Land-to-water hydrological connections represent a key regulatory mechanism of carbon transport, controlling carbon dioxide (CO2) emissions from lakes; however, as of yet, there is no assessment of its role at a pan-Arctic scale across large climatic and topographical gradients. We hypothesized that hydrologically well-connected lakes in wetter regions are CO2 sources fueled by stronger lateral fluxes of external carbon relative to drier regions. However, based on data from >200 Arctic lakes, we found that lakes in drier regions have higher and more variable annual CO2 emissions (37.0(6.2 )(146.0)gC m(-2) yr(-1), median(Q3) (Q1)) compared to lakes in wetter regions (8.017.3 1.7 gC m(-2 )yr(-1)), with both the lowest and the highest fluxes recorded among dryland lakes. We hypothesize that with increasing wetness, the relative proportion of fluvial emissions increases, whereas in drier landscapes where lakes often have limited stream export, carbon inputs can be retained and more efficiently emitted from lakes.

Measurements of surface-atmosphere carbon dioxide (CO2) and methane (CH4) fluxes have been relatively sparse across the Arctic tundra and boreal biomes, causing significant uncertainties in carbon budget estimates from the region. While the availability of Arctic-boreal carbon flux data has increased substantially over the past decade, the data have remained spread across different repositories, scientific articles, and unpublished sources, making it difficult to leverage. Here we present a new dataset of monthly Arctic-boreal carbon fluxes (ABCFlux v2) across terrestrial (wetlands and uplands) and freshwater (lakes and rivers) ecosystems compiled from previous syntheses including the Arctic-boreal CO2 flux database (ABCFlux v1), the Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4), and the Global River Methane Database (GRiMeDB). In addition, we consider data from general-purpose (e.g., Zenodo) and flux network repositories, literature, and site principal investigators. The dataset includes surface-atmosphere CO2 fluxes of gross primary production (GPP), ecosystem respiration (Reco), and net ecosystem exchange (NEE), alongside CH4 fluxes. For aquatic ecosystems, we split CH4 fluxes into diffusive and ebullitive flux pathways, and included potential emissions from transient storage in the water column (“storage fluxes”), alongside CO2 and CH4 concentrations dissolved in the surface water. Fluxes are measured through a variety of methods including chamber and eddy covariance techniques alongside bubble traps, ice-surveys, and concentration-based turbulence-driven modelling in aquatic ecosystems. The monthly flux data are reported together with supporting methodological and environmental metadata. The resulting ABCFlux v2 has 23 847 flux site-months, 8182 concentration site-months, and 199 seasonal observations from 1024 sites, and includes 56 139 reported fluxes (i.e. sum of GPP, Reco, NEE, and CH4 fluxes) from the years 1984 to 2024. The majority of monthly observations occurred after 1999. Wetlands had the highest number of site-month observations (8758), followed by boreal forest (6981), lotic ecosystems (6275), lentic ecosystems (3799) and upland tundra (3308). Measurements of CO2 dominated the dataset across most ecosystem types (25 222) except for lentic ecosystems, where CH4 flux site-months (3098) were more frequent than CO2 flux site-months (2915). Overall, ABCFlux v2 includes 160 % more site-months for terrestrial CO2 flux data compared to ABCFlux v1. Integrating and updating BAWLD-CH4 flux data from growing season averages to monthly fluxes resulted in 5671 site-months of chamber CH4 data compared to 762 site-years. This collaborative initiative, involving contributions from over 260 researchers, provides a comprehensive overview of the current state of the Arctic-boreal carbon flux network and its data, and serves as an important step in reducing uncertainties in Arctic-boreal carbon budgets and in enhancing our understanding of climate feedbacks. The data can be accessed at ORNL DAAC at https://doi.org/10.3334/ORNLDAAC/2448 (Virkkala et al., 2026).

For many Arctic rivers and streams, climate-driven intensification of permafrost thaw slumping is a major source of disturbance to aquatic habitats. Thaw slumps are dynamic landforms that severely increase total suspended solids (TSS) and nutrients in downstream reaches and can persist over decades. Effects may differ in magnitude as slumps cycle through periods of higher and lower activity, with expansion of retrogressive slumps increasing over time. Increases in TSS are known to cause reduced invertebrate abundance and diversity in impacted watersheds; however, it remains unclear if water quality and critical aquatic biodiversity have recovered after prolonged exposure to slumps. Here, we examined decadal-scale effects of slumps and environmental change on benthic macroinvertebrates (BMI) by comparing environmental and BMI data collected between 2010-2014 and a recent sampling campaign from 2021. High TSS and nutrient concentrations observed during 2010-2014 persisted in slump-impacted sites in 2021, with no significant change in TSS and total nutrient concentrations after the 10-year exposure period. TSS continued to act as a nonspecific stressor on BMI, as abundance remained significantly lower in impacted streams compared to reference streams. Although total abundance within reference and impacted sites did not differ significantly between sampling periods, abundance and richness of disturbance tolerant taxa was greater in 2021 as compared to 2010-2014 across all sites, with differences linked to lower precipitation in 2021. These community compositional changes were reflected in increased Shannon-Weiner diversity between sampling campaigns. Overall, the number of thaw slumps upstream was an important driver of both BMI abundance and diversity across sampling periods and will likely continue to be an important determinant of benthic macroinvertebrate communities as the number and size of thaw slumps continues to increase across the circumpolar Arctic.

Coastal waters are increasingly susceptible to low-oxygen conditions where circulation or nutrient inputs have been altered by anthropogenic activities, leading to increased occurrence of coastal hypoxia worldwide. Low-oxygen conditions also occur naturally in coastal settings, typically in bottom water or landward of sills where circulation is weak. Here we identify and characterize a persistent, low-oxygen layer in the mid-water column of a remote fjord of the northeast Pacific Ocean that uncharacteristically lacks an entrance sill. This feature is sustained by weak reoxygenation processes constrained by fjord and watershed morphology and geography, and expands seaward into a neighboring channel during summer and autumn. Marine CO2 system conditions characteristic of adverse habitat for local calcifiers also occurred within the low-oxygen layer, demonstrating that such features are multi-stressor habitats more typical of deeper bottom waters or eutrophied settings. Seasonal movement of this layer overlapped habitat margins for some sensitive local species, indicating risk from interannual variability or response to ongoing climate change. These results provide an improved understanding of fjord habitat that can be applied to other fjords where similar characterization of morphological features and water properties can be evaluated.

Stream dissolved organic matter (DOM) provides a link between terrestrial and aquatic ecosystems and controls many aquatic functions. Spatial and temporal variability of stream DOM differs amongst regions due to interactions between climate and physiogeography, and few studies have investigated stream DOM characteristics in the Boreal Plains ecozone in western Canada. The Boreal Plains has a subhumid climate, flat terrain, widespread wetlands, sedimentary bedrock and thick heterogeneous glacial deposits which cause complex surface water-groundwater interactions. To describe the spatial and temporal variability and main controls on stream DOM in this region, we monitored dissolved organic carbon (DOC) concentrations and DOM composition (assessed through UV-vis absorbance and fluorescence spectroscopy) in 17 streams with wetland-dominated catchments over 3 years (2018-2020). In general, our study streams had higher DOC concentrations, lower DOM aromaticity, and lower DOC export when compared to streams draining wetland-dominated catchments in other boreal regions. Spatial differences in DOC concentration and DOM composition were primarily related to the presence of lakes and whether fine- or coarse-textured glacial deposits dominated. Catchments with lakes and coarse-textured glacial deposits had lower DOC concentrations and lower DOM aromaticity. The seasonal variation in DOC concentration and DOM aromaticity was primarily controlled by temperature, reflecting increased DOC production in wetlands during warmer periods. Runoff was a secondary influence on the temporal variability, with dilution in wetland-dominated catchments during annual high flows in summer. However, wetlands became hydrologically disconnected during seasonal dry periods, and streams either dried up or became dominated by groundwater sources with very low concentration and aromaticity of DOM. Our results show that DOM in streams in the Boreal Plains has both similarities and differences with other boreal regions. The observed temperature sensitivity suggests that DOC concentration and DOM aromaticity in Boreal Plains streams may increase as summer temperatures continue to rise, although hydrological thresholds may cause a shift toward low DOC concentration and aromaticity during droughts.

Abstract. Increased land-water connectivity of northern landscapes driven by permafrost thaw is shifting the bioavailability of dissolved organic matter (DOM) in surface waters, with implications for northern food webs and regional and global carbon balances. However, sorption of DOM to previously frozen sediments has received little attention as a mechanism of regulating the bioavailability of organic matter in thaw-affected freshwater ecosystems. Using batch sorption experiments, we assessed sorption potential, water-extractable dissolved organic carbon (DOC) concentration, and the impact of sorption on DOM composition of six different permafrost sediment types common throughout northwestern Canada, reflecting variation in geologic and permafrost histories. A principal component analysis revealed that sediment biogeochemical characteristics reflected geologic origin, and past thaw increased within-type variation. Sorption was positively correlated with organo-reactive forms of Al and Fe and negatively correlated with sediment pH. Proportion of bulk sediment organic carbon released as water extractable DOC ranged from 1.0 % to 62.0 %, with yedoma sediments from the Klondike region releasing substantially more than sediments from other regions. Preferential sorption of larger, humic-like compounds and displacement of mineral-bound small, aliphatic molecules enriched the DOM pool in labile compounds. Bio-incubations verified that exposure to sediments increased rates of biodegradation, corresponding with shifts in DOM composition and increased nutrient concentrations. Our experiments demonstrate that organo-mineral interactions have the potential to decrease DOC concentrations while increasing DOM bioavailability following exposure to permafrost-origin sediments, but that the strength of this response varies with sediment characteristics that are reflective of landscape history.

Abstract Anticipating the environmental and societal consequences of climate-driven permafrost thaw requires knowledge of terrain and subsurface conditions, which prove challenging to obtain at spatial scales necessary for rigorous prediction and decision-making. Analysis of a systematic inventory of permafrost landforms across northwestern Canada demonstrates that landform assemblages co-develop with ecosystems, distinguishing fundamental permafrost properties across a continental-scale ecoclimatic gradient (106 km2) and among finer-scale ecological regions (103 to 104 km2). This approach quantifies variation in geological and climatic legacies and delineates the diverse consequences of thaw. Here we show that permafrost landsystems, defined by characteristic landform assemblages, express spatial variation in soil, ground ice, geochemical, and carbon characteristics, enabling these intrinsic conditions to be inferred at regional scales through integrated mapping and analyses. Permafrost landsystems also provide a conceptual framework to inform predictions of thaw-driven change, and to formulate, share, and apply permafrost knowledge across scales, disciplines, and ways of knowing.

Permafrost thaw can drastically alter dissolved organic matter (DOM) composition within fluvial networks, and simultaneously affect the microbial communities that degrade DOM. However, it is unclear how coupled thaw-induced change in DOM and microbes might affect microbial decomposition of permafrost-origin DOM (biodegradation), and therefore possible mineralization to carbon dioxide. Here, we use a series of incubations to explore how biodegradation varies with DOM and microbe source, and how microbial community composition changes following incubation with thaw-origin DOM. We undertake this work using leachates from different stratigraphic units across a series of retrogressive thaw slumps on the Peel Plateau, Canada, and microbial communities from upstream of, and draining, slumps. DOM composition and biodegradation varied by stratigraphic unit and across sites that were only tens of kilometers apart, but situated along different recessional fronts of the Laurentide Ice Sheet. Permafrost leachates from paleo-active layers were generally more biolabile than leachates from deeper, unmodified tills, and both were more labile than active layer leachates. Biodegradation also tended to be slightly greater for incubations inoculated with microbes from unimpacted stream water. These results emphasize that permafrost thaw-derived DOM composition and biolability will vary across stratigraphic, landscape, and regional scales, and that the composition of the recipient microbial community may play a role in determining immediate DOM fate.

Research in geocryology is currently principally concerned with the effects of climate change on permafrost terrain. The motivations for most of the research are (1) quantification of the anticipated net emissions of CO 2 and CH 4 from warming and thaw of near-surface permafrost and (2) mitigation of effects on infrastructure of such warming and thaw. Some of the effects, such as increases in ground temperature or active-layer thickness, have been observed for several decades. Landforms that are sensitive to creep deformation are moving more quickly as a result, and Rock Glacier Velocity is now part of the Essential Climate Variable Permafrost of the Global Climate Observing System. Other effects, for example, the occurrence of physical disturbances associated with thawing permafrost, particularly the development of thaw slumps, have noticeably increased since 2010. Still, others, such as erosion of sedimentary permafrost coasts, have accelerated. Geochemical effects in groundwater from trace elements, including contaminants, and those that issue from the release of sediment particles during mass wasting have become evident since 2020. Net release of CO 2 and CH 4 from thawing permafrost is anticipated within two decades and, worldwide, may reach emissions that are equivalent to a large industrial economy. The most immediate local concerns are for waste disposal pits that were constructed on the premise that permafrost would be an effective and permanent containment medium. This assumption is no longer valid at many contaminated sites. The role of ground ice in conditioning responses to changes in the thermal or hydrological regimes of permafrost has re-emphasized the importance of regional conditions, particularly landscape history, when applying research results to practical problems.

Anthropogenic climate warming is amplified in the Arctic, impacting the Arctic carbon cycle and its role in regulating climate and global biogeochemical cycles. In this Review, we provide a quantitative and comprehensive overview of the present-day Arctic carbon cycle across the land–ocean continuum. Terrestrial soil stocks total 877 ± 16 Pg C, with upper marine sediments containing 82 ± 35 Pg C. Overall, the integrated Arctic system is a carbon sink, driven by oceanic uptake of CO2 (127 ± 36 Tg C year−1) and organic carbon burial in shelf sea sediments (112 ± 41 Tg C year–1). Terrestrial systems, including inland waters and disturbance, are a net source of CH4 (38 (21, 53) Tg C year–1) and CO2 (12 (–606, 661) Tg C year–1). The Arctic carbon sink will likely weaken under continued warming, owing to factors such as increased coastal erosion, outgassing of riverine organic carbon and enhanced nearshore carbon turnover lowering shelf sediment burial. Arctic greening and increases in terrestrial carbon sinks will be substantially offset by increases in soil respiration, disturbance from extreme events and enhanced emissions from inland waters. Future research should prioritize enhanced coverage of small catchments and nearshore regions, and inclusion of non-linear responses in biogeochemical models. Anthropogenic warming is perturbing the Arctic carbon cycle. This Review provides an overview of contemporary carbon stocks and fluxes across terrestrial, aquatic and oceanic components of the integrated Arctic system.

Global trends in river nitrogen yields reflect human distortion of the global nitrogen cycle. Climate change and increasing agricultural intensity are projected to enhance river nitrogen yields in temperate watersheds and impair downstream water quality. However, little is known about the environmental drivers of nitrogen yields in major Arctic rivers, which have experienced rapid climatic changes and are important conduits of nutrients and organic matter to the Arctic Ocean. Here we analyze trends in nitrogen yields in the six largest Arctic rivers between 2003 and 2023 and develop generalized additive models to elucidate the watershed characteristics and climatic processes associated with observed spatial and interannual variability. We found significant increases in dissolved organic nitrogen yield and/or declines in dissolved inorganic nitrogen yield in four of the six rivers. While temperature and precipitation, via their relationships to discharge, enhance dissolved nitrogen yields, we attribute the diverging trends to the responses of inorganic and organic nitrogen to temperature via effects on permafrost free extent. Spatially, we attribute differences in nitrogen yields across watersheds to differences in land cover and temperature. Shifts in the amount and composition of river nitrogen yields will impact the balance between primary productivity and heterotrophy in nitrogen limited coastal Arctic Ocean ecosystems. Results from this work highlight the importance of climate‐driven changes in temperature and precipitation on river nitrogen yields in large Arctic rivers and motivate further investigation into how permafrost loss and hydrological shifts interact to drive water quality and biogeochemical cycling in the region.

Abstract. Despite low temperatures that slow chemical reactions, geochemical weathering can be pronounced in glacial rivers due to large quantities of fresh comminuted sediments (glacial flour). We assessed the types and magnitude of geochemical weathering across multiple seasons and years in three proglacial rivers (Sunwapta-Athabasca, North Saskatchewan, and Bow) on the eastern slopes of the Canadian Rocky Mountains, as they meandered from their alpine glacial origins to the montane altitudinal life zone up to 100 kms downstream. To overcome the inherent ecological complexity of our study region, multiple lines of evidence were used to quantify geochemical weathering along river transects and across seasons. Carbon dioxide (CO2) was highly undersaturated and instantaneous CO2 fluxes mostly net consumptive at sampling sites nearest source glaciers. Basic geochemical parameters and a large suite of isotopes (87Sr/86Sr, δ34S-SO4, δ18O-SO4, δ13C-PIC, δ13C-DIC, and Δ14C-DIC) were used to dissect general trends in weathering geochemistry. These trends were supported by an inversion model and an inorganic-organic carbon mass balance model, which together found that while carbonate weathering dominated at all sampling sites and times, silicate weathering and organic carbon contributions to the dissolved inorganic carbon pool increased with distance downriver of glaciers regardless of season. Globally, we suspect these spatiotemporal patterns in the type and magnitude of geochemical weathering are common across glacierized watersheds. Therefore, as glaciers continue to retreat, we can expect to see an encroachment of downriver altitudinal life zones concurrent with glacier mass loss and an evolution of in-river geochemical weathering processes, with direct implications for present-day regional and global carbon budgets.

Small coastal watersheds (< 10,000 km2) can play a large role in forming biogeochemical linkages between land and sea, yet the spatial heterogeneity of small watershed ecosystems is poorly understood due to sparse observations in many regions. In this study, we examined the spatial heterogeneity of water quality exported from diverse watersheds in two rainforest fjordland complexes. Samples were collected about monthly for a year from the outlets of 56 watersheds spanning from high mountains to low islands. Many (20) water quality properties varied significantly across six previously established watershed types defined by 12 easily computed geospatial variables. For example, organic matter concentrations ranged from very low in a Glacierized Mountains watershed type (1.2 ± 0.1 mg L−1 DOC; 28.5 ± 4.6 µg L−1 DON) to very high (15.1 ± 1.0 mg L−1 DOC; 215.6 ± 20.4 µg L−1 DON) in a Rain Lowlands type. Along this gradient, the dominant form of dissolved nitrogen switched from inorganic to organic and the dominant form of phosphorous switched from particulate to dissolved. Watershed type alone explained 67

Climate change is causing mountain glacial systems to warm rapidly, leading to increased water fluxes and concomitant export of glacially derived sediment and organic matter (OM). Glacial OM represents an aged but potentially bioavailable carbon pool that is compositionally distinct from OM found in non-glacially sourced waters. Despite this, the composition of riverine OM from glacial headwaters to downstream reaches and its possible role in structuring microbial assemblages have rarely been characterized in the Canadian Rockies. Over three summers (2019–2021), we collected samples before, during, and after glacial ice melt along stream transects ranging from 0 to 100 km downstream of glacial termini on the eastern slopes of the Canadian Rocky Mountains. We quantified dissolved and particulate organic carbon (DOC, POC) concentrations and used isotopes (Δ14C–OC, δ13C–OC) and dissolved OM (DOM) absorbance and fluorescence to assess OM age, source, and character. Environmental data were combined with microbial 16S rRNA gene sequencing to assess controls on the composition of stream water microbial assemblages. From glacial headwaters to downstream reaches, OM showed a clear transition from being aged and protein-like, with an apparent microbial source, to being relatively younger and humic-like. Indicator microbial species for headwater sites included chemolithoautotrophs and taxa known to harbour adaptations to cold temperatures and nutrient-poor conditions, suggesting some role of glacial seeding of microbial taxa to the headwaters of this connected riverine gradient. However, physical and chemical conditions (including water temperature; POC concentration; protein-like DOM; and deuterium excess, an indicator of water source) could only significantly explain ∼ 9 % of the observed variation in microbial assemblage structure. This finding, paired with the identification of a ubiquitous core microbial assemblage that comprised a small proportion of all identified amplicon sequence variants (ASVs) but was present in large relative abundance at all sites, suggests that mass effects (i.e., whereby high dispersal rates cause homogenization of adjacent communities) largely overcome species sorting to enable a connected microbial assemblage along this strong environmental gradient. Our findings suggest that a loss of novel glacial and microbial inputs with climate change, coupled with catchment terrestrialization, could change OM cycling and microbial assemblage structure across the evolving mountain-to-downstream continuum in glacierized systems.

Rivers and streams are an important pathway in the global carbon cycle, releasing carbon dioxide (CO2) and methane (CH4) from their water surfaces to the atmosphere1,2. Until now, CO2 and CH4 emitted from rivers were thought to be predominantly derived from recent (sub-decadal) biomass production and, thus, part of ecosystem respiration3-6. Here we combine new and published measurements to create a global database of the radiocarbon content of river dissolved inorganic carbon (DIC), CO2 and CH4. Isotopic mass balance of our database suggests that 59 ± 17% of global river CO2 emissions are derived from old carbon (millennial or older), the release of which is linked to river catchment lithology and biome. This previously unrecognized release of old, pre-industrial-aged carbon to the atmosphere from long-term soil, sediment and geologic carbon stores through lateral hydrological routing equates to 1.2 ± 0.3 Pg C year-1, similar in magnitude to terrestrial net ecosystem exchange. A consequence of this flux is a greater than expected net loss of carbon from aged organic matter stores on land. This requires a reassessment of the fate of anthropogenic carbon in terrestrial systems and in global carbon cycle budgets and models.

Accurate accounting of greenhouse‐gas (GHG) emissions and removals is central to tracking progress toward climate mitigation and for monitoring potential climate‐change feedbacks. GHG budgeting and reporting can follow either the Intergovernmental Panel on Climate Change methodologies for National Greenhouse Gas Inventory (NGHGI) reporting or use atmospheric‐based “top‐down” (TD) inversions or process‐based “bottom‐up” (BU) approaches. To help understand and reconcile these approaches, the Second REgional Carbon Cycle Assessment and Processes study (RECCAP2) was established to quantify GHG emissions and removals for carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O), for ten‐land and five‐ocean regions for 2010–2019. Here, we present the results for the North American land region (Canada, the United States, Mexico, Central America and the Caribbean). For 2010–2019, the NGHGI reported total net‐GHG emissions of 7,270 TgCO 2 ‐eq yr −1 compared to TD estimates of 6,132 ± 1,846 TgCO 2 ‐eq yr −1 and BU estimates of 9,060 ± 898 TgCO 2 ‐eq yr −1 . Reconciling differences between the NGHGI, TD and BU approaches depended on (a) accounting for lateral fluxes of CO 2 along the land‐ocean‐aquatic continuum (LOAC) and trade, (b) correcting land‐use CO 2 emissions for the loss‐of‐additional‐sink capacity (LASC), (c) avoiding double counting of inland water CH 4 emissions, and (d) adjusting area estimates to match the NGHGI definition of the managed‐land proxy. Uncertainties remain from inland‐water CO 2 evasion, the conversion of nitrogen fertilizers to N 2 O, and from less‐frequent NGHGI reporting from non‐Annex‐1 countries. The RECCAP2 framework plays a key role in reconciling independent GHG‐reporting methodologies to support policy commitments while providing insights into biogeochemical processes and responses to climate change.

The winter hydrological period is in transition across the Canadian subarctic, as climate warming is shifting precipitation regimes, thawing permafrost, and altering active layer dynamics, and thus increasing the overall amount, and variability, of winter streamflow. Effects of these changes are poorly understood on the Taiga Shield, which comprises similar to 20% of North America's permafrost-covered area, and is characterized by a unique 'fill-and-spill' hydrology whereby runoff generation requires the exceedance of lake basin storage thresholds. Here, we assessed lake hydrostatic levels and used trail camera images of icings, which are sheet-like masses of layered ice that are common manifestations of wintertime flow on the Taiga Shield, to understand landscape controls on winter water movement in this region. We further used paired geochemical measurements to explore how source water characteristics affect icing chemistry, and the degree to which icings may modify the chemical composition of active winter flow. We undertake this work over 2 years, and across watersheds of different sizes and lake basin characteristics. We show that icing growth is driven by hydroclimatic controls that include fill-and-spill hydrologic constraints and winter air temperatures, and that pre-freshet pulses of water flow are common within this landscape. Across winters with variable antecedent precipitation levels, a larger catchment was able to support icing growth via continued runoff generation, while small catchments were not. Icings were often chemically dilute compared with source waters, indicating that solute exclusion may actively enrich geochemical concentrations in flowing water. Across icings, chemical variation appeared related to source water type (groundwater versus lake; lake size) and apparent redox conditions. These results highlight that streamwater hydrology and biogeochemistry can be dynamic during the understudied winter period, and illustrate that icings may alter the composition of wintertime flow as it moves through fluvial networks.