High-latitude ecosystems will play a pivotal role in determining the response of the terrestrial carbon cycle to a rapidly changing climate. The arctic and boreal regions hold between 20–60% of global terrestrial organic carbon (C) in cold or permanently frozen soils [Gorham, 1991; Post et al., 1982]. Arctic and boreal regions warmed significantly during the last half of the 20th century [Serreze et al., 2000; Chapin et al., 2005], and current warming is altering vegetation, decreasing permafrost, increasing soil temperatures, and changing fire and hydrologic regimes in the pan-arctic [Hinzman et al., 2005]. These changes are likely to influence the role of high-latitude ecosystems as sources or sinks of atmospheric CO2.
 Lateral fluxes of dissolved organic carbon (DOC) from land to water are an especially important and sensitive component of high-latitude carbon cycles. Large northern rivers have the highest average concentrations of DOC of any region, and riverine transport to the Arctic Ocean is 11% of the global DOC flux [Köhler et al., 2003], representing a significant fraction of terrestrial NEP. The dynamics of boreal and arctic ecosystems and their connections to aquatic systems are thus an important facet of the contemporary carbon cycle and are crucial for projecting impacts of future change.
 Climate change at high latitudes has the potential to greatly affect river DOC fluxes because factors affecting both DOC production and transport are climate sensitive. Production in soils is positively affected by temperature and CO2 [Hobbie et al., 2002; Neff and Hooper, 2002; Freeman et al., 2004; Prokushkin et al., 2005], and thus it is widely expected that arctic DOC fluxes will increase in the future. This hypothesis is supported by stream flux studies in north-temperate watersheds [Freeman et al., 2001; Tranvik and Jansson, 2002] and a study of 96 Siberian peatland watersheds where temperature was correlated with summertime stream DOC concentration [Frey and Smith, 2005]. This study suggests that expected warming of permafrost soils would significantly increase stream DOC export and reduce storage of soil organic carbon. Another recent study, however, suggests an alternate hypothesis that warming could increase summer active layer thickness in permafrost soils, thus deepening water flow paths below organic-rich soils and reducing summertime DOC fluxes from high-latitude watersheds [Striegl et al., 2005].
 The contrasting results of these and other recent studies demonstrate that current understanding of DOC dynamics are insufficient to predict responses to warming and hydrologic change at high latitudes. A major limitation is that flux estimates for arctic rivers are largely based on a very small number of studies from the middle of last century. These estimates were made with very small sample sizes, with especially limited data for springtime conditions [Köhler et al., 2003]. In addition, lab and field process studies focus primarily summertime conditions. However, runoff, an important factor controlling DOC flux, is dominated by snowmelt at high latitudes [Lammers et al., 2001]. Moreover, warming and hydrologic change (e.g., increasing river discharge) are most intense during fall and winter [Symon et al., 2005]. However, despite the well-known runoff patterns of northern watersheds, and evidence for intensified climate change during winter, DOC sources and transport during spring have received little study. For example, we know of no detailed time series of DOC in large arctic rivers during this period [Köhler et al., 2003]. Thus the spring period is a critical period for improving estimates of contemporary fluxes to the Arctic Ocean and for understanding effects of warming and hydrologic change on carbon transport. Here we examine river DOC concentration and flux from a large watershed completely underlain by permafrost in northeast Siberia. We then synthesize existing high-resolution flux data from arctic and boreal rivers to assess the importance of hydrologic and landscape controls on the timing of river DOC flux, and their importance to understanding of DOC transport in high latitude watersheds.