River channel bifurcations are crucial for distributing water and sediment on floodplains and deltas, but estimating discharge ratios between branches remains challenging. Using satellite imagery and in‐situ discharge data, we demonstrate that bifurcate channel widths can estimate discharge ratios at 23 of 27 bifurcations in 11 rivers worldwide, with good accuracy ( R 2 = 0.80) in 26 of 33 measurements. An empirical width‐discharge equation derived from 5,740 United States Geological Survey gauging stations further improves accuracy ( R 2 = 0.82). For best results, branch widths should be measured within one channel width of the bifurcation. The method is ineffective in cases influenced by tributaries, avulsion, or multiple branches. We conclude that channel width is effective for estimating discharge ratios, especially when paired with an empirical width‐discharge equation, potentially enhancing river discharge estimates from the Surface Water and Ocean Topography satellite mission, which currently lacks flow partitioning capabilities for bifurcations.

Earth-observing satellites have revolutionized the field of fluvial geomorphology by providing large-scale and spatially contiguous observations. The recently launched Surface Water and Ocean Topography (SWOT) satellite’s novel interferometric synthetic aperture radar (inSAR) instrument delivers global measurements of several key geomorphic parameters, such as river surface water elevation, slope, and width, and thus presents the opportunity to study fluvial processes in new ways. Here we explore the utility of the SWOT satellite for advancing understanding of fluvial geomorphology across river systems in the United States, specifically focusing on water surface elevation variations in large braided rivers, temporally dynamic shear stress in bedrock rivers, and the processes associated with knickpoints and dam failures. We also discuss other relevant potential applications of SWOT satellite data related to fluvial geomorphology beyond the scope of these early explorations. By providing global multitemporal observations of several key variables in fluvial geomorphology, SWOT represents a major advance in our ability to quantify, monitor, and understand fluvial systems and their dynamics.

Abstract. Increasing outburst flood hazards from melting glaciers threaten Himalayan communities but are difficult to assess. On 16 August 2024, a catastrophic Glacial Lake Outburst Flood (GLOF) occurred unexpectedly in the Bhotekoshi River Valley, in the Mt. Everest region of Nepal. Using this disaster as an illustration, we demonstrate that combining new and legacy satellite remote sensing technologies for lake water level, turbidity, and extent can detect potential GLOF hazards and help identify these risks.

Ponding of meltwater on the surface of the Greenland Ice Sheet has the potential to reduce ice sheet albedo and amplify mass loss. However, this process remains poorly constrained and is absent from models that project ice sheet mass balance. Here we demonstrate that meltwater ponding considerably increases the amount of energy available for melting the Greenland Ice Sheet. We first use satellite-derived products to show that meltwater ponding has a significant impact on spatial albedo patterns, particularly in the lower percolation zone. We then use drone imagery to demonstrate that, in the upper ablation zone, there are thousands of narrow streams and small pools (<100 m²) that collectively account for >50% of the total meltwater area. These small meltwater features are not resolved by surface water maps derived from medium-resolution satellite imagery, signifying that the radiative effect of meltwater ponding is three to four times stronger than predicted by satellite-based approaches. Our findings therefore place lower bounds on the radiative effect of meltwater ponding that could be used to advocate for the inclusion of this process into models that forecast Greenland Ice Sheet's contribution to sea-level rise.

West Siberia contains some of the largest soil carbon stores on Earth owing to vast areas of peatlands and permafrost, with the region warming far faster than the global average. Organic matter transported in fluvial systems is likely to undergo distinct compositional changes as peatlands and permafrost warm. However, the influence of peatlands and permafrost on future dissolved organic matter (DOM) composition is not well characterized. To better understand how these environmental drivers may impact DOM composition in warming Arctic rivers, we used ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry to analyze riverine DOM composition across a latitudinal gradient of West Siberia spanning both permafrost-influenced and permafrost-free watersheds and varying proportions of peatland cover. We find that peatland cover explains much of the variance in DOM composition in permafrost-free watersheds in West Siberia, but this effect is suppressed in permafrost-influenced watersheds. DOM from warm permafrost-free watersheds was more heterogenous, higher molecular weight, and relatively nitrogen enriched in comparison to DOM from cold permafrost-influenced watersheds, which were relatively enriched in energy-rich peptide-like and aliphatic compounds. Therefore, we predict that as these watersheds warm, West Siberian rivers will export more heterogeneous DOM with higher average molecular weight than at present. Such compositional shifts have been linked to different fates of DOM in downstream ecosystems. For example, a shift toward higher molecular weight, less energy-rich DOM may lead to a change in the fate of this material, making it more susceptible to photochemical degradation processes, particularly in the receiving Arctic Ocean. West Siberia is warming faster than other regions and contains vast areas of peatlands and permafrost, which contain vast stores of carbon. This carbon is transported off the landscape by rivers and the composition of this exported carbon is likely to change with continued warming, but there is no consensus on exactly what changes will occur. To study these potential changes, we used ultrahigh resolution mass spectrometry to analyze molecular-level organic matter composition across a gradient of permafrost influence and peatland cover in West Siberian watersheds. Warm permafrost-free watersheds had organic matter that was more diverse, of higher molecular weight, and had unique molecular composition compared to cold permafrost-influenced watersheds. We also found that while peatland cover explained much of the compositional diversity between rivers, permafrost ultimately controlled the influence of peatland cover on dissolved organic matter composition, effectively acting as a switch on the compositional signal from peatlands. We predict that as West Siberia warms, the fate of organic matter transported by rivers in the region will thus change and the role of photochemical degradation processes may become more important. West Siberian watersheds exhibit distinct dissolved organic matter (DOM) composition related to permafrost influence and peatland cover Permafrost acts as a switch controlling the influence of peatland cover on the molecular composition of watershed DOM Warming West Siberian watersheds may export more heterogeneous DOM with ramifications for its fate in the Arctic Ocean

Abstract. Meltwater runoff from the Greenland Ice Sheet (GrIS) is an important contributor to global sea level rise, but substantial uncertainty exists in its measurement and prediction. Common approaches for estimating ice sheet runoff are in situ gauging of proglacial rivers draining the ice sheet, and surface mass balance (SMB) modeling. To obtain hydrological and meteorological datasets suitable for both runoff characterization and SMB model validation, we established an automated weather station (AWS) and cluster of traditional and experimental river stage sensors on the Minturn River, the largest proglacial river draining Inglefield Land, NW Greenland. Secondary installations measuring river stage were installed in the Fox Canyon River and North River at Thule Air Base, NW Greenland. Proglacial runoff at these sites is dominated by supraglacial processes only, uniquely advantaging them for SMB studies. The three installations provide rare hydrological time-series and an opportunity to evaluate experimental measurements of river stage from a harsh, little-studied polar region. The installed instruments include submerged vented and non-vented pressure transducers, a bubbler sensor, experimental bank-mounted laser rangefinders, and time-lapse cameras. The first three years of observations (2019 to 2021) from these stations indicate a) a meltwater runoff season from late June to late August/early September, roughly synchronous throughout the region; b) early onset (~ June 23 to July 8) of a strong diurnal runoff signal in 2019 and 2020, suggesting minimal meltwater storage in snow/firn; c) one-day lagged air temperature displays the strongest correlation with river stage; d) river stage correlates more strongly with ablation zone albedo than with net radiation; and e) late-summer rain-on-ice events appear to trigger the region’s sharpest and largest floods. The new gauging stations provide valuable in situ hydrological observations from a little-studied, rapidly changing area and are freely available through the PROMICE network (https://promice.org/weather-stations/).

Supraglacial stream/river catchments drain large volumes of surface meltwater off the southwestern Greenland Ice Sheet surface. Previous studies note a strong seasonal evolution of their drainage density ( D d ), a classic measure of drainage efficiency defined as open channel length per unit catchment area, but a direct correlation between D d and surface meltwater runoff ( R ) has not been established. We use 27 high‐resolution (∼0.5 m) satellite images to map seasonally evolving D d for four GrIS supraglacial catchments, with elevations ranging from 1,100 m to 1,700 m. We find a positive linear correlation ( r 2 = 0.70, p < 0.01) between D d and simulations of runoff production from two climate models (MAR v3.11 and MERRA‐2). Applying this R ‐ D d empirical relationship to climate model output enables parameterization of spatial and temporal changes in supraglacial drainage efficiency continuously throughout the melt season, although temporal and spatial skewness of D d observations likely affects the application of this R‐D d relationship on crevasse fields and snow/firn surfaces. Incorporating this information into a simple surface routing model finds that high runoff leads to earlier, larger diurnal peaks of runoff transport on the ice surface, owing to increased D d . This effect progressively declines from low (∼1,100 m) to high (∼1,700 m) elevation, causing a roughly order‐of‐magnitude reduction in diurnal runoff variability at the highest elevations relative to standard climate model output. Combining intermittent satellite D d mapping with climate model output thus promises to improve characterization of supraglacial drainage efficiency to the benefit of supraglacial meltwater routing and subglacial hydrology models.

Light transmission into bare glacial ice affects surface energy balance, biophotochemistry, and light detection and ranging (lidar) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present measurements of spectral transmittance at 350–900 nm in bare glacial ice collected at a field site in the western Greenland ablation zone (67.15∘ N, 50.02∘ W). Empirical irradiance attenuation coefficients at 350–750 nm are ∼ 0.9–8.0 m−1 for ice at 12–124 cm depth. The absorption minimum is at ∼ 390–397 nm, in agreement with snow transmission measurements in Antarctica and optical mapping of deep ice at the South Pole. From 350–530 nm, our empirical attenuation coefficients are nearly 1 order of magnitude larger than theoretical values for optically pure ice. The estimated absorption coefficient at 400 nm suggests the ice volume contained a light-absorbing particle concentration equivalent to ∼ 1–2 parts per billion (ppb) of black carbon, which is similar to pre-industrial values found in remote polar snow. The equivalent mineral dust concentration is ∼ 300–600 ppb, which is similar to values for Northern Hemisphere warm periods with low aeolian activity inferred from ice cores. For a layer of quasi-granular white ice (weathering crust) extending from the surface to ∼ 10 cm depth, attenuation coefficients are 1.5 to 4 times larger than for deeper bubbly ice. Owing to higher attenuation in this layer of near-surface granular ice, optical penetration depth at 532 nm is 14 cm (20 %) lower than asymptotic attenuation lengths for optically pure bubbly ice. In addition to the traditional concept of light scattering on air bubbles, our results imply that the granular near-surface ice microstructure of weathering crust is an important control on radiative transfer in bare ice on the Greenland Ice Sheet ablation zone, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model development and validation.

Glacial environments offer the opportunity to study the incipient stages of chemical weathering due to the high availability of finely ground sediments, low water temperatures, and typically short rock-water interaction times. In this study we focused on the geochemical behavior of germanium (Ge) in west Greenland, both during subglacial weathering by investigating glacier-fed streams, as well as during a batch reactor experiment by allowing water-sediment interaction for up to 2 years in the laboratory. Sampled in late August 2014, glacial stream Ge and Si concentrations were low, ranging between 12–55 pmol/L and 7–33 µmol/L, respectively (Ge/Si = 0.9–2.2 µmol/mol, similar to parent rock). As reported previously, the dissolved stable Ge isotope ratio (δ 74 Ge) of the Watson River was 0.86 ± 0.24‰, the lowest among global rivers and streams measured to date. This value was only slightly heavier than the suspended load (0.48 ± 0.23‰), which is likely representative of the bulk parent rock composition. Despite limited Ge/Si and δ74Ge Ge fractionation, both Ge and Si appear depleted relative to Na during subglacial weathering, which we interpret as the relatively congruent uptake of both phases by amorphous silica (aSi). Continued sediment-water interaction over 470–785 days in the lab produced a large increase in dissolved Si concentrations (up to 130–230 µmol/L), a much smaller increase in dissolved Ge (up to ∼70 pmol/L), resulting in a Ge/Si decrease (to 0.4–0.5 µmol/mol) and a significant increase in δ 74 Ge (to 1.9–2.2‰). We argue that during the experiment, both Si and Ge are released by the dissolution of previously subglacially formed aSi, and Ge is then incorporated into secondary phases (likely adsorbed to Fe oxyhydroxides), with an associated Δ 74 Ge secondary−dissolved fractionation factor of −2.15 ± 0.46‰. In summary, we directly demonstrate Ge isotope fractionation during the dissolution-precipitation weathering reactions of natural sediments in the absence of biological Ge and Si uptake, and highlight the significant differences in Ge behavior during subglacial and non-glacial weathering.

The Pleistocene sand sea on the Arctic Coastal Plain (ACP) of northern Alaska is underlain by an ancient sand dune field, a geological feature that affects regional lake characteristics. Many of these lakes, which cover approximately 20 % of the Pleistocene sand sea, are relatively deep (up to 25 m). In addition to the natural importance of ACP sand sea lakes for water storage, energy balance, and ecological habitat, the need for winter water for industrial development and exploration activities makes lakes in this region a valuable resource. However, ACP sand sea lakes have received little prior study. Here, we collect in situ bathymetric data to test 12 model variants for predicting sand sea lake depth based on analysis of Landsat-8 Operational Land Imager (OLI) images. Lake depth gradients were measured at 17 lakes in midsummer 2017 using a Humminbird 798ci HD SI Combo automatic sonar system. The field-measured data points were compared to red–green–blue (RGB) bands of a Landsat-8 OLI image acquired on 8 August 2016 to select and calibrate the most accurate spectral-depth model for each study lake and map bathymetry. Exponential functions using a simple band ratio (with bands selected based on lake turbidity and bed substrate) yielded the most successful model variants. For each lake, the most accurate model explained 81.8 % of the variation in depth, on average. Modeled lake bathymetries were integrated with remotely sensed lake surface area to quantify lake water storage volumes, which ranged from 1.056×10-3 to 57.416×10-3 km3. Due to variations in depth maxima, substrate, and turbidity between lakes, a regional model is currently infeasible, rendering necessary the acquisition of additional in situ data with which to develop a regional model solution. Estimating lake water volumes using remote sensing will facilitate better management of expanding development activities and serve as a baseline by which to evaluate future responses to ongoing and rapid climate change in the Arctic. All sonar depth data and modeled lake bathymetry rasters can be freely accessed at https://doi.org/10.18739/A2SN01440 (Simpson and Arp, 2018) and https://doi.org/10.18739/A2HT2GC6G (Simpson, 2019), respectively.

Large, complex supraglacial river networks are widely distributed on the northwestern Greenland Ice Sheet (GrIS) each summer. Owing to the absence of moulins and crevasses on the ice surface, meltwater is continuously routed on the ice surface by supraglacial river networks to feed proglacial rivers on land. This continuous supraglacial-proglacial river system controls the magnitude and timing of surface meltwater runoff on the northwestern GrIS but remains poorly studied. In this study, we first mapped the supraglacial-proglacial river system across the Inglefield Land on the northwestern GrIS during 2016–2019 melt seasons using ninety Sentinel-2 and forty-five Landsat-8 images. Then, we proposed two quantitative river metrics, i.e., surface meltwater area fraction and proglacial river width, to quantify the seasonal and annual evolutions of the supraglacial-proglacial river system. Next, we correlated these satellite-derived river metrics with surface meltwater runoff estimated by two Surface Mass Balance (SMB) models (MAR v3.11 and MERRA-2), and estimated the optimal meltwater routing lag times. Our results showed that: (1) two satellite-derived river metrics, surface meltwater area fraction and proglacial river width, are strongly and positively correlated, indicating that the northwestern GrIS supraglacial-proglacial river system can efficiently route surface meltwater from the ice surface to the proglacial zone; (2) these two satellite-derived river metrics are also positively correlated with simultaneous surface runoff simulated by MAR and MERRA-2, indicating that SMB models can capture the general runoff pattern but exhibit considerable discrepancy with satellite observations; and (3) delayed MAR surface runoff better match two satellite-derived river metrics than simultaneous MAR surface runoff, and the optimal lag times are both two days, suggesting that supraglacial routing accounts for most of the lag time whereas rapid proglacial routing accounts for short lag time. Overall, the northwestern GrIS supraglacial-proglacial river system is a unique and efficient meltwater routing system, and multi-temporal satellite observations of this river system raise prospects for directly estimating surface meltwater runoff on the poorly-studied northwestern GrIS.

In situ river discharge estimation is a critical component of studying rivers. A dominant method for establishing discharge monitoring in situ is a temporary gauge, which uses a rating curve to relate stage to discharge. However, this approach is constrained by cost and the time to develop the stage‐discharge rating curve, as rating curves rely on numerous flow measurements at high and low stages. Here, we offer a novel alternative approach to traditional temporary gauges: estimating Discharge via Arrays of Pressure Transducers (DAPT). DAPT uses a Bayesian discharge algorithm developed for the upcoming Surface Water Ocean Topography satellite (SWOT) to estimate in situ discharge from automated water surface elevation measurements. We conducted sensitivity tests over 4,954 model runs on five gauged rivers and conclude that the DAPT method can robustly reproduce discharge with an average Nash‐Sutcliffe Efficiency (NSE) of 0.79 and Kling‐Gupta Efficiency of 0.78. Further, we find that the DAPT method estimates discharge similarly to an idealized temporary gauge created from the same input data (NSE differences of less than 0.1), and that results improve significantly with accurate priors. Finally, we test the DAPT method in nine poorly gauged rivers in a realistic and complex field setting in the Peace‐Athabasca Delta, and show that the DAPT method largely outperforms a temporary gauge in this time and budget constrained setting. We therefore recommend DAPT as an effective tool for in situ discharge estimation in cases where there is not enough time or resources to develop a temporary gauge.

This dataset contains georeferenced three-band orthomosaics of green, red, and near-infrared (NIR) digital imagery at 1m resolution collected over selected surface waters across Alaska and Canada between July 9 and August 17, 2017. The orthomosaics were generated from individual images collected by a Cirrus Designs Digital Camera System (DCS) mounted on a Beechcraft Super King Air B200 aircraft from approximately 8-11 km altitude. Flights were over the following areas: Saskatchewan River, Saskatoon, Inuvik, Yukon River including Yukon Flats, Sagavanirktok River, Arctic Coastal Plain, Old Crow Flats, Peace-Athabasca Delta, Slave River, Athabasca River, Yellowknife, Great Slave Lake, Mackenzie River and Delta, Daring Lake, and other selected locations. Most locations were imaged twice during two flight campaigns in Canada and Alaska extending roughly SE-NW then NW-SE up to a month apart. The data were georeferenced using 303 ground control points (GCPs) across the study region.

Abstract. Greenland ice sheet surface runoff is evacuated through supraglacial stream networks, which influence surface mass balance as well as ice dynamics. However, in-situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discharge measurements and continuous water level measurements for 62 days spanning 13 June to 13 August 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedented long record of supraglacial discharge capturing both diurnal and seasonal variability. By comparing in situ hydraulic geometry parameters with previous studies, we find that significant heterogeneity exists such that estimating stream discharge using these parameters over ungauged supraglacial catchments could lead to substantial errors. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting (78 % of melt energy). However, during high melt episodes, the shortwave only contributes to 50 % of melt energy. Instead, the relative contribution of longwave radiation, sensible and latent heat fluxes to overall melt increases by 16.5 %, 4 %, and 7 % respectively. Our data also show a seasonal variation in the timing of daily maximum discharge during clear sky days, shifting from 16:00 local time (i.e., 2.75 hours after solar noon) in late June to 14:00 in late July, then rapidly returns to 16:00 in early August coincident with an abrupt drop in air temperature. These changes in peak daily flow timing can be attributed to a changing effective catchment area, resulting in a smaller stream network supplying water to the outlet at the end of the season throughout the melt season. Further work is needed to uncover how widespread rapid shift in the timing of peak discharge is across Greenland supraglacial streams, and thus their potential impact on meltwater delivery to the subglacial system and ice dynamics.

Rivers are commonly used to define political borders, but no global study has quantified the importance of rivers on territorial delimitation at subnational scales. This paper presents Global Subnational River-Borders (GSRB), a first comprehensive geospatial dataset of subnational, as well as national, political borders set by large rivers. GSRB incorporates three previous vector datasets (GAUL, GRWL + +, and WDBII) to map and quantify the use of large rivers as political borders at local, state, and national scales. GSRB conservatively finds that at least 58,529 km (23%) of the world's interior (non-coastal) national borders, 188,436 km (17%) of the world's interior state/province borders, and 442,046 km (12%) of the world's interior local-level political borders are set by large rivers. GSRB finds 219, 2,267, and 13,674 dyads sharing river-borders at these three administrative scales, respectively. While previous studies have emphasized transboundary rivers separating nations, GSRB highlights the abundance of river-borders at subnational scales, where numerous domestic stakeholders share jurisdiction in water resource management. These participants, identified with GSRB, ought not to be ignored when crafting water policy and instituting whole-basin management regimes. GSRB should prove useful for global, geospatial analyses of riparian stakeholders across administrative scales. The GSRB dataset (DOI: 10.5281/zenodo.3906566) can be found via the following linkhttps://zenodo.org/record/3906567#.XvN-GGhKjIU.

Abstract. Light transmission into bare glacial ice affects surface energy balance, bio-photochemical cycling, and light detection and ranging (LiDAR) laser elevation measurements but has not previously been reported for the Greenland Ice Sheet. We present in-ice solar irradiance measured over the spectral range 350–900 nm and 12–77 cm depth collected at a site in the western Greenland ablation zone. The acquired spectral irradiance measurements are used to calculate flux attenuation coefficients using an exponential decay Bouguer law model and are compared to values calculated from two-stream radiative transfer theory. Relative to asymptotic two-stream theory, our empirical attenuation coefficients are up to one order of magnitude larger in the range 350–530 nm, suggesting light absorbing particles embedded in ice enhance visible light absorption at our field site. The empirical coefficients accurately describe light attenuation in the ice interior but underestimate light attenuation near the ice surface. Consequently, Bouguer’s law overestimates transmitted flux by up to 50 % depending on wavelength. Refraction is unlikely to explain the discrepancy. Instead, vertical variation in the ice microstructure and the concentration of light absorbing particles appears to enhance near-surface attenuation at our field site. The magnitude of this near-surface attenuation implies that optical penetration depth is lower by up to 19 cm (28 %) at wavelengths relevant to visible-wavelength lidar altimetry of ice surface elevation (e.g. 532 nm for the Ice, Cloud, and Land Elevation Satellite-2) than is suggested by e-folding depths inferred from two stream theory for optically pure glacier ice. This enhanced near-surface attenuation implies shallower light transmission and therefore lower subsurface light availability for subsurface radiative heating and bio-photochemical cycling. We recommend radiative transfer models applied to bare ice in the Greenland Ice Sheet ablation zone account for vertical variation in light attenuation due to the vertical distribution of light absorbing particles and ice microstructure, and we provide new values of flux attenuation, absorption, and scattering coefficients to support model validation and parameterization.

Meltwater runoff from the Greenland Ice Sheet (GrIS) significantly contributes to sea level rise and is the dominant driver of enhanced mass loss. While most melt occurs during summer, little is known about its seasonal and/or interannual retention within the GrIS. Here, we document evidence of runoff during winter, ~4 months after summer melt. Ground‐penetrating radar and borehole surveys in the proglacial Isortoq River reveal slowly flowing water beneath >0.5 m of river ice. Geochemical analysis of this water indicates previous contact with the ice sheet bed. Comparable surveys in proglacial rivers draining four neighboring catchments found no winter drainage, despite a brief surface melt event ~10 days prior. We attribute the observed runoff to residual meltwater storage and release enabled by a >600 m deep trough beneath Isunguata Sermia, but not neighboring glaciers. We conclude that the GrIS bed can stay wet and drain small amounts of meltwater year‐round.