The globe and in particular, the Arctic and subarctic regions of the world are experiencing enhanced warming and increased movement of atmospheric and terrestrial water through the hydrologic system (Huntington, 2006; England et al., 2021; Chylek et al., 2022; Wendisch et al., 2022), along with accelerated thaw of the permafrost (ground that remains frozen for more than 2 years, Rowland et al., 2010; Lawrence et al., 2012; Chadburn et al., 2017; Nitze et al., 2018; Biskaborn et al., 2019). As these changes occur, complex processes result in deeper soil active layers (the seasonally thawed layer at the top of the permafrost) and thinner permafrost (Hayes et al., 2014), more extensive talik (i.e., holes in the permafrost) formation (Parazoo et al., 2018; Liu et al., 2021), larger areas of discontinuous permafrost (Biskaborn et al., 2019), and less ground ice (Liljedahl et al., 2016).

In the Arctic and subarctic, hydrological intensification also results in increased water flow, both off the land surface and in rivers, deeper snowpack, more summer rain, and more extreme events (Peterson et al., 2002; McClelland et al., 2006; Rawlins et al., 2010; Prowse et al., 2015). Both hydrologic cycle rates and permafrost thaw have positive feedbacks to the vegetation, water quality (i.e., heat, sediment, and carbon/mercury mobilization), and energy exchanges (i.e., water vapor feedback, permafrost extent, snowpack, evapotranspiration, and soil moisture changes) across the Arctic and subarctic landscapes and oceans (Peterson et al., 2002; Rawlins et al., 2010; Bring et al., 2016; Stuefer et al., 2017; Durocher et al., 2019; Zheng et al., 2019; Koenigk et al., 2020; Feng et al., 2021). These changes are anticipated to enhance positive feedbacks to global climate impacts, leading to even more rapid changes and further increasing greenhouse gas concentrations in the atmosphere (Schuur et al., 2015).

Arctic and subarctic river outflows to the Arctic Ocean have been noted to be increasing over the past 50 years (Peterson et al., 2002; McClelland et al., 2006; Bring et al., 2016; Durocher et al., 2019). A particular focus has been on the annual discharge of the largest Eurasian rivers (e.g., Lena, Ob, and Yenisei) as these rivers input a large percentage (55%) of fresh water annually to the Arctic Ocean compared with 28% from North American (NA) systems (Shiklomanov et al., 2021). While Eurasian rivers are unquestionably increasing in annual mean streamflow (Shiklomanov et al., 2021), if and how NA Arctic river discharges are changing remains in question. For example, in the Mackenzie River basin, no obvious trends were found in terms of changing annual flow magnitudes from 1973 to 1999, although variability was noted to be increasing (Zhang et al., 2001; Woo and Thorne, 2003). Updated work showed increases over a longer time frame (early-to-mid 1900's-2013; Rood et al., 2017). On the other hand, Déry and Wood (2005) found trends of decreasing annual streamflow discharge from 1964 to 2003 at coastal stations in Arctic Canada. An updated paper in 2009 extended the time series by 4 years (1964–2007), which revealed a trend reversal toward increasing annual streamflows and streamflow variability (Déry et al., 2009). NA river basin annual discharges to the Arctic Ocean were noted to be decreasing from 1964 to 2000 by McClelland et al. (2006) while Zhang et al. (2001) found increasing trends in some basins from 1967 to 1996. More recent work by Durocher et al. (2019) showed that annual river discharges around the Hudson, James, and Ungava bays in northern Canada increased significantly while other NA systems increased as well (although results were not significant at p-values < 0.05). Déry et al. (2016) noted increasing annual flows in northern Canada (18%) over the latter part of the 1964–2013 (1989–2013) time period. Annual discharge in 2020 from the two large NA rivers contributing to the Arctic (Mackenzie and Yukon) was ~25% greater than the 1981–2010 average, largely owing to a very wet summer in 2020 (Druckenmiller et al., 2021). Overall, the consensus is toward increasing NA river flows on the order of 1 km³ per year from the larger NA river systems (Druckenmiller et al., 2021; Shiklomanov et al., 2021).

Previous research has focused on the causal relationships for changing streamflow patterns. Some authors have linked changing streamflow to specific climate variability indexes, such as the Pacific Decadal Oscillation (Rood et al., 2017) or the Arctic Oscillation (Déry and Wood, 2004), although no significant relationships were found by Durocher et al. (2019) for either climate variability index Peak streamflow within high latitude systems is largely driven by snowmelt processes, a seasonal phenomenon that is highly influenced by climate warming (Wang et al., 2016). Peak streamflow carries with it important constituents, including sediment, carbon, oxygen, and nutrients from river systems into the ocean, which may strongly affect nearshore environments and deltas (Holmes et al., 2008, 2012; Rawlins et al., 2010). Low flows, which occur during summer and fall in Arctic and subarctic systems, are largely driven by summer rainfall and baseflow conditions. Deeper active layers and subsidence from permafrost thaw may change low flows by increasing the storage of water in the active layer and the movement of water through the soil profile and into rivers (Walvoord and Striegl, 2007; St. Jacques and Sauchyn, 2009; Walvoord and Kurylyk, 2016). Low flow increases in permafrost regions also have important ramifications for exports of nutrients from Arctic systems (McNamara et al., 2008; Frey and McClelland, 2009; Neilson et al., 2018). Finally, streamflow may be changing in the Arctic due to shifting climate and water balance relationships over long timeframes. Rivers in northwestern Canada were observed to have declining magnitudes of annual snowmelt peaks (Cunderlik and Ouarda, 2009). Further, precipitation minus evaporation (P-E) relationships in these basins are likely changing (Serreze et al., 2000; Zhang et al., 2000; Huang et al., 2022).

Challenges in defining streamflow trends and linkages to increased precipitation may be due in part to sparse measurements and cold season biases in precipitation observations (Adam and Lettenmaier, 2003; Yang et al., 2005; Cherry et al., 2007). Indeed, sparse spatial and temporally short observations, low-quality data, and intermittency in the records limit the ability of researchers to conclusively determine the direction and magnitude of changes and to properly attribute these changes (Ahmed et al., 2020). While many studies on changing Arctic streamflow use established methods to infill data (Pigott, 2001; Teegavarapu and Nayak, 2017; Dembélé et al., 2019) and new techniques utilizing machine learning and modeling methods to gap fill records show promise (Zhang and Post, 2018; Arriagada et al., 2021), no current review of infilling in Arctic systems data has been conducted. Thus, determining trends in streamflow records accurately using high quality streamflow gage records along with process-based and statical tools to model streamflow is considered a grand challenge for cold regions hydrological sciences (Blöschl et al., 2019).

Our objective in this work is to consider NA streamflow trends for recent and longer-term periods within gaged basins affected by varying amounts of permafrost coverage. We aim to (1) quantify differences and trends in annual and seasonal streamflow discharges for mean, minimum, and maximum streamflow and (2) assess whether these vary with permafrost coverage across their basins. We look at 74 gaged sites in North America, ranging from 1 to 90% permafrost coverage. We consider the seasonal and annual response in these differences and trends for varying amounts of permafrost coverage and use multiple tools along with newly available datasets and machine learning methods to examine the drivers of change within the basins. Our goal is to highlight changes in streamflow, identify the drivers of shifts within these systems, and to discuss the challenges associated with identifying trends and their drivers in Arctic and subarctic hydrologic systems.

Table 1 includes an overview of the different sites utilized in this study, based on rabpro and gage metadata analysis and GF3.1 reanalysis data, and Supplementary Table 1 includes all the observational gages used in this work, ordered by permafrost coverage (%). Figure 1 illustrates the site locations across the NA Arctic/subarctic. Gages in the non-dominant permafrost group (n = 28) were generally at the lowest latitudes (average 59.06° N) with the lowest average drainage areas (26,859 km²) and an average elevation of 781.4 m. Gages in the moderate permafrost group (n = 23) were at moderate latitudes (average 61.51° N), at an average elevation of 884.2 m, and had average drainage areas similar in size to the non-dominant systems (26,493 km²). Gages in the dominant-permafrost group (n = 23) were highest in latitude (average 65.52° N), at an average elevation of 555.6 m, and had the largest drainage basin areas (44,291.3 km²). Average annual temperatures range from −2.6, −4.4, and −7.6°C, while precipitation (rain) values range from 16.8 (9.8), 15.8 (9.4), and 21.1 (12.2) km³ yr⁻¹ in the non-dominant, moderate, and dominant permafrost groups, respectively.

Seasonal and annual streamflow patterns within permafrost-affected systems shifted in unique ways which can be examined using various methods, as considered in this work. Minimum streamflow increased across all permafrost systems regardless of the percentage of permafrost coverage, particularly for winter seasons, and annually; trends that held whether we considered individual systems or regional patterns. Annual trends in maximum streamflow were decreasing in the recent-46 (for all permafrost-affected systems) and over the recent-32 period (for the >25% permafrost systems). However, for regional trends, the only significant changes (increases) in maximum streamflow was for permafrost-dominant systems during the winter and fall, a new finding that may be reflective of increasing minimum streamflow patterns, owing to increases in storage within deepening active layers, as presented by other authors (St. Jacques and Sauchyn, 2009; Walvoord et al., 2012; Bring et al., 2016; Walvoord and Kurylyk, 2016; Cooper et al., 2023). The changes in annual maximum streamflow, which is indicative of changing peak streamflow, likely reflect a lower snowpack during the spring snowmelt period, and a shift toward an earlier or “flatter” spring streamflow peak, with the non-dominant permafrost systems experiencing no change in timing and a higher peak (Bokhorst et al., 2016; Rixen et al., 2022). As temperatures continue to warm in high latitude regions and peak streamflow moves earlier in the spring season, the streamflow hydrograph may become flatter as observed in our systems (increasing minimum flows and decreasing maximum flows in the spring), leading to a less dramatic peak flow and a flatter hydrograph overall (Stadnyk et al., 2021).

Overall, mean streamflow increased in our permafrost systems and most prominently in permafrost-dominant systems (regional and individual systems), with more significant results compared to maximum streamflow trends across the metrics we considered. This finding is well-documented for permafrost systems across the Arctic (McClelland et al., 2006; Durocher et al., 2019; Ahmed et al., 2020; Shrestha et al., 2021). The strong shift toward increasing flows may be indicative of accelerated streamflow runoff conditions in permafrost dominated systems. Permafrost-dominant systems are very likely changing with regards to their active layers and surface conditions (Makarieva et al., 2019; Wang et al., 2021; Wright et al., 2022), a feature that appears to be already leading to increased mean streamflow now, with stronger increases anticipated in the future (Stadnyk et al., 2021). Interestingly, our overall trend results for permafrost-dominated systems align very closely with recent estimates of increased streamflow for the Mackenzie and Yukon River systems (Druckenmiller et al., 2021; Shiklomanov et al., 2021).

Aside from differences in permafrost extent, the unique changes in permafrost-dominant systems compared to non-dominant permafrost systems could be in part due to differences in elevation and precipitation between permafrost systems. More of the non-dominant permafrost systems are headwater (higher elevation) vs. lowland systems and snowpack change under warming is elevation-dependent (Solander et al., 2018; Rottler et al., 2019), leading to unique changes in streamflow between permafrost systems. Further, permafrost-dominant systems considered here also had higher precipitation compared to the other systems.

The drivers of streamflow changes within our permafrost-dominated systems can be examined using reanalysis tools and models such as ERA5-Land and ERA5-GloFAS datasets in conjunction with machine learning techniques, although these results will be affected by biases within the climatic drivers and modeled streamflow at ungaged basins. As well, our results come with a caveat that the methodology is susceptible to data availability, randomness in the tools we applied, and methodological approach overall. Nevertheless, we saw notable differences in the grouping of climate trends and static features driving streamflow trends in our models. Overall, climate appeared to be a much stronger feature in the streamflow trends examined in these systems compared to the basin characteristics such as permafrost percent, with precipitation being ranked as more important than temperature features. Further, in the different records, basin characteristics did not usually outrank temperature variables, except for a few cases. Only for trends in streamflow in the most recent time period did we see permafrost percent start to become an important variable, which may be indicative of enhanced thawing of permafrost in more recent years, as this process begins to play an increasingly focal role in changing streamflow trends across all components of the streamflow hydrograph.

An important issue in streamflow trend analysis for Arctic and subarctic systems is the high variability that confounds the signal-to-noise within these systems. Part of this is due to the lack of streamflow gages, data intermittency, and lack of long term (i.e., 100+ years) data sets. Intermittency arises from the wintertime freezing of rivers and difficulties associated with maintaining and recording remote streamflow gage data. Longer records were associated with significant increases in statistically significant trends in our data sets; but at the same time, the more recent period showed shifts that may be indicative of enhanced warming and thawing within permafrost basins of the high latitudes. Further, as temperatures rise, precipitation and rain/snow partitioning are shifting, along with changes in the permafrost conditions, all of which can cause increases and decreases in streamflow that obscure changes in the systems. Thus, considering observations alone may be insufficient to clearly evaluate streamflow trends in a statistically robust manner. However, these limitations will likely diminish in the future given the work being done on new datasets, infilling techniques, and improved remote sensing and modeling tools (Muskett and Romanovsky, 2009; Biancamaria et al., 2016; Solander et al., 2017; Harrigan et al., 2020; Feng et al., 2021).

Our conclusions for NA streamflow system affected by permafrost are:

The datasets presented in this study can be found in online repositories. The names of the repositories and accession numbers can be found at: https://data.ess-dive.lbl.gov/view/doi:10.15485/1985921.

KB conceived of the work, performed data collection, analysis, original writing, and editing of the paper. JSc and JSt curated all datasets used in analysis except streamflow time series and contributed to writing and editing of the original text. CB provided original writing and edits to the text. EG provided data analysis, figure development, and editing of original text and through the review phases. WB provided feedback on concepts, data consultations, and review of original text and revisions. JR provided feedback on work concepts, review of original text and revisions, and funding as project PI. All authors contributed to the article and approved the submitted version.