discovery that several reaches of the Mississippi\nbasins are anabranching because in several reaches the Mississippi is\nmeandering, which is true, and it does not disagree with our results.\nc) The reviewer raises questions on how the authors are handling some\nspecific types of patterns usually classified as “wandering” by some\nauthors.\nWe attended several of the suggestions of the reviewer and included them in the\nmanuscript. We explained in more detail wandering systems were categorized and\ncataloged into some of the four classes. For the Brahmaputra and Indus, we included\nsome references such as Ashworth and Lewin 2012 (Ashworth and Lewin 2012), and\n1\ndata on the islands that sustain the classification like anabranching for several\nreaches. We also explained why the examples used of the Huallaga and Ene are\nanabranching. To avoid misunderstandings, we made clearer in the manuscript what\nreaches of the Mississippi we consider anabranching.\nWe also included several comments and suggestions for the other two reviewers\nwhich were positive on the quality and relevance of our manuscript.\nBelow we sandwiched our answer to each comment-criticisms by the reviewer.\n1-REVIEWER: The authors begin their Abstract with a declaration about the\nessentialness of such quantification. They then move to focus on, and in effect\npromote, the case for anabranching as a separable pattern – seen here as one of four\nonly, and co-equal with straight, meandering and braiding. Anabranching itself refers\nto situations where two or more channels branch and rejoin ‘back-to-back’. By\ncontrast, the other three types relate to variously defined geometries within\nindividual channel reaches. Moving on from the data analysis, this paper concludes\nthat anabranching is way dominant globally over all the others. This viewpoint builds\nfrom, and echoes, the (supervising?) senior authors’ previous research, especially in\nusing data from remote sensing sources, and as in South America and the Amazon\nin particular.\nANSWER: The four major end-member categories of channel patterns in alluvial\nrivers we used are the most typically used in fluvial geomorphology, and grouping\nchannels into these four big groups is considered the most adequate strategy.\nRegarding anabranching, yes, as politely mentioned by the reviewer, previous studies\nby Latrubesse demonstrated that anabranching patterns were dominant among the\nlargest rivers of the world and even among the top 20 largest rivers on the planet in\nwater discharge (Latrubesse 2008, Latrubesse 2015).\nHowever, as usually happens with any innovative results that challenge established\nconcepts, many fluvial scientists still argue that a) anabranching patterns (alluvial\nmultichannel) are not as common and b) that they are only dominant among the\nmega rivers and some large rivers, c) that they are minority among the majority of\nthe wide spectrum range from large to small alluvial rivers, and d) that they are\nconstrained to be a representative group of multichannel patterns in some extreme\ngeographic territories such as Australia where they were originally defined and\nstudied by Dr. Gerald Nanson and collaborators.\n2-REVIEWER: The authors’ well-laid out procedures amount to a valuable advance\n2\ninto global perspectives, but I have strong reservations. These concern the four\nalternative ontological categories recognised, and then the labelling of anabranching\nas ‘dominant’. Categorisation is indeed carefully based on criteria using Landsat\nimagery: size (over 0.01 km2 ) and vegetation presence is used to distinguish ‘Islands’\nfrom exposed river sediment ‘bars’ (although their exposure depends on the timing\nof the imagery used in terms of river discharge). This presence/absence of vegetated\nislands then dominates categorisation (e.g., line 585): all island-present rivers are\nsolely categorised as ‘anabranching’. Their branches were not otherwise\ncharacterised by their characteristics such as meandering, braiding or straight (lines\n532-536) as were single channels. Furthermore, meandering is defined as sinuosity\n1.3; some would restrict such a term as applicable if involving an evolution-based\nprocess of spatially repeated geometry, and something rather more than mere\nsinuosity. If it’s sinuosity that’s here measured, why not label their pattern as such?\n(‘wandering’ is another declared form of channel pattern not used here; others of\ncourse also write of ‘patterning continua’).\nANSWER: We would, kindly have to say that it is unclear what the question or\ncriticism by the reviewer is and why it triggers the reviewer's “strong reservations”. To\nput it straight, there is a global acceptance that we can enclose channel patterns in\nthese four major categories. Regarding the selected four ontological planform\ncategories, they are typically used worldwide and included in every textbox on fluvial\ngeomorphology or even in sedimentology books today. Thus, if we use these\ncategories in thousands of papers (a quick search in Google Scholar, Science Direct\nor any search platform demonstrates that); faculty even teach their students each of\nthese river categories, worldwide, with specific chapters and sections, and we devote\nspecific chapters on each of these channel patterns in the Treatise of Geomorphology\n(Elsevier 2022); also those categories of channel patterns are the focus of numerous\nspecific scientific sessions in meetings such as AGU, RCEM, and many others on fluvial\nsystems (particularly uncountable meetings and sessions on meandering and braided\nrivers); it is difficult to comprehend why the four “envelope” categories we propose\ncan scientifically trigger “strong reservations” in the reviewer.\nBeyond the diversity of styles rivers exhibit, there is no doubt we can catalog major\nchannel pattern groups/categories. Like in biology, despite the huge biodiversity of\nsome groups, if we describe fish there is an agreement on how to define a fish, or\nwhat family of fishes we are talking about, just to use one example.\nSo, for geomorphologists, beyond some strange and exceptional cases in nature,\nthere is no doubt that, for the sake of building a global assessment and database,\nthe users will understand how we classify meandering, braided, straight and\nanabranching river reaches.\ngeomorphologists use sinuosity as the key element to define in planform what is (or\nis not) a meandering river. The most common use is >1.3 sinuosity value as pointed\nout many years ago by Stanley Schumm (Schumm 1985), as older pioneer authors\nlike Leopold et al (Leopold and Wolman 1957) originally used 1.5 which has been\nconsidered a too high value. Thus >1.3 is the most appropriate to define a single\nmeandering channel pattern. The meandering rivers include a diversity of sinuous\nplanforms (compose, tortuous, harmonic, with sandier load, with more suspended\nload, in peat, or whatever) but all of them are considered “meandering patterns”\nbecause they are single channels and sinuous. As we worked on alluvial patterns,\nparaphrasing the reviewer, we made a meticulous analysis of the presence of an\nalluvial belt, thus we discarded incised meanders in rocks although they are also\nsinuous meandering rivers. Any other previous work at a global scale made such a\ncareful analysis by discriminating alluvial from rocky channels. The same we made to\navoid including rocky multichannel patterns.\nRegarding braided, of course, there are varieties of braided rivers too. They can be\nmore or less laterally confined, gravel or sandy braided rivers, etc, but there is a clear\nagreement among geomorphologists and fluvial scientists on how we define and\nwhat is the planform of braided rivers.\nThe second criticism is why we don’t classify the style of each branch in the\nanabranching reaches. The answer is simple: because, the objective was, for the first\ntime under a geomorphological perspective, to identify and measure how many\nanabranching reaches exist on the planet and where they occur. As the reviewer can\nappreciate, it was a huge effort and demanded complex geospatial-computational\nwork. We are not focusing on building a classification of the huge diversity of\nanabranching channels and branch styles.\nAnabranching are multiple-channel systems and, as the reviewer knows very well, the\n“sine qua non” condition to classify a pattern as “anabranching” is that a) the river\nexhibits two or multiple channels divided by alluvial islands that can be stable for a\nfew years to thousands of years, and b) the branches have individual morpho-\nhydraulic identity even at bankfull discharge. Any type of alluvial river reach that\nattends that definition is classified as anabranching. The definition we used, strictly\nfollows the definition of anabranching rivers by Gerald Nanson (Nanson and\nKnighton 1996), the author mentioned by the reviewer.\nWe appreciate the comments of the reviewer on the methodology we developed and\napplied. Categorisation is indeed carefully based on criteria using Landsat imagery:\nsize (over 0.01 km2) and vegetation presence is used to distinguish ‘Islands’ from\n4\nexposed river sediment ‘bars’ (although their exposure depends on the timing of the\nimagery used in terms of river discharge).\nBecause of the robust methods we applied, data are consistent and strong on how\nrivers are classified, and how we processed and extracted the data. We also explained\nthe criteria and how we discriminated cases of potential transitional patterns,\nparticularly in smaller systems. The innovative and well-based methodology covers\nmultidecadal records of imagery (multitemporal for water occurrence and recurrence\nand spatial distribution, channel positions, vegetation) allowing us to discriminate\nwith a high level of confidence islands from the mobile channels and sandy fluvial\nbars. We also have been very open when explaining the methodological efficiency\nand accuracy but also pointing out the limitations on the scale, explaining why we\ndid not force results and why we did not assess smaller rivers and features below the\nexplained threshold of channel width and island areas of 0.01km2. It strengthens our\nscientific integrity by being clear and scientifically transparent.\nFurthermore, the reviewer mentions wandering bed-river channels, which are a\nlimited number of cases on Earth and are indecisively defined in the literature. Almost\n^90% of the papers on wandering rivers in the international literature are from the\nreaches of a limited number of Canadian rivers, and a few mainly from China, the\nUSA, and Europe. They are rivers dominated by bed load and fluvial bars exposed at\nlow water levels, such as a typical braided, but have some sinuosity and even\nislands. Thus, they were traditionally considered a transitional pattern between\nbraided and meandering (Church 1983, Desloges and Church 1989).\nIn our study, sinuous wandering channels without islands (>1.3 sinuosity) are\nincluded in meandering channels and bed load-dominated rivers with exposed\nbars and no-islands are recorded as braided. When multiple channels and stable\nislands with vegetation were present, then the reaches were classified as\nanabranching. To clarify this concern raised by the reviewer, we have included the\nexplanation in our manuscript.\n3-REVIEWER: Nanson & Knighton (1996) proposed what many others would now\ngenerally accept as a decent binary classification system: straight, meandering and\nbraided on the one hand, and on the other whether these are present as single-\nchannels or come in a range of multi-channel combinations. Individual component\nchannels around islands can be each of straight, meandering and braided, or plurally\nof several kinds alongside each other within valley floors (e.g., one branch being\nrelatively straight, another actively meandering. Dominating main channels may be\nany of all three). Lower-level processes such as bar-bend evolution, avulsion, in-\n5\nchannel aggradation and extra-channel drainage are differently involved in each.\n[Relevant but unmentioned recent literature includes Ashworth & Lewin (2012) Earth-\nScience Reviews 114, 84-107, and Lewin & Ashworth (2014) Geomorphology 215, 83-\n98, wherein it is channel pattern rather than island presence that is used as the key,\nand processing, discriminator.\nANSWER: We consider that the reviewer is, in part, misconstruing the papers and\nwords of Gerald Nanson and Knighton (Nanson and Knighton 1996) (as well as\nAshworth and Lewin (Ashworth and Lewin 2012)) to justify that it is useless or\nconflictive to discriminate single-channel reaches from multichannel (anabranching)\nchannel reaches in our study, by avoiding citing further discoveries and papers by\nthat author (Nanson).\nAlthough the terminology anabranching was previously used generically and\ndescriptively (as reviewed by Carling (Carling, Jansen and Meshkova 2014), for\nexample), it was in disuse for long decades. Thus, making justice and merit, Gerald\nNanson can be considered the father of the modern concept of anabranching rivers\nand the pioneer in building their morphodynamics theories. Gerald Nanson has been\nfighting for years on the preconceptions by fluvial geomorphologists to accept his\npostulations on anabranching and the relevance of anabranching patterns in fluvial\ngeomorphology and has worked for decades in collaboration with his colleague\nHuang and others to build new paradigms and theories on the morphodynamics\nfunctioning and physical understanding of anabranching rivers. The old paper by\nNanson and Knighton 1996 (Nanson and Knighton 1996) - mentioned by the\nreviewer and written at the early stages of the development of the studies of\nanabranching patterns-, showed early discoveries and tried to face the establishment\nby showing a simple schematic figure (non-sustained by data) that branches can be\nof any type (meandering or braided or whatever) and that we can have anabranching\nreaches with different styles of branches.\nToday, as for all of us and in our paper, and contrary to what the words of the reviewer\nsuggest, Nanson considers that anabranching rivers are a complex and distinct group\nof channels and that because they are multichannel patterns they cannot be simply\nclassified like braided and meandering, the two traditional patterns (binary\nclassification), a classification based on rivers early described and studied by\nEuropean and North American scientists.\nThe reference to Ashworth & Lewin (Ashworth and Lewin 2012), and Lewin &\nAshworth (Lewin and Ashworth 2014) is non-conflictive at all to our study and the\nclassification of anabranching channels. Both respected colleagues (senior members\nof our team know them very well), have made important contributions to the\n6\nunderstanding of large rivers and floodplain evolution, but their papers do not intend\nto criticize or specifically classify anabranching channel patterns but focus on\nassessing the complex processes on large rivers, settings, and time scale among\nothers. They also discuss how floodplain environments and channels interact in large\nrivers. Although they did not conduct any field work at that time in the Amazon,\nthey published their results showing the morphology of the Amazon where islands can be\nas much older as thousands of years, and used as examples several of the articles by\nLatrubesse. The authors use the terminology anabranching and do not question the\nuse of an “anabranching” pattern class. Their figure 4 (Ashworth and Lewin 2012)\nindicates with asterisks anabranching rivers and the caption reads “Most big rivers\nare anabranching with a system of multiple channels that divide and rejoin but\nindividual branch channel patterns may be straight, meandering, braided,\nwandering or anastomosing”. What they said confirms what Latrubesse\n(Latrubesse 2008, Latrubesse 2015) and our manuscript says (the biggest rivers\nare dominantly anabranching) and when mentioning styles, they support that\nthere is a huge morpho-diverse world of anabranching patterns and processes\ninvolved in creating and sustaining multiple branches with a variety of patterns.\nEven the Brahmaputra is labelled by Ashworth and Lewing (Ashworth and Lewin\n2012) as anabranching in their figure 4, agreeing with the crucial presence of\nislands in that system. We don’t see any conceptual or conflict with our article\nwhen reading paper from Ashworth and Lewin (Ashworth and Lewin 2012). Even\nmore, they use similar end members, enclosing meandering rivers in a “single\nchannel dominated” system (figure 8) when confronting them with all the multiple\nchannels (anabranching) dominated systems when assessing a variety of\ngeologic and geomorphologic settings for large rivers.\nOn the other hand, our first objective was to globally understand the amount and\ndistribution of anabranching channels. For decades, many fluvial scientists have\nquestioned the relevance of anabranching rivers without presenting or processing\nglobal data. Far from that, our results show that they are the most common pattern\nin Earth. Secondly, we are not sure why the reviewer mentions channel discriminators\nhere. Despite it being a secondary objective, we presented for the first time a pioneer\nglobal assessment of alluvial channel patterns and channel discriminators, using\ntypical factors and parameters used in geomorphology (slope, discharge, stream\npower, etc). We imagined that it would be appreciated by fluvial specialists as it is the\nfirst time this kind of global geomorphological data on meandering, braided and\nanabranching patterns can be assessed by the fluvial community.\n4-REVIEWER: Anabranching, as a visual geometric style, actually dates back in the\nliterature as far as 1841. Here this is used to mean ‘interconnected channels separated\nby islands’, mentioning (line 57), but later ignoring (lines 581, 586), the more recent\n7\nuse of ‘anastomosing’ as a named separate and well-accepted multi-channel style in\nlow-energy rivers (with straight to meandering channels). These enclose floodbasins\nrather than upstanding islands [see Makaske Earth-Science Reviews 53 (2001) 149-\n196]. Floodbasins are negative rather than being positive relief features on\nfloodplains; they may or may not be vegetated, crossed by levee’d channels, or\nlacustrine.\nANSWER: Yes, as mentioned before, anabranching was a generic term used back in\ntime, but it was forgotten for decades. Our colleague and respected fluvial specialist\nPaul Carling (Carling et al. 2014) made a classical review of the terminology of\nmultichannel systems. But the term anabranching was reborn and redefined by\nGerald Nanson and collaborators in the latest 90s and early 2000´s.\nHowever, it is difficult to understand the criticism raised in our article. First, the\nreviewer claims that in our paper ““anabranching“ is used to mean ‘interconnected\nchannels separated by islands’, mentioning (line 57).” Well, it is explained and\nreferenced in the paper, but clarifying the reviewer and editor, the definition we use\nis the standard definition of anabranching as defined by Gerald Nanson (Nanson and\nKnighton 1996) (and referenced in the paper), which is repeatedly published and used\nin the literature. It is not a “sui generis” classification we are proposing here.\nSecondly, the reviewer claims…. “but later ignoring (lines 581, 586), the more recent\nuse of ‘anastomosing’ as a named separate and well-accepted multi-channel style in\nlow-energy rivers (with straight to meandering channels).” The asseveration by the\nreviewer is incorrect. Anastomosing is not “a more recent terminology” and we are\nnot ignoring anastomosing rivers. Rather we are incorporating them in the\nanabranching rivers category because they are alluvial multichannel systems. Indeed,\ncontrary to that mentioned by the reviewer, we assume the reviewer very well knows\nthat anastomosing is of older use than the current concepts and definition of\nanabranching as defined by Nanson and the following papers on anabranching rivers,\nincluding the largest rivers on Earth. Again, we emphatically clarify that we are not\nignoring anastomosing patterns. Anastomosing patterns (a multi-channel style in\nlow-energy rivers) are part of the anabranching family. Anastomosing are alluvial\nmultichannel systems, so they are anabranching. Anastomosing was described mainly\nbased on a particular type of multiple channels rivers dominated by vertical accretion,\nlow energy and surrounded by flood basins as described by early works of\nsedimentology and geomorphology in some rivers of Canada (outstanding\npioneering work by D.G Smith and N.D Smith (Smith 1983, Smith and Smith 1980)),\nthe lower Magdalena river (Smith 1986) and later by Makaske (Makaske 2001) and\nmany others in Niger, Rhine, and other systems. Anastomosing rivers have multiple\nchannels and islands, and that is the crucial point. For that, they are classified as\n8\nmultichannel (anastomosing) systems, and consequently are anabranching systems.\nThe flood basins mentioned by the reviewer help in the occurrence of splays and\navulsions and create anabranching patterns. The point is that “anastomosing” rivers,\nas defined by Makaske (Makaske 2001) and others, are just a small fraction of the\nhuge diversity of alluvial anabranching patterns family we have on Earth. But the most\nimportant is that there is no conflict at all between our paper and the traditional\nclassification of some peculiar rivers as anastomosing. We clarify that absolutely ALL\nthe anastomosing rivers reaches described in the literature were included in the\nanabranching category. To address the reviewer's concern, we expanded the\nexplanation in the improved version of the manuscript.\nOn the other hand, as previously mentioned, there is no conflict in having straight,\nmeandering, or whatever branches morphologies in anabranching patterns. The\ngreat point here is that anabranching reaches indicate that rivers are incapable of\nsustaining, in that particular reach, a single channel pattern. As Nanson clearly\nexplained, even Latrubesse (Latrubesse 2008) and many others working in\nanabranching patterns such as Ashworth and Lewin (Ashworth and Lewin 2012) (cited\nby the reviewer), branches can have different styles in each anabranching reaches.\nThe critical point is that when a river cannot sustain a single channel pattern the\nresponse is an anabranching reach and vice versa.\n5-REVIEWER: Others researchers have happily accepted braiding or meandering as\nhaving occasional islands (as enclosed following cutoffs and avulsions or emerging\nthrough aggradation), but without changing their categorical naming. Many\ngeomorphologists would probably agree that a four-fold classification (line 44) is just\nnot adequate; it really hasn’t been used ‘traditionally’ since the late 1970s, nor is\nbraiding manifestly that scarce in large rivers (Brahmaputra/Jamuna, Indus?). And\nwho has claimed that anabranching is anomalous anyway (line 89)? To take a specific\nexample (lines 193ff.), seventeen of the twenty largest basins ‘exhibit anabranching\npatterns’: but equally, and here ‘under cover’, these are variably also straight,\nmeandering or braiding. Is the ‘classically’ meandering Mississippi really\n‘characterised by anabranching patterns’ (line 200) – because it has some ‘islands’?\nANSWER: We differ with the reviewer that using a four-end members categorization\nand channel grouping is not illustrative and efficient to conduct a global assessment,\nand that enclosing channels into those four big families of categorization has not\nbeen in use since the 1970s. We challenge the reviewer to use any scientific search\ntool to search for braided and meandering, or to argue that using those four end\nmembers when describing channel patterns is in disuse.\nhighlight the relevance of using new technologies and remote sensing information\nto identify channel partners “ it will be important that these data are consistently\nobtained using standard methodology (within a Geographic Information System\nplatform) if meaningful comparisons between river reaches are to be made\n(Molloy and Stepinski, 2007; Thommeretetal.,2010,Baillyetal.,2011). Specifically,\nmetrics are highly sensitive to river stage for some rivers (van der Nat et al., 2002;\nLuchi et al., 2010b). Such an approach should supersede the currently limited\nand simplistic extraction of planform metrics… and that is what we did, processing\nchannel networks data, and a combination of remote sensing data (multitemporal-\nmultidecadal images, geospatial processing and generation of statistics by assessing\nchannel networks, floodplains, water river stage, water recurrence, morphological\nand morphometric parameters, vegetation indexes, and other factors).\nAlso, the reviewer suggests something could be wrong in our approach because\nsome large rivers despite having islands should be classified by others like braided\nor meandering …”Others researchers have happily accepted braiding or meandering\nas having occasional islands (as enclosed following cutoffs and avulsions or emerging\nthrough aggradation), but without changing their categorical naming. …… nor is\nbraiding manifestly that scarce in large rivers (Brahmaputra/Jamuna, Indus?). ….”\nLet´s start with the first comment ” Others researchers have happily accepted braiding\nor meandering as having occasional islands”. First, the readers can check carefully\nhow we defined each pattern in the extensively and detailed methodological section,\nso they are aware how the reaches were classified. Second, yes, similarly to the\nreviewer, we too would consider a river to be dominantly braided if a big part of its\nlength is braided and only some reaches had “occasional” islands.!! There is no\nconflict here between the reviewer and us. The wrong approach is attributing a single\nstyle to the whole river length. We are assessing all the reaches along the total river\nextension and quantifying what kind of channel patterns reaches are dominant along\neach river and even each basin.\nThus, a river can be dominantly braided along the assessed length extension but\nperhaps can also have some reaches with islands. In our study, all the reaches without\nislands would be computed as braided and the occasional reach with islands (stable\nstable\nareas identified by the analyses of the multidecadal record of vegetation cover and\npermanence and occurrence of water) would be classified as anabranching.\nTherefore, the river would be dominantly braided along a certain percentage of its\ntotal extension and have occasional anabranching reaches in smaller percentages.\nSo, once again, there is no conflict because we are not classifying a whole river as\nbraided or meandering or anabranching because of the “general perception of the\n10\nplatform” but computing reaches and quantifying in reaches the proportion of the\nriver lengths that exhibit one or another channel pattern.\nThe second comment relates to two large rivers, Jamuna-Brahmaputra and Indus,\nsuggesting that for the same reasons discussed above, the rivers could be classified\nas braided for other authors .…”nor is braiding manifestly that scarce in large rivers\n(Brahmaputra/Jamuna, Indus?). “\nAgain, the first assumption on our results by the reviewer is inaccurate when\nmentioning “rivers”. There is no controversy that some reaches along a river can be\nbraided and others with islands are anabranching. We are not generically classifying\n“rivers” but assessing river reaches. Having reaches with different patterns is the more\ngeneral situation and not the exception. That is clearly explained in our\nmethodological section on how we are classifying channel patterns.\nBut moving back to the examples used by the reviewer of the two large rivers (Indus\nand Brahmaputra/Jamuna), in these cases there are no “grey” shadows or dubious\nmethods to sustain how we classified those rivers' reaches.\nThe attribution of braided patterns by the reviewer in the Brahmaputra deserves\nspecial treatment. Since the early work by Coleman (Coleman 1969) and the seminal\nwork by British scientists such as Thorne, Best, Ashworth, Bristow, and others, mainly\nin the 1990s and early 2000s, when anabranching concepts and theories were almost\nnot even born, the British authors used concepts and classifications they knew from\nbraided rivers on the northern Hemisphere and focused on the study of in channel\nprocesses (fluvial bars, confluences) as the main morphological components. Thus,\nthe Brahmaputra, with large suspended sediment transport and a huge sandy bed\nload looked different from the traditional “anastomosing” vertical accretion, low\nenergy, fine-grained rivers described at the time in Canada, and as a consequence,\nthey usually called the Brahmaputra braided. Since that early time, the classification\nand theories of anabranching started to develop to classify multiple-channel alluvial\nrivers across the world. Thus, although the river tends to have braided reaches (that\nwere identified in our results) and even braided morphologies in the main and\nsecondary branches, also the river has many reaches with islands that are even\nseparating branches and flows even at bankfull discharge making those reaches of\nthe river to be classified as anabranching. Consequently, the Brahmaputra has many\nlarge islands and some are among the largest fluvial islands of the world such as\nMajuli Island (Sarma et al (Sarma 2005) among many others).\nThe first-rate paper by Thorne (Thorne, Russell and Alam 1993) on the Brahmaputra\ndescribes the difference between braid bars, which are highly unstable, changing\ntheir size and position between flood seasons, and islands, which survive for\n11\nseveral decades with perennial vegetation, cultivation, and human settlements.\nDunne and Aalto (Dunne and Aalto 2013) also rediscuss the pattern of the\nBrahmaputra and agree that despite its braided mobile bed the general pattern is\nanabranching in several reaches because of the islands. Also recognized Indian\nfluvial specialists classify long reaches of the Brahmaputra like anabranching.\nBy the way, in the paper mentioned by the reviewer (Ashworth and Lewin 2012) those\nauthors also classified the Brahmaputra as anabranching. They textually affirm… “The\nJamuna is an example of an anabranching river (Nanson and Knighton,1996)\nwith braided and occasional anastomosing branches that divide around\nvegetated, stable alluvial islands with heights up to the adjacent floodplain (\nBest et al.,2007).”\nNotice also that Ashworth and Lewin (Ashworth and Lewin 2012) acknowledge\nthe existence of “ … branches that divide around vegetated, stable alluvial islands\nwith heights up to the adjacent floodplain citing the work of J. Best 2007”. As\nadditional support to the discussion, despite Ashworth and Lewin did not mention\nit in 2012, Latrubesse (Latrubesse 2008) also classified the Brahmaputra as an\nanabranching river, so both authors/papers attributed to the river to the same\nclassification. Latrubesse (Latrubesse 2008) wrote… “ Numerous sand bars,\nhowever, can also be characteristic of some anabranching rivers, such as the\nBrahmaputra and Orinoco, where extensive channel bars form and result in local\nchannel braiding. Nevertheless, the overall pattern is anabranching, with large\nstable vegetated islands that do not seasonally adjust with annual flow variability,\nas with unconsolidated sediments comprising bars in a braided river. Examples\nof anabranching patterns with large islands and sand bars resembling braiding\npatterns are shown in Fig. 7. Several reaches of the Brahmaputra River (Fig. 7B)\n(Bristow, 1987, Sarma, 2005).”\nImportantly, related to the previous discussion above, notice that Ashworth and\nLewin (Ashworth and Lewin 2012) when classifying the major pattern\nJamuna/Brahmaputra as “Anabranching” they also classify the internal style of\nthe branches as braided and anastomosing, so anastomosing is considered by\nthem a sub-category and comprised into the envelope anabranching style.\nThe crystal-clear point here is that based on the anabranching definition\n(multichannel with islands and branches with morpho-hydraulic identify even at\nbankfull), the dominant pattern for several reaches of the Brahmaputra is\nanabranching and some reaches without islands are classified as braided.\nThe Indus River mentioned by the reviewer also deserves special attention. Back\nin time also was considered “braided”. But today, because of the islands and\nbranches, several reaches of the Indus are classified as anabranching. For\nexample, the mentioned specialist Paul Carling and collaborators (including co-\nauthor H.Q Huang, one of the founders of the theories of anabranching river, S.\nchannel) because of the presence of islands (Carling et al. 2018).\nThe same happens in the middle-lower Mississippi, a river traditionally described as\nmeandering. The Mississippi is considered the ultimate single-thread meandering\nriver on the planet in water discharge (Latrubesse 2008). But the Mississippi basin\nwas assessed all along its extension. So, yes, we have several sectors of the Mississippi\nbasins that have rivers with anabranching patterns and even the main tract of the\nMississippi has some anabranching reaches. Even in the middle-lower reaches the\nMississippi has large islands that divide the flows even at bankfull and the islands are\nrelevant geomorphic features. Historically, because of its huge dimensions and large\nw/d rations, in many cases currently ranging from 70-90 after human modifications,\nthe river used to have w/d rations of 100 to 200 in natural conditions from Vicksburg\nto Natchez. Consequently, the Mississippi has struggled to maintain a “pure” single\nmeandering channel, a fact early indicated by Schumm and also mentioned later by\nLatrubesse (Latrubesse 2008) and Knox and Latrubesse (Knox and Latrubesse 2016),\namong others. Historic maps indicate that, prior to human modifications, the lower\nMississippi had a substantial number of small Channel Islands, especially in\nmeander bends. This was particularly common in the middle reaches of the\nalluvial valley (Schumm, Rutherfurd and Brooks 1994), which underwent high\nrates of meander bend migration (Hudson and Kesel 2000). Nearly all bends\nincluded an island or\nThus, answering your question, the Mississippi is meandering on many reaches, but\nsome reaches have islands and more than one coetaneous functional channel.\nBecause we don’t discriminate different sub-categories of anabranching, the\nmultichannel sinuous reaches with islands of the Mississippi are categorized as\nanabranching reaches.\n7- REVIEWER: Unfortunately (given the careful and extensive data collection), there\nseems in the paper to be a purposefully preferred, & subjective, categorisation choice\nin favour of designating rivers solely as anabranching if islands are present. This\ngreatly expands Latrubesse’s admirable pioneering advocacy on behalf of multi-\nchannel systems [in Geomorphology 101 (2008) 130-145]. But to my mind a frame of\nmind has now, in this new paper, come to bias the way in which very useful data-\nderivation project has come to be used, classified, interpreted and invoked – for an\nover-egged promotion of ‘anabranching’ (lines 293-299; 335-337). Brief attention\nonly was given to ‘mixed patterns’ (line 651 ff.)\nANSWER: We appreciate the words on Latrubesse´s pioneer work (Latrubesse 2008).\nToday, after that paper, almost every fluvial geomorphologist consents that the\n13\nlargest rivers are dominantly anabranching. We are not over egged promoting\nanabranching. We are showing that the global data demonstrate that, when\nidentifying channel reaches, a variety of multichannel alluvial patterns are the most\ncharacteristics channel pattern arrangements in Earth.\nHence, regarding the comment by the reviewer, we have some difficulty\nunderstanding the criticism. What is indeed subjective in our categorization choice?\nThe categorization in braided meandering and anabranching groups is far from\nsubjective. Every year in AGU, RCEM, and any geomorphology fluvial session around\nthe world we have specific sessions devoted to discussing channel patterns,\nmeandering or braided rivers. Hundreds of oral presentations, abstracts, and posters\nare presented at those meetings every year on field data, flume experiments,\nnumerical and physical modelling of braided, meandering, and, at a very much\nsmaller scale, some on multichannel systems. Thousands of papers are published\nclassifying the rivers as meandering or braided and more recently as anabranching\nor at least using the word “multi-channels”.\nIf we understand the criticism correctly, then the reviewer suggests that because\nrivers are a continuum among end members it would be difficult to discriminate some\ntransitional channel styles and put/sort out some of those transitional cases into one\nof the four categories. But the reality is that for a global analysis like the one we\nconducted, that is not a major concern nor compromise the results and conclusions\nand how we discriminate patterns is clearly explained in the methodology. There is\nalso not brief explanation on ‘mixed patterns’ (line 651) but extensive explanations in\nthe methodology on how we defined and extracted patterns.\nOf course, we can have different types of meandering systems, more sinuous less\nsinuous more tortuous less tortuous, with more or fewer migration rates, with more\nor fewer sediments transport. Again, hundreds of papers and posters on meandering\nmodelling every year in AGU and other meetings are titled using the words “meander\nor meandering”, and everybody knows and understands that a meandering system\nis a sinuous single-channel pattern and massively considered by the fluvial\ncommunity a sinuosity minimum threshold of 1.3. That is a fact, and sinuosity is the\nstarting point to discuss any varieties and sub-types of meandering rivers we can\nidentify and classify on our planet, and even meanders on Mars. There is no\ncontroversy when we mention meanders on Mars or Earth about what type of\nchannel patterns we are talking about and why we identify them as meandering.\nThe same can apply to braided rivers, sessions, and books. Any geomorphologist\nknows from the early papers from sedimentologists and geomorphologists from the\nNorthern Hemisphere, that braided rivers are not characterized by islands but by\n14\nactive beds with dynamic bedload, multiple bars exposed at low stages, shallow\nmultiple channels surrounding sediment bars, and at a bank full discharge/stage, the\nbars are not acting as individual branches and the cross section tends to morpho-\nhydraulically homogenize. Thus, other processes and factors are secondary to\ncharacterizing the planform of braided rivers. For many decades thousands of papers\nhave been produced on braided rivers with a focus on fluvial bars and\nmodelling/flume experiments on fluvial bars. In all those papers, those channels are\nclassified as braided independently of the type of experiments or the characteristics\nof the fluvial bars. The relevance here is that there is a river planform where there\nare no islands but the dominant presence of exposed sediment bars at low stages,\nshowing in planform shallow multiple channels at low water stage, so shallow that\nthey are not acting as fluvial branches at all at higher water stages. In any definition\nof braided rivers, islands are included as a relevant or distinctive feature.\nEven this kind of classification in braided meanders and anabranching is used in\nfluvial sedimentology and has been a matter of discussion on the evolution of fluvial\nchannel patterns on Earth since the Palaeozoic to today\nThus, beyond trying to criticize why we discriminate single channel patterns from\nanabranching/multichannel patterns, we consider that a main interesting point here\nis that our results open a new horizon in fluvial science because there is a huge\namount of anabranching reaches and their geomorphic diversity and sub-patterns\nhave to be yet classified, categorize and, new theories and models build.\n8- REVIEWER: In claiming that about half of global rivers as being ‘anabranching’\n(and ignoring their other characteristics, however dominant they and their processes\nmay be), the authors downgrade the straightness, meandering and braiding that\nclearly enough co-exist alongside channel multiplicity. For example, the authors\nprefer to call the illustrated images of the Huallaga and Ene Rivers in the Andes\n(supplementary Figure 4) ‘anabranching’ rather than ‘braiding’ with islands. Braided\nchannel processes may produce vegetated islands quite commonly; meandering\nbifurcations and rejoinings also create binary courses through crevassing and\navulsions in anastomosing systems, too. When it comes to explaining pattern\nmechanisms, failing to distinguish low-power anastomosis from island-branching (or\neven meander-branching from braiding-branching) as separable ontologies seems\nregrettable.\nANSWER: So, the rationale of the reviewer is that because we have branches with\ndifferent styles we cannot demonstrate and, affirm that multichannel anabranching\nreaches are more dominant on the planet than single pattern reaches?\nworthy because branches can exhibit channel styles that, of course can be braided,\nstraight, meandering, and even anabranching. That is not the essence of the study.\nThe point is that we assume the reviewer was also shocked that there is such a huge\namount of anabranching reaches in Earth.\nYes, we know that some branches can be meandering straight or braided and even\nanabranching. What is the point again with anastomosis if there is no conflict at all\nwith what we explained before? There is a limited amount of planform arrangements\ncategories we already use in rivers and for instance, branches are part of that limited\nuniverse and can exhibit different patterns. Then, why is it a concern to the reviewer\nif all of us (the reviewer, authors) and specialists in fluvial geomorphology and\nanabranching rivers agree with that? The reviewer knows very well that characterizing\nthe patterns of all the branches of the world and trying to understand all the\nmultichannel arrangements of each identified anabranching reach in the planet and\nproducing some sort of classificatory system is a challenge that can take many years\neven with the advanced computational tools currently available. Our question is: Is\nnot the reviewer considering that our study opens a vast field of research for the\nyounger generation and new studies and models have to be built to understand why\nunder certain conditions branches will have similar or different patterns or how\nelements mentioned by the reviewer such in channel processes or channel-floodplain\ninteractions would control the enormous varieties of planforms arrangements we can\nsee in anabranching reaches?\nRegarding the critique of the Huallaga and Ene Rivers, it lacks substance. We are not\narbitrarily and subjectively “preferring” to classify those examples as anabranching.\nThey are not classified as braided with islands because as clearly explained in the\ndefinition of patterns and the methods, once you have islands and more than one\nactive channel in a reach, then the reach is anabranching and not braided. Contrarily,\nthe reaches with fluvial bars and no-islands are braided.\nSo, the reviewer says that we “prefer” … but we don’t prefer, we classified in function\nof the data and applied method. So why the examples of the Huallaga and Ene River\nreaches are classified as anabranching? First, because of the data sourced by the\ngeoprocessing as explained in the methods. Second, the islands of Huallaga and Ene\nRivers in the selected examples, are islands covered by dense 25-35meters tall\ntropical jungle trees in the Amazon basin. How it can be braided and not classified\nas anabranching despite the reviewer seeing fluvial bars in the channel branches?\n16\n9- REVIEWER: Particularly considering the praiseworthy effort and labour taken in\nobtaining this large data set, I am so sorry to find that it has been (in my opinion) \nmisdirected. It could have been (and adjusted, still could be) so useful without its\nmethodological pre-determination in favour of the conclusions drawn.\nANSWER: We are sorry the reviewer is not sensitive enough to value our contribution\nas relevant to be published and beneficial to open new discussions on channel\npatterns to the geomorphological and fluvial community. We cannot understand why\nthe reviewer consider we built under a conceptual predetermination when, in fact, we\nare presenting a concrete data set, with quantifications, spatialization, and statistics.\nThere is no pre-determination to favour anything. There is data and a reality that\ndespite the decades and the efforts by people like Gerald Nanson and some of our\nco-authors, the establishment still resists accepting that multichannel systems are a\nmajor component of the fluvial nature beyond single channel meandering and\nbraided rivers.\nTo review this kind of global paper for a top journal like Nature Communications,\nthere is no doubt that the reviewer is a topmost experienced fluvial specialist. Thus,\nfrom the bottom of our hearts, we appeal its decision and ask the following questions.\nFirst: before reading our manuscript, was the reviewer not happy in having for first\ntime a global assessment of the four major types of channel patterns? Why the result\nis not “per se” a pioneer and substantial discovery, of broad interest and impact, that\ncan positively motivate and shake the fluvial community?\nSecond, being a senior fluvial specialist, is the reviewer not surprised that for the first\ntime we, fluvial lovers have global results from any continent and even that the fluvial\ncommunity is provided by a quantification of the major channel patterns style even\nfor physiographic regions? Based on the personal professional and field experience\nand knowledge of the literature,… was the reviewer not surprised (like we were) by\nsome results from some regions of the world where the reviewer (and we) were\nexpecting different results on what channel patterns were dominant?.\nThird, was the reviewer not surprised that anabranching reaches were more\npresentative than braided or meandering globally in almost any region of the planet?\nFourth, is the reviewer not happy that for the first time, some colleagues were able\nto put some global results on fluvial discriminators for the major categories of\nchannel patterns such as slope, stream power, and others, opening a new opportunity\nto motivate scientific research and discussions on that matter? Does not consider the\nreviewer that these incipient and pioneer results can open neat and very healthy\n17\ndiscussions in our fluvial community and trigger curiosity to study some part of the\nworld yet unexplored?\nIn its introductory words, the reviewer kindly mentions that considers our article\ninnovative, supported by a meticulous methodology, and that the research produced\noriginal results on rivers on a global scale.\nFollowing this open and frank scientific exchange of opinions and arguments, we\nhope and appeal the reviewer can reconsider its first assessment and support the\npublication of our article.\nThis manuscript develops the largest accounting of river channel forms using large\nremote sensing datasets and analyzes the patterns in these forms using\nphysiographic, climatic, and geomorphic variables. The study is well conceived and\nwell designed, with the exception of one concern below. My main concern with this\nmanuscript is the significance given the recent publications with similar findings on\nthe prevalence of multi-channel rivers. The authors do cite these studies, but further\nexplanation of why the other studies’ results were incomplete would bolster the\nsignificance of this manuscript. Besides this concern, the framing of the importance\nof this study is well argued.\nI really enjoyed reading the introduction and review of channel form classifications\nthroughout literature. Similarly, the ‘Widespread Implications’ section is interesting\nand provides good perspective on what could be seen as simply classifying river form\nand instead focuses on the connection between river form and river function.\nANSWER: We appreciate the positive and constructive comments by Reviewer 2.\nWe included the suggestions and improved the manuscript to attend to the\nrecommendations of the reviewer.\nRegarding previous studies, we expanded the reference to the previous studies and\npresented even more detailed information on the methods, coverage, and other\nstrategies that differentiate our research and results.\nWe are confident the reviewer will be satisfied with the improvements we made to\nthe original version.\nAs suggested by the reviewer we also highlighted the differentness when necessary\nwith other studies and also provided the necessary credit to Wang et al (Wang et al.\n2022) and Nyberg et al (Nyberg et al. 2023) when required.\nMajor comments:\n- Lines 78-81: The characterization of Wang et al. 2022 as not globally representative\nand only focused on the largest rivers is slightly misleading as they collect data and\nstudy the entire basins (down to 180m width which is admittedly large) which are\nglobally distributed and include a range of river sizes. Similarly, the methodological\ndifferences in Nyberg et al., 2023 are real and important, but it is notable that they\nhave very similar final percentages of multi-threaded rivers. I don’t have any personal\nstake in these publications, but I think more credit is owed to these papers for\n19\nidentifying the global abundance of multi-channel rivers and the misconceptions\naround channel form occurrence. Further contrast with their findings would help\nexplain the novelty of your work.\nANSWER: We agree with Reviewer 2 and have expanded the references and credit\nto Wang et al (Wang et al. 2022), and Nyberg et al (Nyberg et al. 2023), while also\nhighlighting the differences between their study and our results and approach. We\nexplained the differences in the methodological and conceptual approaches and\nsome other elements as listed below.\nDifferences with Wang et al., 2022.\na) Channel patterns assessment. For the first time, we have a global classification\nand quantification of meandering, braided, anabranching, and straight\npatterns on the planet. It never was produced before and was not produced\nby Wang et al., 2022\nb) Also, it is the first time we have quantitative and spatial results on\nmeandering, braided, anabranching and straight river reaches per continent\nand physiographic regions at a global scale.\nc) Different global coverage. Wang et al 2022 studied the 20 largest basins. Big\nareas of the planet are not covered. Wang et al do not cover Oceania, Europe,\nand extensive areas in other continents.\nd) Wang et al 2022 identify anabranching patterns in the 20 largest basins.\nHowever, they don’t identify nor present quantitative and statistical data on\nthe occurrence and representativity of other types of channel patterns in those\n20 basins (meandering, braided, and straight).\ne) We presented the first global assessment confronting hydro-\ngeomorphological data and discriminators from all the detected rivers\nglobally, comparing meandering, braided, and anabranching rivers. On the\nopposite, Wang et al., 2022 do not identify meandering, braided, and straight\nreaches and only have data from the anabranching reaches of the 20 largest\nbasins, they present some correlations between large anabranching rivers\nwith stream power and sediment supply from anabranching reaches.\nNyberg et al 2023,\na) Nyberg et al 2023 have a major global coverage but it is even conceptually\nmore limited and geomorphologically poorest than Wang et al., 2022 and our\npaper. Nyberg et al 2023 only classify two categories, multi-treated versus\nsingle-channel belts, without using sustained geomorphological\nclassifications and criteria to catalogue channel patterns. Even worse, they\nenclose braided and anabranching in the same single category of multi-\n20\nthreaded channels, a serious conceptual mistake in geomorphology. They also\ndon´t identify and discriminate between meandering and straight channels\nbut group all of them as a single-threated category. Also, they don´t identify\nand separate alluvial channels from rocky channels. Because of the lack and\ninadequacy of a geomorphological conceptual framework, they also are not\nable to produce an assessment of channel patterns but a simplistic two\ncategories classification that erroneously mixes different types of channels in\nthe same class, and without presenting any global analyses of geomorphic\ndiscriminators to understand global channel patterns.\nLines 111-116. I think the combination of mountainous and unconsolidated sediment\nareas is confounding, as mountains have high slope and uplift in addition to bedrock\nconstraints. Further, I’m not sure the GUM dataset captures what you are looking for\nin terms of substrate control on morphology. For example, the Canadian Shield\nregion is heavily controlled by bedrock joints and fractures. GUM classifies this as\n‘glaciated’, and I didn’t read further to look into this, but it may just require tweaking\nwhich sediment types in GUM you consider unconsolidated sediment. Similarly parts\nof the lower Congo that are influenced by bedrock. I would like to see a map of just\nthe reach classifications in the supplemental.\nJust saw that you recognized this in methods section (lines 753-755), but I still think\nthis needs to be either recognized in the main text as a limitation of the classifications\nor addressed by revising the method. Given the importance of the physiographic\nclassification importance to the results, I would really encourage revisiting this\nclassification to see if you can improve it.\nANSWER: We agree with the reviewer that it needed better explanation and\nimproved the manuscript. Beyond GUM (Börker et al. 2018), we expanded the\nexplanation on what we considered non-mountain areas to make clear that cratonic\nareas such as the Canadian Shield and large areas of South America, Africa, and areas\nof plateaus and hilly regions (but not classified as mountains in GMBA mountain\ndataset (Snethlage et al. 2022)) are included in our non-mountain terrains.\nWe defined mountainous alluvial reaches as regions that fall within both GMBA\nmountain dataset and GUM dataset; and non-mountainous reaches are those not\nintersected by GMBA mountain dataset. Thus, non-mountainous rivers include those\nin lowlands, low dissected terrains, cratons, plateaus, and hilly regions that are not\nclassified as mountains. Cratonic Shields or Plateaus, like the Guiana Shield in South\nAmerica or parts of the Canadian Shield, represent large stable regions, with flat, hilly,\n21\nand some flat-shaped highlands, which are neither covered by the GMBA mountain\nmaps nor by the unconsolidated sediments map, and are therefore included in our\nnon-mountainous regions.\nMinor comments:\nI don’t often see “cumecs” written, mostly as a spoken shorthand. I think m3/s is more\ninterpretable and succinct, but you don’t need to address this you prefer cumecs.\nANSWER: We thank Reviewer 2 for pointing out the use of \"cumecs\". We have\nreplaced it with m3/s which is more interpretable and succinct.\nLine 76: “who” should likely be “which”\nANSWER: We also agree with Reviewer 2 highlighting the incorrect use of “who”. We\nhave changed the word \"who\" to \"which\" in the revised manuscript.\nFigure 1: Should the composited channels be their own inner grouping? (at the AN,\nBR, ME, ST level)\nANSWER: We appreciate the reviewer that a clearer explanation was needed to\nstrengthen the manuscript. Regarding the term “composited channels”, we have\nminimized its use to avoid confusion. We explain what these straight channels are\ndefined as “river reaches where straight channels connect two or more anabranching\nor meandering morphological structures”. Some examples are provided in Extended\nData Fig. 6.\nThe manuscript makes a massive effort to quantify prevalence of four channel\npatterns in global rivers. The methods are sound, and the results are encouraging for\nadvancing river studies. I only have some minor comments:\nANSWER: We deeply appreciate the review and comments. We are glad the reviewer\nvalued the effort and merit of our research, and the sympathetic assessment,\nconsidering our results of broad interest for the fluvial community and further\ndevelopments. We incorporated the suggestions and corrections made by the\nreviewer.\nLines 102-104: Please double check the SWORD. It is incorrect to state that it\n“excludes widths narrower than 150m”. Many channels in SWORD are actually less\nthan 100-m wide.\nANSWER: We appreciate reviewer’s suggestion to recheck the SWORD. Upon review,\nwe recognize that our previous description may have been unclear and could mislead\nreaders into thinking that SWORD itself does not include reaches narrower than 150\nm. To clarify, we intended to convey that we exclude reaches narrower than 150 m in\nour analysis. In this revised version of the manuscript, we have updated the text to\nexpress more clearly: “This is complemented by SWORD which serves as our primary\nreference for global river networks, with each reach spanning ~10km in length and\nexcluding widths narrower than 150m” has been revised to “This is complemented\nby SWORD, our primary reference for global river networks with each reach spanning\n~10 km in length, from which we exclude reaches flowing on bedrock (see Methods)\nor narrower than 150 m”. We hope this revised phrasing eliminates the potential\nconfusion.\nFig.2:I The percentage numbers in the histograms appear a bit crowded. It might be\nbetter to adjust the font position or reduce the size slightly for better readability.\nANSWER: We agree with the reviewer’s suggestion about changing the size of the\nfont in the histograms. The percentage numbers appeared crowded, making them\ndifficult to read clearly. It is a very important information. To address it, we have\nreduced the size slightly for better readability.\nLines 202-205: Do you have a figure and statistical data to support this argument\nabout small and medium-sized river basins?\nstatistical data to support our argument about small and medium-sized river basins.\nIn response, we calculated the anabranching proportion for small and medium- sized\nbasins in greater detail and listed examples of smaller basins with high anabranching\nproportion to support this argument. Specifically, we now state: “In addition to the\n20 large river basins mentioned above, we observed that in half of the remaining\nsmaller-sized river basins, anabranching channels constitute over 40% of the reach\nlength within their non-mountainous basin areas. The highest fraction of\nanabranching channels relative to total non-mountainous reach length is found in\nthe Ili River basin, Kazakhstan, where they account for 84% of reach length within its\nnon-mountainous basin areas. This pattern is similarly prominent in other smaller\nbasins, such as the São Francisco and Magdalena basins, where anabranching\nchannels comprise over 70% of reach length within their non-mountainous basin\nareas.”\nExtended Data Fig. 5: Please change Wang to Wang et al., 2022 and include the\ncitation in the caption.\nANSWER: Thank you and we have changed Wang to Wang et al., 2022 and included\nthe citation.\nLine 485-Data sources: Could you clarify the data sources used for floodplain width\nand discharge?\nANSWER: We appreciate the reviewer for raising this important point about\nclarifying our data sources to strengthen our manuscript. The exact datasets and\nprocessing steps for floodplain width and discharge are detailed in the\nSupplementary Information. To enhance clarity, we have added sentences about the\nhydro-geomorphic factors we used in the \"Data sources\" section. For detailed\ncalculations of these variables, please refer to Supplementary Note 3. Additionally,\nwe have updated the flowchart (Extended Data Fig. 1) of the hydro-geomorphic\nfactors to visually clarify the various datasets and complex steps in the methodology.\nIt would greatly aid in simplifying the workflow for readers.\nLines 784-786: This section is a bit unclear to me. The numbers presented here (i.e.\n1846 and 1935) seem quite low on a global scale. Given SWORD includes over\n210,000 reaches, are you suggesting only 1846 and 1935 reaches are fully\nanabranching and meandering, respectively?\nto carefully clarify these numbers. Since the paragraph refers to straight channels, the\nnumber presented here is straight channels connected with two or more\nanabranching or meandering morphological structures (i.e., scenarios of c and d, see\nExtended Data Fig. 6), rather than the total length of anabranching or meandering\nchannels. The number of straight channels we presented here is not low on a global\nscale, on the contrary, it accounts for a high proportion with 31% of straight channels,\nhighlighting that a lot of straight channels connect two or more anabranching or\nmeandering morphological structures.\nTo further clarify, we have added the number of remaining SWORD reaches at each\nprocessing step to our flowchart (Extended Data Fig. 1). This provides a clear overview\nof the total population initially considered and the filtering process that led to the\nfinal numbers.\nAshworth, P. J. & J. Lewin (2012) How do big rivers come to be different? Earth-\nScience Reviews, 114, 84-107.\nBörker, J., J. Hartmann, T. Amann & G. Romero‐Mujalli (2018) Terrestrial\nsediments of the earth: development of a global unconsolidated sediments\nmap database (GUM). Geochemistry, Geophysics, Geosystems, 19, 997-\n1024.\nCarling, P., J. Jansen & L. Meshkova (2014) Multichannel rivers: their definition\nand classification. Earth Surface Processes and Landforms, 39, 26-37.\nCarling, P., H. Trieu, D. Hornby, H. Q. Huang, S. Darby, D. Sear, C. Hutton, C.\nHill, Z. Ali & A. Ahmed (2018) Are equilibrium multichannel networks\npredictable? The case of the regulated Indus River, Pakistan.\nGeomorphology, 302, 20-34.\nChurch, M. (1983) Pattern of instability in a wandering gravel bed channel.\nModern and ancient fluvial systems, 169-180.\nColeman, J. M. (1969) Brahmaputra River: channel processes and\nsedimentation. Sedimentary geology, 3, 129-239.\nDesloges, J. R. & M. A. Church (1989) Wandering gravel‐bed rivers. Canadian\nGeographer/Le Géographe canadien, 33, 360-364.\nDunne, T. & R. Aalto. 2013. 9.32 Large river floodplains.\nHudson, P. F. & R. H. Kesel (2000) Channel migration and meander-bend\ncurvature in the lower Mississippi River prior to major human modification.\nGeology, 28, 531-534.\nKnox, R. L. & E. M. Latrubesse (2016) A geomorphic approach to the analysis of\nbedload and bed morphology of the Lower Mississippi River near the Old\nRiver Control Structure. Geomorphology, 268, 35-47.\nLatrubesse, E. M. (2008) Patterns of anabranching channels: The ultimate end-\nmember adjustment of mega rivers. Geomorphology, 101, 130-145.\n--- (2015) Large rivers, megafans and other Quaternary avulsive fluvial systems:\nA potential “who's who” in the geological record. Earth-Science Reviews,\n146, 1-30.\nLeopold, L. B. & M. G. Wolman. 1957. River channel patterns: braided,\nmeandering, and straight. US Government Printing Office.\nLewin, J. & P. J. Ashworth (2014) Defining large river channel patterns: alluvial\nexchange and plurality. Geomorphology, 215, 83-98.\nMakaske, B. (2001) Anastomosing rivers: a review of their classification, origin\nand sedimentary products. Earth-Science Reviews, 53, 149-196.\nNanson, G. C. & A. D. Knighton (1996) Anabranching rivers: their cause,\ncharacter and classification. Earth surface processes and landforms, 21,\n217-239.\nNyberg, B., G. Henstra, R. L. Gawthorpe, R. Ravnås & J. Ahokas (2023) Global\nscale analysis on the extent of river channel belts. Nature\nCommunications, 14, 2163.\nSarma, J. (2005) Fluvial process and morphology of the Brahmaputra River in\nAssam, India. Geomorphology, 70, 226-256.\nSchumm, S., I. Rutherfurd & J. Brooks. 1994. Pre-cutoff morphology of the lower\nMississippi River. In The variability of large alluvial rivers, 13-44.\nSchumm, S. A. (1985) Patterns of alluvial rivers. Annual Review of Earth and\nPlanetary Sciences, Vol. 13, p. 5, 13, 5.\nWestern Canada. Modern and ancient fluvial systems, 155-168.\n--- (1986) Anastomosing river deposits, sedimentation rates and basin\nsubsidence, Magdalena River, northwestern Colombia, South America.\nSedimentary Geology, 46, 177-196.\nSmith, D. G. & N. D. Smith (1980) Sedimentation in anastomosed river systems;\nexamples from alluvial valleys near Banff, Alberta. Journal of Sedimentary\nResearch, 50, 157-164.\nSnethlage, M. A., J. Geschke, A. Ranipeta, W. Jetz, N. G. Yoccoz, C. Körner, E.\nM. Spehn, M. Fischer & D. Urbach (2022) A hierarchical inventory of the\nworld’s mountains for global comparative mountain science. Scientific\ndata, 9, 149.\nThorne, C. R., A. P. Russell & M. K. Alam (1993) Planform pattern and channel\nevolution of the Brahmaputra River, Bangladesh. Geological Society,\nLondon, Special Publications, 75, 257-276.\nWang, B., L. C. Smith, X. Yang, T. M. Pavelsky, E. H. Altenau, C. J. Gleason, A.\nPietroniro, E. Rodriguez & P. D. Bates (2022) Remote sensing of broad-\nscale controls on large river anabranching. Remote Sensing of\nEnvironment, 281, 113243.\n27"}, {"round": 2, "reviewer_comments": {"reviewer_2": "I appreciate the authors' efforts to address my comments, and those of the other reviewers. I am satisfied with their addressing of the difference with previous work and clarifications on classifications.\n\nI also feel compelled to comment on previous comments and the authors' responses to Reviewer 1. My reading of this exchange is largely a disagreement on methodologies or definitions of river morphology classifications. I am more aligned with one of Reivewer 1's comments that the 4 end-member classification system breaks down and fails to be useful when considering the true diversity of real river systems. Nevertheless, I understand the utility of simplifying real systems into digestible classes, especially when working at the global scale. Given the adequate description by the authors of their methods and classification scheme, I do not find issue with the presentation of their results in these categories.\n\nTo this classification question, I would like to see the data in the form it will be released publicly (before accepting the manuscript). It would add great value if, in addition to final classifications, the derived properties that were used for the authors ~specific~ classification scheme were made available. This would allow the hard work in creating this dataset to be reused under different schemes, which may highlight differences that are useful to specific processes. I.e. river discharge algorithms have different geomorphological classifications and are affected by river form differently than sediment transport or grain size analyses.", "reviewer_3": "The authors have answered all my questions. I have no further comments on the manuscript.", "reviewer_4": "I was requested to review this manuscript as Reviewer 1 was unavailable for the second round of evaluation. The original request explicitly mentioned that the manuscript had already undergone one round of review and that I should consider the response to the first review while also having the freedom to provide my own feedback. Given this, my evaluation focuses primarily on the points raised by the previous reviewer, although I also include some of my own observations.\n\nThe central discussion in the previous review revolved around the classification of major channel patterns, and I sympathize with arguments made by both the reviewer and the authors. Much of the disagreement appears to stem from the inherent continuum of fluvial patterns, where rigid classification frameworks struggle to capture transitional cases. Despite this, I agree with the authors that classification is still meaningful, but given that a dominant pattern exits. In this sense, labeling a river reach based on its prevailing morphology—whether anabranching, braided, meandering, or straight—without going into the secondary characteristics of individual channels within that structure is a reasonable approach.\n\nHowever, I also understand the concerns raised by the reviewer. In cases where multiple patterns coexist within a reach, the presence of a single characteristic, such as an island, should not necessarily dictate the classification unless it is a dominant feature. As an extreme hypothetical example, if a 100 km reach (the reach definition is arbitrary so let me make it extreme) contains a single 1 km island, in my opinion, it would be inappropriate to classify the entire reach as anabranching. The presence of such features needs to be weighed carefully in determining the primary pattern.\n\nThis brings me to a broader issue regarding global-scale analyses. These studies have been instrumental in advancing our understanding of geomorphic systems by showing us global patterns, but these analyses require automating classification processes and minimizing human intervention. Their reliability depends on the robustness of both the methodology and its underlying assumptions. The present study relies on 10 km segmentation of rivers (reaches) from the SWORD dataset, and its pattern classification framework, that is defined in the manuscript. In their response to Reviewer 1, the authors assert that their classification of anabranching patterns is methodologically objective and clear. If I understand their methods correctly (I hope I am mistaken and that this can be solved with a better description of the method), any reach of 10 km in length containing an island of at least 0.01 km² would be classified as either braided or anabranching. A rough back-of-the-envelope calculation suggests that only about 0.07% of the wet area of the reach needs to be occupied by an island for it to be denoted as multithreaded. Similarly, in meandering reaches, cut-off channels or oxbow lakes (remnants of former meander loops that have been bypassed by the active channel but may still hold water) may influence classification. These are precisely the cases that I suspect concerned the previous reviewer and that I share major reservations about. The methodology, as currently described, appears to introduce a bias toward overrepresenting anabranching and braided systems, which could affect the study's conclusions.\n\nThis issue is fundamental, as the validity of the study's claims depends on the robustness of its classification approach. In some cases, a strong methodology might yield mixed classifications rather than forcing each reach into a single discrete category. Many geomorphologists recognize that fluvial patterns are not mutually exclusive but exist along a continuum. While the authors correctly argue that these major pattern types are widely accepted and taught, I would disagree with the implicit notion that they always exist as fully distinct entities (in most of the textbook examples and well-studied cases, one pattern dominates)\n\nAn additional side-concern is whether this rigid classification framework contributes to the relatively weak results obtained from the PCA analysis. If the imposed classification does not adequately capture real-world variability, it is unsurprising that the PCA fails to yield strong differentiation between pattern types.\n\nIn a different note, another point that should be more explicitly stated is the dataset's coverage limitations. Due to the 150 m minimum width resolution, the study only captures 38% of global alluvial reaches from the SWORD dataset and 58% of alluvial reaches excluding intermittent arid channels. This is an important limitation and should be highlighted more prominently in the manuscript.\n\nAs a minor methodological point, I wonder why the authors opted to use NDVI and IR sequentially rather than jointly when identifying islands. Some islands may exhibit high NDVI values yet be frequently inundated, potentially affecting classification.\n\nFinally, regarding the accuracy assessment presented in Extended Data Table 1, the error matrices indicate relatively high classification accuracy, except for braided systems. However, I would be interested to know whether these classification errors are spatially autocorrelated—i.e., whether misclassification tends to persist across contiguous reaches within specific river systems. If errors cluster in certain areas due to regional factors (e.g., incorrect NDVI-based assumptions), they could introduce significant biases into the overall analysis and affect the conclusions drawn from the dataset.\n\nBased on these concerns, unless I have misunderstood aspects of the methodology, I am inclined to support some of the reservations raised by the previous reviewer. Given the potential for classification biases to impact the study’s conclusions, I am unable to recommend this manuscript for publication in Nature Communications in its current form."}, "author_response": "ANSWER TO REVIEWERS:\nREVIEWER 2\nWe welcome that the reviewer considers our paper can now be accepted for publication.\nIn response to the Reviewers' request, we are expanding the information on Data\nAccessibility. The global alluvial channel pattern (GACP) established in this study is\navailable via figshare at https://figshare.com/s/db4065c089d798f6177c\nREVIEWER 4\nWe appreciate the positive review and analysis of our paper by Reviewer 4 and the\nvaluable recommendations and suggestions. We made a significant effort to attend to all\nthe recommendations and to answer the questions raised by the Reviewer. We consider\nthat our improved version meets the standard required by the reviewer, answers and\nclarifies the major questions on methods, and provides, with the available state-of-the-art\ntechnology, robust results, never processed previously at a global scale by another\nresearch group. As previously stated, we are confident our paper will be a worthy\ncontribution to the global community of fluvial specialists and will open exciting\ndiscussions among scientists in our discipline.\n1-Reviewer: I agree with the authors that classification is still meaningful, but given that\na dominant pattern exists. In this sense, labeling a river reach based on its prevailing\nmorphology—whether anabranching, braided, meandering, or straight—without going\ninto the secondary characteristics of individual channels within that structure is a\nreasonable approach.\nAnswer: We appreciate the reviewer´s understanding that our approach to classifying\nfour major categories of channel patterns is valid and appropriate.\n2-Reviewer. The major questions by the reviewer are:\n1) In cases where multiple patterns coexist within a reach, the presence of a single\ncharacteristic, such as an island, should not necessarily dictate the classification\nunless it is a dominant feature….If I understand their methods correctly (I hope I\nam mistaken and that this can be solved with a better description of the method),\nany reach of 10 km in length containing an island of at least 0.01 km² would be\nclassified as either braided or anabranching. A rough back-of-the-envelope\ncalculation suggests that only about 0.07% of the wet area of the reach needs to\nbe occupied by an island for it to be denoted as multithreaded.\n2) regarding the accuracy assessment presented in Extended Data Table 1, the error\nmatrices indicate relatively high classification accuracy, except for braided\nsystems”; and that “As a minor methodological point, I wonder why the authors\nopted to use NDVI and IR sequentially rather than jointly when identifying\nislands. Some islands may exhibit high NDVI values yet be frequently inundated,\npotentially affecting classification”.\n3) “In meandering reaches, cut-off channels or oxbow lakes (remnants of former\nmeander loops that have been bypassed by the active channel but may still hold\nwater) may influence classification. These are precisely the cases that I suspect\nconcerned the previous reviewer and that I share major reservations about. This\nissue is fundamental, as the validity of the study's claims depends on the\nrobustness of its classification approach.”\n4) The methodology, as currently described, appears to introduce a bias toward\noverrepresenting anabranching and braided systems, which could affect the\nstudy's conclusions.\nAnswer: We understand the criticisms of our previous GACP method and the necessity\nto explain in more detail and precision how to distinguish channel patterns at the reach\nscale. The reviewer suggested that our method, relying on the absolute area of islands and\nbars, would be unreliable to discriminate some transitional patterns of\nstraight/meandering and anabranching/braided channels at our nearly 10 km reach scale.\nParticularly, the reviewer is concerned that our method could overestimate the presence\nof islands when attributing a channel pattern to a particular reach. Also, the reviewer\nsuggested using in a combined way, NDVI and IR. To address the requirements of the\nreviewer, we incorporated all the suggestions and made a very substantial effort to\nstrengthen the methods and validate the results. We included the new methodological\ninformation spread throughout the main manuscript, but mainly in the Methods section.\nAccordingly, several figures and tables were updated and incorporated Fig. 1 – 4 in\nmanuscript file; Extended Date Fig. 1, 2, 5, 7, 8, and Extended Data Table 1 in Extended\nData file; Supplementary Fig. 3 and Supplementary Table 1 – 3 in Supplementary\nInformation file.\nPlease allow us to make a detailed step-by-step description of the methodology, as it has\nbeen the crucial point we needed to elaborate on and answer you. The new figure below\nexplains the improved methodological framework.\nWe called our earliest method GACP. A very important fact on the method, to be\nconsidered by the Reviewer, is that we selected 4413 reference reaches over high-\nresolution (sub-meter) Google Earth images to evaluate the accuracy of our classified\nchannel types. The selection was carried out through a stratified random sampling strategy\nto ensure representation across different physiographical regions and channel types. The\ntotal number of validation samples for anabranching, braided, meandering, and straight\nreaches was 2153, 225, 1060, and 975, respectively, spanning various regions globally,\nincluding non-mountainous (3091 reaches), mountainous (491 reaches) and coastal (831\nreaches) regions. For each selected reference reach, we thoroughly examined all available\nhigh-resolution historical images during the wet season on Google Earth. Comparing\nthese referenced reaches with the channel classifications, we achieved an overall accuracy\nof 88.4%. Please, be aware and take in mind that the study by Wang et al., 20221, used to\nvalidate uncertainty of Ai calculation (only anabranching) using RivWidthCloud\nalgorithm, only 200 reaches from all reaches in the 20 river basins and manually digitize\nall visible channels around the selected reaches in the same water mask they use as input\nfor RivWidthCloud algorithm. Nyberg et al2 used a total of 790 localities for training, but\n370 were riverine examples, and an additional 420 localities were non-riverine regions.\nThey used only 245 manually described river morphologies from Hartley et al3 and the\nsingle- versus multi- threaded prediction of the GCB model achieved an 84% accuracy.\nAlso, remember that Nyberg et al do not discriminate alluvial from non-alluvial channels\nbut only a generic and geomorphologically useless two-group classification: single-\nthreaded channels and multi-threaded channels (in rock or alluvium). Thus, we draw the\nreviewer’s attention to the remarkable differences in the amount of data and processing\n(4413 reaches) used to validate the results in our first GACP approach, and also the\nappropriate alluvial channel patterns categories we used and discriminated in comparison\nto previous studies.\nThe primary distinction between straight/meandering and anabranching/braided channels\nlies in sinuosity for single-threaded patterns, and in the presence of mid-channel islands\nor bars for multi-threaded systems. Accordingly, we first identified islands and bars fully\nenclosed within the channel, excluding those in contact with the channel margin (i.e.,\nlateral and point bars). To minimize noise and ensure robust detection, we applied an area\nthreshold of 0.01 km2 (equivalent to 11 pixels), discarding any islands or bars below this\nsize (Extended Data Fig. 9).\nHowever, despite being GACP a well-calibrated method, the Reviewer considered that,\nas presented, relying on islands and bars (plus sinuosity) was not considered enough to\ndiscriminate with precision between straight, meandering, anabranching, and braided\nchannels at our nearly 10 km reach scale because some transitional patterns would\npotentially be misclassified. Therefore, to attend the questioning raised by the Reviewer,\nwe additionally introduced and calculated an Islands and Bars Index (IBi) to identify and\nclassify reaches with islands and bars, adapted from the approach of Wang et al (2022)1.\nFor each reach containing islands or bars, we generated a bounding circle with a diameter\nequal to the corresponding SWORD reach length. IBi was defined as the ratio between\nthe total active channel length within this circle and the original SWORD reach length:\nIBi = LT,\nwhere L is the total length of extracted centerlines—derived from the 25%\nT\nLR\nwater occurrence maps using the RivWidthCloud algorithm—and L is the SWORD\nR\nreach length. We excluded islands or bars with a surface area < 0.3 km2 to reduce\ncenterline complexity, smoothed the extracted centerlines using the Smooth Line function\nto eliminate sharp angles, and removed short spurious branches (< 300 m), aligning with\nWang et al.\nThus, for reaches with IBi ≤ 1, we further calculated a relative area (RA) metric to\nevaluate whether the presence of islands or bars was morphologically significant. RA was\ndefined as the ratio of the total area of extracted islands and bars to the total channel area:\nA\nRA = islands and bars . This additional filter helped differentiate reaches where minor\nA\nchannel\nfeatures might otherwise lead to misclassification. Specifically, reaches with IBi ≤ 1 and\nRA < 1% were deemed to lack geomorphologically meaningful islands or bars and were\nsubsequently reclassified as straight or meandering, rather than multi-threaded types.\nFor anabranching and braided channels, the primary diagnostic criterion lies in the\ndominance of either stable vegetated islands or dynamic sediment bars within the reach.\nBraided bars are typically unstable, exhibiting substantial shifts in size, position, and\nmorphology between flood events, whereas islands are geomorphologically stable\nfeatures that persist over years to millennia. These islands often support perennial\nvegetation, agricultural use, or even permanent human settlement4-6.\nOverall, a fundamental distinction between anabranching and braided channels lies in\ntheir in-channel features: islands in anabranching systems are typically vegetated and\nmorphologically stable, whereas bars in braided systems tend to be barren and more\ndynamic7,8. A second key difference is that vegetated islands are more resistant to\ninundation, while sediment bars are more frequently and easily flooded. To quantitatively\ndifferentiate these two channel types, we employed two diagnostic metrics: the\nnormalized difference vegetation index (NDVI) and the inundation ratio (IR). NDVI is a\nwidely used proxy for vegetation cover and greenness, computed as: NDVI =\nSR(near infrared) – SR(red)\n, where SR(λ) is the atmospherically corrected surface\nSR(near infrared) + SR(red)\nreflectance (dimensionless) at a specific Landsat band. We derived NDVI using the full\narchive of Landsat surface reflectance products from 1984 to 2020, including Landsat-4\nTM, Landsat-5 TM, Landsat-7 ETM+, and Landsat-8 OLI, all accessed through the\nGoogle Earth Engine platform. Low-quality observations—such as those affected by\nclouds, cloud shadows, snow, ice, or radiometric saturation—were excluded using the\nCFMask quality assurance (QA) flags9,10. To characterize long-term vegetation presence,\nwe applied a maximum value composite (MVC) approach, selecting the highest NDVI\nvalue per pixel across all valid observations. The median NDVI value across all extracted\nisland or bar pixels within each reach was then computed to represent the overall\nvegetation status of in-channel features. To assess the inundation behavior of in-channel\nfeatures, we defined the inundation ratio (IR) as: IR = 1−\nTC0\n, where TC is the total\nnumber of island and bar pixels with zero water occurrence (i.e., never inundated), and\nTC is the total number of pixels classified as islands or bars. The term,\nTC0\nthus represents\nTC\nthe fraction of island and bar pixels that remain exposed even during the flood season\n(i.e., where GSWO = 0), relative to the total area of these features exposed during the\nbroader wet season (i.e., GSWO < 25). Consequently, IR serves as a proxy for the degree\nof inundation, with higher IR values indicating more frequent submergence and lower\nvalues suggesting greater geomorphic stability and flood resistance.\nAs requested by the Reviewer, we jointly used the NDVI (i.e., median of MVC) and IR\nto classify anabranching and braided. To differentiate anabranching from braided\nchannels, we employed a support vector machine (SVM) classifier, a widely used\nsupervised machine learning technique effective for binary classification tasks11. The\nSVM seeks an optimal decision boundary that maximizes the margin between classes\nwhile minimizing hinge loss associated with misclassified samples12. We used the median\nNDVI (derived from the maximum value composite) and the IR as classification features,\ncalculated for 2,529 representative reaches distributed across diverse environmental and\nphysiographic regions (Extended Data Fig. 2). Of these, 70% were used to train the model\nand 30% were reserved for testing its performance. Model optimization was conducted\nusing a grid search approach to identify the best-performing hyperparameters. The final\nmodel used a radial basis function (RBF) kernel with a gamma value of 0.5, which\ncontrols the influence of individual data points, and a regularization parameter (C) of 0.1\nto balance model complexity and accuracy. The classifier was implemented using the\nScikit-learn library13 and achieved a test accuracy of 0.94. We then applied the trained\nSVM model to all global river reaches containing mid-channel islands or bars, enabling\nreliable and consistent classification of anabranching versus braided systems based on\ntheir vegetation cover and inundation behavior. Thus, the suggestion by the Reviewer was\nattended.\nThe combined use of NDVI and IR effectively captures the contrasting characteristics of\nanabranching and braided channels. In general, braided bars exhibit low NDVI and high\nIR values, reflecting sparse vegetation cover and frequent inundation, while anabranching\nislands tend to show high NDVI and low IR values, indicating greater vegetative stability\nand resistance to flooding (Extended Data Fig. 2b; located in the upper left and lower\nright quadrants, respectively). Nonetheless, several important exceptions merit careful\nconsideration. First, anabranching channels with both low NDVI and IR values are\ncommonly found in cold or arid regions, where stable islands are seldom inundated but\nsupport only sparse vegetation due to environmental constraints (Extended Data Fig. 3e).\nSecond, although anabranching islands are generally more resistant to inundation, they\nmay still be submerged during rare and high-magnitude flood events. For instance, in\nsome tropical river systems, such as those in the Amazon basin, anabranching islands can\nexperience inundation with return intervals of approximately 40 years, yet remain\ngeomorphologically stable and sustain dense flood-adapted vegetation (e.g., igapó and\nvárzea forests). Third, to minimize misclassification of island inundation caused by\nhuman activities—such as aquaculture ponds, which often appear as low NDVI but high\nIR features in anabranching systems in regions like the Mekong Delta or southern\nChina—we incorporated artificial impervious area data as an additional diagnostic layer.\nReaches containing islands intersected by impervious areas were interpreted as\nanthropogenically stabilized and thus classified as anabranching, recognizing the\nstabilizing role of human land use. This supplemental step enhances classification\naccuracy by accounting for both natural and anthropogenic influences on island stability,\nthereby improving the distinction between anabranching and braided channels. Thus,\nreaches revealed as potential “mixed pattern”, which posed challenges in\ndifferentiating them, were classified as either anabranching-dominant or braided-\ndominant based on our systematic classification rather than arbitrarily.\nFor straight and meandering channels, sinuosity—an established morphological index—\nwas used to differentiate these two types, given their distinct morphological differences.\nSinuosity was calculated as the ratio of the reach length (i.e., the SWORD reach length)\nto the valley length (i.e., the straight-line distance between the reach endpoints):\nSinuosity = LR,\nwhere L is the SWORD reach length while L is the valley length. We\nR V\nLV\nutilized the sinuosity threshold of 1.3, in line with previous studies93-95, to distinguish\nbetween straight (low sinuosity) and meandering (high sinuosity) channels.\nConsequently, all the reaches with sinuosity ≥ 1.3, IBi ≤ 1, and RA < 1% were classified\nas meandering, and those with sinuosity < 1.3, IBi ≤ 1, and RA < 1% were classified as\nstraight. These criteria were incorporated into the classification algorithm used to assign\nchannel types globally.\nAlthough we made a huge effort to run again global data and to apply new techniques and\nmethods to address the reviewer's suggestions, there were no significant quantitative\ndifferences from the previous results. Nothing has changed in the main conclusion of our\narticle, regarding the representativeness and global distribution of the different types of\nchannel patterns (Table below). In conclusion, when comparing the results of the GACP\nmethod with this current improved and definitive four-step method, the results are quite\nsimilar. Thus, the fourth question by the Reviewer is answered. There is no bias in the\nmethod, classification, or numbers.\nConsidering that we attended to all the suggestions by the reviewer, and the conclusions\nare sustained by applying the methodological approaches suggested by the referee, we\nlook forward to the final acceptance of our manuscript.\nAnabranching +\nanabranching braided meandering straight total\nbraided\nVersion 1 30555 3401 33956 13079 11755 58790\nVersion 2 29768 3782 33549 13286 11954 58790\nComparison of the number of reaches from the previous version and the new multi-step\napproach results.\n3-Reviwer: another point that should be more explicitly stated is the dataset's coverage\nlimitations. Due to the 150 m minimum width resolution, the study only captures 38% of\nglobal alluvial reaches from the SWORD dataset and 58% of alluvial reaches excluding\nintermittent arid channels. This is an important limitation and should be highlighted more\nprominently in the manuscript.\nAnswer: We appreciate the review’s suggestion to more explicitly highlight the dataset’s\ncoverage limitations. In response, we would like to clarify that this limitation has already\nbeen explicitly stated in several parts of the manuscript:\nIn the Results section: “The total length of alluvial channel reaches investigated in our\nGACP datasets is 0.69 million km, with individual reaches ranging from 0.3 km to 38 km,\ncovering 38% of alluvial reaches in the SWORD dataset.”\nIn the Methods section about developing our algorithm: “The final analysis encompassed\n58,790 alluvial river reaches globally, representing 38% of alluvial reaches in the\nSWORD dataset, or 58% when excluding intermittent arid reaches.”\nIn the Methods section about the methodological uncertainty and limitations:\n“Additionally, extracting smaller bars and islands in narrow reaches, where the width is\ntoo small to detect such features from the 30 m GSWO map that led us to study only 38%\nof alluvial reaches in the SWORD dataset or 58% when excluding intermittent arid\nreaches, presents another challenge.”\nWe believe these statements clearly acknowledge the coverage limitations of our study\nand have presented them prominently throughout the manuscript.\n4-Reviewer: Regarding the accuracy assessment presented in Extended Data Table 1,\nthe error matrices indicate relatively high classification accuracy, except for braided\nsystems. However, I would be interested to know whether these classification errors are\nspatially autocorrelated—i.e., whether misclassification tends to persist across\ncontiguous reaches within specific river systems. If errors cluster in certain areas due to\nregional factors (e.g., incorrect NDVI-based assumptions), they could introduce\nsignificant biases into the overall analysis and affect the conclusions drawn from the\ndataset.\nAnswer:\nWe thank the reviewer for raising this important point regarding the potential spatial\nautocorrelation of classification errors. Our accuracy assessments were shown in\nExtended Data Fig. 5 and Extended Data Table 1. Among the 4,413 reaches used to assess\nthe accuracy of our alluvial channel pattern classification algorithm, 88.4% (3,903\nreaches) were correctly classified, while 11.6% (510 reaches) were misclassified.\nTo investigate whether misclassification errors are spatially clustered, we examined their\nspatial distribution across continents and physiographic regions. At the continental scale,\nmisclassifications appeared to be relatively evenly distributed, with a slight concentration\nin Europe (Table 1). At the physiographic scale, errors were also broadly dispersed,\nalthough a higher frequency of misclassification was observed in mountainous regions\n(Table 2). This may be attributed to relative narrower channel widths or lower water\noccurrence values in these areas, which can affect the reliability of features such as islands\nor bar extraction, NDVI, and inundation ratio calculation. Overall, we found no strong\nevidence of systematic spatial clustering of errors across contiguous reaches within\nspecific river systems. Although some misclassifications do occur, they are not appear to\nintroduce regional biases that would significantly impact the global conclusions of our\nanalysis.\nExtended Data Fig. 5 | Accuracy assessments. Accuracy of the channel morphology\ncompared to reference sample reaches based on Google Earth images.\nTbale 1. Accuracy assessments at continent scale.\nTotal reaches Correctly Incorrect % of incorrect\nContinent\nused in test set reaches reaches reaches\nAfrica 406 361 45 11.1%\nAsia 2202 1774 228 10.4%\nEurope 340 291 49 14.4%\nNorth America 800 690 110 13.7%\nOceania 68 62 6 8.8%\nSouth America 797 725 72 9.0%\nTotal 4413 3903 510 11.6%\nTable 2. Accuracy assessments at physiographical scale.\nPhysiographical Total reaches Correctly Incorrect % of incorrect\nregions used in test set reaches reaches reaches\nNon-mountain 3091 2767 324 10.5%\nMountain 491 406 85 17.3%\nCoastal 831 730 101 12.2%\nTotal 4413 3903 510 11.6%\nReferences\nanabranching. Remote Sensing of Environment 281, 113243 (2022).\nscale analysis on the extent of river channel belts. Nature Communications 14,\n2163 (2023).\ndistributive fluvial systems: characteristics, distribution, and controls on\ndevelopment. Journal of Sedimentary Research 80, 167-183 (2010).\ngeomorphology, 544 (2022).\nsystems: A potential “who's who” in the geological record. Earth-Science\nReviews 146, 1-30 (2015).\nevolution of the Brahmaputra River, Bangladesh. Geological Society, London,\nSpecial Publications 75, 257-276 (1993).\nand classification. Earth surface processes and landforms 21, 217-239 (1996).\nGeomorphology 9, 289-312 (2013).\nLandsat imagery. Remote sensing of environment 118, 83-94 (2012).\noperational Landsat data products. Remote sensing of environment 194, 379-390\n(2017).\nsensing. International journal of remote sensing 26, 1007-1011 (2005).\napplications for engineers and system designers 39-66 (Springer, 2015).\nmachine Learning research 12, 2825-2830 (2011).\napplications of the river styles framework. (John Wiley & Sons, 2013).\nthe Devonian of Spitsbergen. Journal of Sedimentary Research 36, 1102-1117\n(1966).\nand Wetlands International European Association, 8p (2014)."}, {"round": 3, "reviewer_comments": {"reviewer_2": "For background, I had previously approved this article for acceptance in a previous round of reviews and am now being asked to weigh in on disagreement between the authors and Reviewer 4. I have already expressed my own concerns with the paper and had those concerns addressed to my satisfaction, and as such will only comment on these disagreements.\n\nI understand Reviewer 4’s reservations about the disagreement between common understanding or interpretation of river morphology and the metric provided by the authors here. However, I am of the belief that this is largely a result of the inexactness of morphological classes rather than a failure of the author’s methodology. I believe there is no algorithm that would exactly reproduce the classes that an expert geomorphologist would define by eye. The author’s methodology is based in previous, established, large-scale geomorphology studies and makes a very robust attempt to progress these methods and quantify river morphology classes at the global scale. In fact, I believe the authors have produced the most rigorous and defensible definitions of anabranching rivers, and opposition to this is rooted in imprecise definitions and preconceptions. It is not perfect, but unlike Reviewer 4’s worry that this paper will add noise to the field instead of clarity, I think this paper adds clarity to the noise of geomorphic classifications. With one caveat that these classifications are not the exact same as previous definitions. They are however a good approximation and a more rigorous definition.\n\nI still recommend acceptance of this manuscript and applaud the amount of work the authors have done to address the concerns of all reviewers.", "reviewer_4": "I am providing a final report on this manuscript. I have carefully considered the authors' revisions and their rebuttal. While I acknowledge their efforts to refine their methodology, my fundamental concerns about the paper's central claims and its suitability for Nature Communications remain.\n\nMy position is not a critique of the authors' technical competence but a critical assessment of the manuscript's premise and its conclusions. In my professional and honest opinion, this work does not represent a major scientific advancement that challenges or transforms our understanding of fluvial systems. Instead, its primary finding—the global dominance of anabranching channels—is a direct and unavoidable artifact of the classification scheme the authors have developed.\n\nThe core issue is one of circular reasoning. The paper redefines the boundaries of what constitutes an anabranching river, adopting a broader and more inclusive set of criteria suitable for automated, large-scale mapping. The subsequent \"discovery\" that this pattern is globally dominant is, therefore, a foregone conclusion, predetermined by the initial definition. It does not unveil a hidden geomorphological reality but rather demonstrates the outcome of applying a new classificatory lens to the globe.\n\nA new classification can be a valuable contribution if it provides a more robust, process-based understanding of the system in question. However, this is not the case here. The proposed scheme, by lumping a vast diversity of multichannel forms into a single \"anabranching\" category, does not offer new insights into the mechanistic controls on channel pattern formation. It does not help us better predict when or why a river will anabranch. This weakens the justification for its adoption and undermines the claim that it will advance fluvial theory.\n\nConsequently, the manuscript's claims of transformative impact are not supported by the work. Statements about \"transforming a fundamental perspective\" and posing \"new questions and challenges for the fluvial geomorphology community\" are significant overstatements. The questions raised are not about river dynamics but about the semantics of classification.\n\nThe manuscript presents two potential contributions:\n1. A new methodology for global river planform classification. As a methodological paper, it could be a solid contribution to a specialist journal in geomorphology or remote sensing. There, the focus would be on the method's strengths, limitations, and its comparison to established frameworks.\n2. A transformative discovery about the Earth's river systems. This is the framing the authors have chosen, and it is the basis for submission to a high-impact journal like Nature Communications. In my view, the work does not support this framing. The impact is manufactured by the methodology itself, not discovered by it.\n\nAs a reviewer, my role is to provide an honest and expert evaluation of the work's scientific merit and impact. It is my firm opinion that the manuscript's central claims are not substantiated and are a byproduct of its definitional framework. The work adds a new dataset, but it does not warrant a paradigm shift in our understanding of fluvial geomorphology.\n\nTherefore, I can not provide recommendation for publication."}, "author_response": "Review 4: I acknowledge the effort the authors have put into addressing my previous comments and trying to improve the robustness of their approach to overcome the issues I raised earlier. Indeed, they have refined their methods to define anabranching structures, although I am still not convinced that these improvements are sufficient to truly depict the dominant pattern, as I elaborate below. Thus, I still have some reservations about whether the statements — such as: “The prevalence of anabranching channel systems transforms a fundamental perspective on global river systems, posing new questions and challenges for the fluvial geomorphology community.” and claims like: “While fluvial theory has focused for decades on meandering systems and, to a lesser extent, on braided systems, theoretical developments for anabranching have been largely confined to maximum flow efficiency (MFE) and the least action principle. Considering that their distribution is pervasive, with proportions of 50%, 49%, and 54% observed in non-mountainous, mountainous, and coastal regions, respectively, the widespread prevalence of anabranching channels underscores their pivotal role in advancing fluvial geomorphological theories.” (to name a few) — are truly as claimed, or if these results are largely a by-product of their definitions and the thresholds chosen.\nLet me put it plainly: there are many reaches that any geomorphologist would agree belong to the anabranching category. The authors themselves use some of these clear examples (e.g., Fig. 2c,d) to illustrate anabranching patterns—similar to cases shown in the first two references in the rebuttal letter (Fig. 1a-c in Wang et al. 2022, or Fig. 7d in Nyberg et al. 2023).\nMy main concern can be summarized as follows: are these archetypical examples truly representative of the almost 50% of global reaches the authors classify as anabranching? Based on the thresholds chosen—IBi ≥ 1 (I think there was a typo in the rebuttal and the paper stating IBi ≤ 1) and RA > 0.01—it seems that either these thresholds are too low, or the typical anabranching patterns shown in the figures are actually extreme cases within their classification. Moreover, some prototypical meandering reaches, such as Fig. 7a in Nyberg et al. (2023), which most geomorphologists would agree are clear examples of meandering patterns, might even fall within the anabranching category using the proposed methodology —at least according to my calculations. This suggests that the conclusions and implications might not be as straightforward as stated: perhaps it is not that geomorphologists have overlooked the prevalence of anabranching patterns, but rather that the authors’ definition of the anabranching class itself shifts the balance between different classes.\nIf the authors can convincingly demonstrate that this is not simply a definition artifact—showing, for example, that typical meandering patterns like Fig. 7a in Nyberg et al. (2023) are not included in their anabranching class, and more generally, that random reaches around the median (or better yet, around the 25th percentile or lower to be convincing about the global claims) according to RA (given IBi > 1) can be comfortably and uncontroversially presented as representative anabranching examples (similar to their Figure 2)—I would be happy to remove my concerns and reconsider my recommendation. But if this is not the case, I must admit that I remain unconvinced that the authors’ claims and methodology align with common definitions of anabranching, and I worry the paper may end up adding noise rather than clarity to the field.\nReply: We thank you and appreciate the input from Review 4.\nIn our previous round, we attended to the suggestions raised by the Reviewer and even improved and made the methodology more robust. The outcomes were similar to those of the first round. However, despite the presented data, the Reviewer is still not convinced that anabranching reaches are globally dominant above meandering and braided reaches.\n1. “I still have some reservations about whether the statements … are truly as claimed, or if these results are largely a by-product of their definitions and the thresholds chosen.”\n2. “the typical anabranching patterns shown in the figures are actually extreme cases within their classification”\n3. My main concern can be summarized as follows: “are these archetypical examples truly representative of the almost 50% of global reaches the authors classify as anabranching? Based on the thresholds chosen—IBi ≥ 1 (I think there was a typo in the rebuttal and the paper stating IBi ≤ 1) and RA > 0.01—it seems that either these thresholds are too low, or the typical anabranching patterns shown in the figures are actually extreme cases within their classification.”\nThus, in this new round of review, the last and decisive pending points raised by the reviewer regard\na) the apprehension to accept an IBi=1 threshold value to start the discrimination and classification of a unitary SWORD reach as anabranching or single channel reach.\nb) To present the statistical distribution and representativeness of branching classes- hierarchies at the global scale (from less branched to more branched).\nThe apprehension to accept an IBi=1 threshold value to start the discrimination and classification of a unitary SWORD reaches as anabranching or single channel reaches.\nPlease allow us to elaborate on your claim on “reservations on the result” as at this stage, the “reservations” seem to focus not on the methods but on the results sourced by the multiple methods applied in our study, including the outcomes sourced by those methodological modifications you previously suggested and that we incorporated.\nA brief compilation:\n1- The reviewer made early reservations about our first GACP method and requested additional processing because the results were not what the reviewer, based on the own perception/experience, expected.\n2- The reviewer suggested using NDVI and IR jointly. We did it. However, it appears that the sourced results by the method you proposed were not what you expected them to be.\n3- In addition to improving the method by incorporating your suggestions on NDVI and IR, we also included the IBi of Wang et al. IBi is a method accepted, published, and useful to separate single channels from anabranching (multiple channels).\n4- But we went even further in robustness by incorporating the use of IBi along with additional components through our four-step methodology, and by including a support vector machine (SVM) trained to distinguish anabranching from braided reaches.\n5- The IBi method is based on derivations from the broadly accepted and used braided index proposed by the fluvial specialists P. Friend (Cambridge) and Rajiv Sinha (IITK) and similar rationale to further adaptation by Kong et al1, (where Latrubesse is co-author). In their methods, Wang et al2 used for the calculations up-to-date available geospatial and computational technology for global analyses. By the way, when confronting the concerns of the reviewer on the set of other methodological steps we applied and also the results sourced by the IBi method by questioning its efficacy, specifically for the low values threshold (concerns on low values threshold only because the method unquestionably detect multichannel patterns), it means that the reviewers disagree as well with the original principles of Wang’s et al. and, transitively, with all the fluvial colleagues who have authored and used it.\n6- The IBi metric, authored by all the co-authors of Wang et al, include researchers from prestigious institutions ranking among the top senior fluvial and remote sensing specialists, such as Paul Bates (Bristol University), Colin Gleason (Un. Massachussets Ambers), Lawrence Smith (Brown Univ), Tamlin Pavelsky (UNC), Ernesto Rodriguez (Jet Propulsory Lab), Alain Pietrorino (Un. Calgary) to mention the most seniors authors. We agree with these authors/colleagues on the utility of the IBi method, and perhaps it is appropriate to add that our group is also formed by top senior scientists with fluvial global experience who consider that the proposal by Wang et al is useful and robust. We support our choice of adopting Wang’s IBi as a complementary parameter in the multi-step methodology we built.\n7- Low threshold: Despite the concerns raised by the reviewer on the low values of IBi threshold to define the separation of single channels from anabranching channels, we, and a significant number of colleagues, consider the method and the low (IBi~1) threshold valid to start discriminating against single channels from multichannel tracts. Thus, we consider the IBi by Wang to be a valid, peer-reviewed, published, and replicable method currently accepted and used in the literature to distinguish single from multiple channel patterns\n8- However, the acceptance of our paper is dependent on the peer anonymous review process, and, as a consequence, we need to source additional arguments to answer the anonymous reviewer (two other reviewers have already accepted the paper for publication). Consequently, we focus now on the only controversial remaining point, which is the reviewer's assumption that the low value (not the highest) IBi threshold is “questionable” to sort the initial stage discrimination threshold between single channel and multichannel.\n9- Thus, we understand the critical point for the reviewer is the potential “too low” threshold mentioned by the reviewer (IBi~1) and what should be the breaking point (threshold) to discriminate and classify a single channel with a few islands as anabranching or single channel pattern (sinuous or straight).\n10- It has to be considered that no data or quantitative analyses support the reviewer´s concerns about what minimum thresholds the reviewer suggests we have to consider/adopt. Also, the reviewer does not provide specific arguments to explain and convince us why and in what dimensions the raised concerns, in fact, could affect or modify our quantitative results and conclusions.\n11- Also, there is no question that the IBi method easily identifies multichannel tracts, and it is unequivocal once we increase the IBi number. Thus, we count on the Reviewer´s agreement on that.\n12- However, our concern is the fact that, by disregarding low IBi as a threshold, the reviewer tends to discriminate patterns on the “visual perception” of the channel and not by proposing, implementing and applying a strict and systematic calculation of metrics, such as a global and comparative study demands. For example, we understand that the reviewer would classify a channel pattern with low IBi (1 to perhaps up to 1.1) as a single channel reach (straight or meandering based on a sinuosity), ignoring in both cases the presence of secondary channels and islands. Thus, paraphrasing the reviewer's words, we can change the dispute by arguing that, by neglecting the presence of islands and the existence of more than one channel in a particular SWORD reach, is the reviewer who is imposing “archetypes” by using a questionable descriptive/qualitative “threshold” based on planform styles that forcedly classify reaches with branches and islands as single channel ones.\n13- So, for the sake of this review, and based on all the above, we understand that the only reviewer´s pending concern about thresholds is just related to the population of low IBi interval ~1-1.1 (low end values) and not to demerit the whole complex methodology we applied.\n14- Because of the concerns/doubts of the reviewer, we will concentrate on the frequency of channels of the lowermost ranks of IBi (low threshold) and not on the whole population of anabranching reaches. From here and ahead, we emphasize our explanations on the population of low anabranching index values.\nON ARCHETYPES AND DISTRIBUTION OF BRANCHING CLASSES\nRegarding the examples in our figures, the reviewer asks:\n4. “are these archetypical examples truly representative of the almost 50% of global reaches the authors classify as anabranching”\nThe clarification raised by the reviewer is relevant, and we incorporated in the paper the required information.\nFirst, we produced a complementary chart with frequency and metrics, aiming to help the reader get a quick visual comparative perception of the classification and IBi values >1.\nTable.1 | A total of 26501 reaches are IBi>1, and from that total population, 28% exhibit a sinuosity ≥1.3.\nNumber of reaches Non mountain Mountain Coastal Total\nAnabranching IBi>1 18075 2641 5785 26501\nSinous anabranching Sinuosity ≥ 1.3 and 5150 862 1654 7666\nIBi >1\nFig.1 | The anabranching reaches with IBi >1. a, Frequency distribution and cumulative proportion of anabranching reaches with IBi >1. b, Histogram distributions of anabranching reaches with IBi >1 across bins. c, Planform examples from individual SWORD reach with increasing IBi values. The first and third rows display high-resolution imagery obtained from ESRI, while the second and fourth rows are the Global Surface Water Occurrence (GSWO) images.\nThe examples we provided in our previous version are no archetypes but good examples, as IBi ≥1.5 represent 50% of the total anabranching population and ≥1.25 represent 69% of the total population. To clarify and illustrate it, we included in the supplemental materials new plots that help demonstrate that the examples shown in the manuscript are not extreme outliers, but rather representative cases within the distribution.\nImportantly, IBi population, ranging from IB ~ 1 to 1.1 is limited to no more than 15% of the total population of the anabranching reaches IBi>1.\nIf by intending to address the concern of the reviewer, we remove the 15% low IBi (1-1.1) population from the anabranching group, then those 15% reaches would have to be forcedly transferred to a single channel (straight or meandering) despite having islands and being anabranching, as indicated by the calculated parameters. But it is also relevant that even transferring all those 15% (3975 reaches) to some single reach category (straight or meandering), it does not modify/alter at all the general conclusion of the paper that anabranching reaches are globally dominant above meandering and braided reaches.\nEven more, from those 15% of anabranching reaches with IB 1-1.1 (3975 reaches, Fig. 1b), only 36% of those reaches are sinuous (≥1.3) (1456 reaches, Fig. 2b).\nIf we attend to the reviewer’s suggestion of ignoring the presence of islands and rejecting the occurrence of secondary channels in the IB 1-1.1 interval because the value is low, those sinuous reaches would hypothetically need to be classified as “single channel meanders” (although they have islands and branches in the reach). Again, doing it does not alter at all the general conclusion that 81% (Fig. 2b) of the remaining sinuous anabranching reaches with higher IBi are undoubtedly classified as sinuous anabranching and that the merged set of single channel meander+ sinuous with low IB indexes (1-1.1) reaches are significantly smaller in number than anabranching reaches with IB>1.1 at the global scale.\nFig.2 | The sinuous anabranching reaches with IBi > 1 and sinuosity ≥ 1.3. a, Frequency distribution and cumulative proportion of sinuous anabranching reaches with IBi >1 and sinuosity ≥1.3. b, Histogram distributions of sinuous anabranching reaches with IBi >1 and sinuosity ≥1.3 across bins. c, Planform examples from individual SWORD reach with increasing IBi values. The first and third rows display high-resolution imagery obtained from ESRI, while the second and fourth rows are the Global Surface Water Occurrence (GSWO) images.\nFig.3 | Histogram distributions of sinuous anabranching reaches grouped by sinuosity and IBi.\nIn conclusion, we attended to the suggestion by the reviewer by presenting\na) A new graphic chart/figure showing a continuous series of examples with increasing IBi patterns (Fig. 1c, 2c).\nb) Frequency diagrams and statistics of the different IBi classes for anabranching reaches (Fig. 1b).\nc) Frequency diagrams and statistics of the different IBi classes for anabranching sinuous reaches (Fig. 2b).\nd) Additional text in the manuscript regarding a, b and c, and an explanation on the concerns raised by the reviewer above the minimum selected thresholds.\nTHE SPECIFIC CASE OF MEANDERS AND THE MADRE DE DIOS REACH (BOLIVIA)\nThe Reviewer used as an example for discussion a reach of the Madre de Dios River in the Amazon basin. Please allow us to elaborate on a long answer as we consider it to be relevant and helpful to clarify our perspective and the approach we used.\nThe reviewer mentions:\n5. Moreover, some prototypical meandering reaches, such as Fig. 7a in Nyberg et al. (2023), which most geomorphologists would agree are clear examples of meandering patterns, might even fall within the anabranching category using the proposed methodology—at least according to my calculations.\nANSWER: Yes, the reviewer is right. It is falling into the sinuous anabranching category. It is not an archetypical meandering reach. We consider this Madre de Dios reach to be a clear example of how susceptible to planform first glance observations geomorphologists are, forcing conclusions on one single parameter (sinuosity in this case) beyond making a more integral analysis that includes a variety of metrics, interpretations of the river's dynamics, and assessing carefully the geomorphology of the channel and floodplain they observe.\nIt is an interesting reach, and one of our authors (Latrubesse) is very familiar with the area selected by the reviewer, as he has conducted extensive fieldwork in that region (Fig. 7a in Nyberg et al3). Thus, the first question is why the Reviewer or some colleague could consider it “clear examples of meandering patterns?”\nSecondly, why do the reviewers or a colleague could visually consider that “as most geomorphologists would agree are clear examples of meandering patterns” thus discrediting the systematic and rigorously quantitative method of classification of sinuous anabranching we proposed for sinuous reaches like this one.\nFig. 4 | The Madre de Dios River in the Amazon basin.\nPlease, allow us some elaboration. The Madre de Dios presents here a complex alluvial belt formed by at least three morpho-sedimentary units. A Late Pleistocene unit (LP) (part of the low Terrace), a middle-early late Holocene unit (also part of the low Terrace) (MH-ELH), and the most recent active fluvial belt, incised on the older units. Notice that the river banks are touching recurrently on the right bank, with the old Tertiary sedimentary units forming banks up to 20 meters in height. Notice also how old landforms prompt the unaware viewer to assume there are ´young` abandoned meanders in the floodplain when, in reality, they are landforms (paleochannels, middle Holocene age) on the terrace, and, for instance, the scars and paleochannel patterns are not related to the present-day dynamic of the river. That mistake is presented by Nyberg, which would demonstrate they are not familiar with the mapping of fluvial landform in complex alluvial belts and the interpretation of Quaternary records.\n[REDACTED]\nFig. 5 | The Rio Madre De Dios in Bolivia plotted by Nyberg et al (2023).\nThus, “meander scars” and oxbow lakes, as pointed out in the Nyberg figure, have conceptual and interpretive problems as they are paleo-landforms that do not relate to the present-day fluvial dynamics but to older units.\nBased on our 40-year GSWO record, the six reaches on the right side of Fig. 6d exhibit multi-threaded channels and are therefore classified as sinuous anabranching. In contrast, the single reach on the left is classified as meandering due to the failure to capture islands. The results show that the IBi is 1.33 ( ), indicating that this reach contains a cumulative total of 26 km of secondary channels over a 78 km main reach length. The relative area is 0.11 ( ), meaning there are 7.52 km² of islands within a 68.65 km² of channels. Based on these metrics (IBi, RA, NDVI, IR), the whole reach can be classified as sinuous anabranching.\nFig. 6 | Detailed illustration of the Rio Madre De Dios River in Bolivia. a, The merged SWORD reach centerline. b, The extracted channel centerline. c, The extracted islands and channels polygons with the reach buffer. d, The corresponding classified channel pattern for each SWORD reach. Notably, we merged seven original SWORD reaches into one representative reach to better illustrate the Rio Madre De Dios River.\n[REDACTED]\nFig. 7 | Islands of the Rio Madre De Dios River in Bolivia. Is= Islands.\nAlso, remember that in our four-step method, our record is multitemporal, adding information on the patterns.\nThe channel has sustained for four decades an anabranching sinuous pattern with multiple islands. Despite the sinuosity of the segment, the sinuous tracts are a kind of “fake” meanders, as they are almost motionless structures with limited lateral migration rates because of the incision of the channel and the lateral control exerted by Tertiary rocks on the right banks. Notice that no substantial platform changes and remarkable floodplain recycling-reworking processes by lateral shifting of free meanders have been detected in four decades (Fig. 8). Only a single loop was abandoned, and that contained islands as well.\nFig. 8 | Water surface occurrence change map using Global Water Surface derived data 1984-2021, Copernicus.\nNotice also the irrelevant migration rates of the river, the lack of capacity for lateral shifting, and the absence of young point bars and scroll morphologies.\nIslands along the reach vary in number through time, but always are representative features, a fact also pointed out in our metrics and multidecadal record.\nAs we work with SWORD, the calculations are for each SWORD length. Here we show below some additional information that demonstrates that, despite the sinuosity of the loops in the reach, even those loops with no islands are not free meanders. So, the sinuosity of the loops “per se” is not a component that allows for classifying the segment as a single channel-meandering SWORD reach, but as a sinuous anabranching one with multiple islands.\n[REDACTED]\nFig. 9 | Google Earth imagery of loop 1 in 1985, 2023, and 2024.\nNotice that Loop 2 is also almost motionless and deformed. It is not a harmonic free meander shifting loop. Observe as well that Reach 3 has plenty of islands. Even in area 4, the most dynamic of the reaches, islands are always present, and a single meandering channel is not the dominant feature but shifting into an anabranching structure, alternating islands and bifurcations. Lateral migration rates in that reach are still very low, and erosional depositional patterns are pretty straight, to the sides of the main axial tract (green color in Fig. 10). One cut-off is detected, but notice that there is never a dominant single channel pattern (without islands), even immediately after the rectification of the loop.\nFig. 10 | Water surface occurrence change map at the location of loop 4 using Global Water Surface derived data 1984-2021, Copernicus.\n[REDACTED]\nFig. 11 | Google Earth imagery of loop 4 in 1988, 2010, and 2024. The loop with islands was cut off in 2007. Islands and secondary channels are always present along the reach, and average lateral migration rates for the whole reach are low.\nIn conclusion, for all the above, we consider that trying to classify this segment of the Madre de Dios River as a “clear example” of a single-channel meandering reach is a remarkable conceptual and interpretative mistake. It is not in agreement with the geomorphological record and history of the alluvial belt (floodplain, lower terrace, channel dynamics) nor with the rigorous and robust metrics we used to discriminate quantitatively and systematically channel patterns, which were plainly explained in our methodology. Thus, we maintain that the classification of the reach is sinuous anabranching.\nFinal comments:\nWe have considered carefully and respectfully all the suggestions and recommendations made by the reviewer and included almost the totality of them. The fact is that even when trying multiple approaches and including additional methodological steps, the results do not quantitatively change, nor does the general conclusion of the paper. We also do not detect any substantial pendency regarding the comments and concerns raised by the reviewer or the need for additional quantifications that could impact the conclusions and main results of our article.\nScience is respected for being open-minded and a stage for intellectual discussions. If some counterargument is still open by the Reviewer, a reply or a new article with data and conceptual arguments contesting our results is the logical arena for a dispute and not a blockage under an anonymous review.\nTwo reviewers have already accepted our manuscript. We expect that, in consideration of:\na) the respectful deliberation and transparent interactions with the reviewer that guided the intellectual exchange, the effort we made, and the willingness we demonstrated to attend to the suggestions by the reviewer; b) the remarkable set of complementary data we generate to strength the discussion and results, as requested by the reviewer; and c) the fact that our claims and results are based on a rigorous systematic, quantifiable and reproducible method; the reviewer will grant us the definitively acceptance of the manuscript.\nReference\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nFor background, I had previously approved this article for acceptance in a previous round of reviews and am now being asked to weigh in on disagreement between the authors and Reviewer 4. I have already expressed my own concerns with the paper and had those concerns addressed to my satisfaction, and as such will only comment on these disagreements.\nI understand Reviewer 4’s reservations about the disagreement between common understanding or interpretation of river morphology and the metric provided by the authors here. However, I am of the belief that this is largely a result of the inexactness of morphological classes rather than a failure of the author’s methodology. I believe there is no algorithm that would exactly reproduce the classes that an expert geomorphologist would define by eye. The author’s methodology is based in previous, established, large-scale geomorphology studies and makes a very robust attempt to progress these methods and quantify river morphology classes at the global scale. In fact, I believe the authors have produced the most rigorous and defensible definitions of anabranching rivers, and opposition to this is rooted in imprecise definitions and preconceptions. It is not perfect, but unlike Reviewer 4’s worry that this paper will add noise to the field instead of clarity, I think this paper adds clarity to the noise of geomorphic classifications. With one caveat that these classifications are not the exact same as previous definitions. They are however a good approximation and a more rigorous definition.\nI still recommend acceptance of this manuscript and applaud the amount of work the authors have done to address the concerns of all reviewers.\nReply: We particularly appreciate your continued support and insightful comments on our manuscript, and thank you for your positive recommendation for acceptance. We concur with the definitive assessment that our work “adds clarity to the noise of geomorphic classifications”.\nReviewer #4 (Remarks to the Author):\nI am providing a final report on this manuscript. I have carefully considered the authors' revisions and their rebuttal. While I acknowledge their efforts to refine their methodology, my fundamental concerns about the paper's central claims and its suitability for Nature Communications remain.\nMy position is not a critique of the authors' technical competence but a critical assessment of the manuscript's premise and its conclusions. In my professional and honest opinion, this work does not represent a major scientific advancement that challenges or transforms our understanding of fluvial systems. Instead, its primary finding—the global dominance of anabranching channels—is a direct and unavoidable artifact of the classification scheme the authors have developed.\nThe core issue is one of circular reasoning. The paper redefines the boundaries of what constitutes an anabranching river, adopting a broader and more inclusive set of criteria suitable for automated, large-scale mapping. The subsequent \"discovery\" that this pattern is globally dominant is, therefore, a foregone conclusion, predetermined by the initial definition. It does not unveil a hidden geomorphological reality but rather demonstrates the outcome of applying a new classificatory lens to the globe.\nA new classification can be a valuable contribution if it provides a more robust, process-based understanding of the system in question. However, this is not the case here. The proposed scheme, by lumping a vast diversity of multichannel forms into a single \"anabranching\" category, does not offer new insights into the mechanistic controls on channel pattern formation. It does not help us better predict when or why a river will anabranch. This weakens the justification for its adoption and undermines the claim that it will advance fluvial theory.\nConsequently, the manuscript's claims of transformative impact are not supported by the work. Statements about \"transforming a fundamental perspective\" and posing \"new questions and challenges for the fluvial geomorphology community\" are significant overstatements. The questions raised are not about river dynamics but about the semantics of classification.\nThe manuscript presents two potential contributions:\n1. A new methodology for global river planform classification. As a methodological paper, it could be a solid contribution to a specialist journal in geomorphology or remote sensing. There, the focus would be on the method's strengths, limitations, and its comparison to established frameworks.\n2. A transformative discovery about the Earth's river systems. This is the framing the authors have chosen, and it is the basis for submission to a high-impact journal like Nature Communications. In my view, the work does not support this framing. The impact is manufactured by the methodology itself, not discovered by it.\nAs a reviewer, my role is to provide an honest and expert evaluation of the work's scientific merit and impact. It is my firm opinion that the manuscript's central claims are not substantiated and are a byproduct of its definitional framework. The work adds a new dataset, but it does not warrant a paradigm shift in our understanding of fluvial geomorphology.\nTherefore, I can not provide recommendation for publication.\nReply: Thanks for the critical comments of our manuscript. However, we firmly believe that, as supported by the other two reviewers, the prevalence of anabranching channels in our findings transforms fundamental perspectives on global river systems, an essential contribution to the fluvial geomorphology community. We also have incorporated an explanatory paragraph into the “Widespread implications” section. “Perhaps some critics can contend that the continuous spectrum of fluvial patterns challenges the effectiveness of a framed four-end-member classification and threshold-based approach, as certain transitional cases may not be adequately categorized. This criticism often arises from those who rely primarily on visual interpretations of channel planforms rather than systematic metric measurements or comprehensive analyses of processes, landforms, and channel-floodplain relationships. Nonetheless, categorizing river reaches according to prevailing morphologies—anabranching, braided, meandering, or straight—is a scientifically validated methodology. To mitigate uncertainty and avoid classification artefacts, we employed a hierarchical algorithm with quadruple control, grounded in established statistical techniques and methods prominent in previous fluvial geomorphology and large-scale geomorphological research. Our results demonstrate a statistically robust classification; for instance, 85% of total anabranching reaches have an Island Buffer Index ≥ 1.1, indicating that low thresholds have not skewed outcomes and thereby affirming the validity of the reported global patterns.”"}]

Decision letter