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<p>Large amounts of methane (<span class="inline-formula">CH₄</span>) could be released as a result of the gradual or abrupt thawing of Arctic permafrost due to global warming. Once available, this potent greenhouse gas is emitted into the atmosphere or transported laterally into aquatic ecosystems via hydrologic connectivity at the surface or via groundwaters. While high northern latitudes contribute up to 5 % of total global <span class="inline-formula">CH₄</span> emissions, the specific contribution of Arctic rivers and streams is largely unknown. We analyzed high-resolution continuous <span class="inline-formula">CH₄</span> concentrations measured between 15 and 17 June 2019 (late freshet) in a <span class="inline-formula">∼120</span> km transect of the Kolyma River in northeast Siberia. The average partial pressure of <span class="inline-formula">CH₄</span> (<span class="inline-formula"><i>p</i>CH₄</span>) in tributaries (66.8–206.8 <span class="inline-formula">µatm</span>) was 2–7 times higher than in the main river channel (28.3 <span class="inline-formula">µatm</span>). In the main channel, <span class="inline-formula">CH₄</span> was up to 1600 % supersaturated with respect to atmospheric equilibrium. Key sites along the riverbank and at tributary confluences accounted for 10 % of the navigated transect and had the highest <span class="inline-formula"><i>p</i>CH₄</span> (41 <span class="inline-formula">±</span> 7 <span class="inline-formula">µatm</span>) and <span class="inline-formula">CH₄</span> emissions (0.03 <span class="inline-formula">±</span> 0.004 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mmol</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="d4c2ee668fea12b7ca233e9f97025a6f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-5059-2022-ie00001.svg" width="67pt" height="13pt" src="bg-19-5059-2022-ie00001.png"/></svg:svg></span></span>) compared to other sites in the main channel, contributing between 14 % to 17 % of the total <span class="inline-formula">CH₄</span> flux in the transect. These key sites were characterized by warm waters (<span class="inline-formula"><i>T</i>&gt;14.5</span> <span class="inline-formula">^∘C</span>) and low specific conductivities (<span class="inline-formula"><i>κ</i>&lt;88</span> <span class="inline-formula">µS cm⁻¹</span>). The distribution of <span class="inline-formula">CH₄</span> in the river could be linked statistically to <span class="inline-formula"><i>T</i></span> and <span class="inline-formula"><i>κ</i></span> of the water and to their proximity to the shore <span class="inline-formula"><i>z</i></span>, and these parameters served as predictors of <span class="inline-formula">CH₄</span> concentrations in unsampled river areas. The abundance of <span class="inline-formula">CH₄</span>-consuming bacteria and <span class="inline-formula">CH₄</span>-producing archaea in the river was similar to those previously detected in nearby soils and was also strongly correlated to <span class="inline-formula"><i>T</i></span> and <span class="inline-formula"><i>κ</i></span>. These findings imply that the source of riverine <span class="inline-formula">CH₄</span> is closely related with sites near land. The average total <span class="inline-formula">CH₄</span> flux density in the river section was 0.02 <span class="inline-formula">±</span> 0.006 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M33" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mmol</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="bc30a35f2d41fc21c297cb749b759172"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-5059-2022-ie00002.svg" width="67pt" height="13pt" src="bg-19-5059-2022-ie00002.png"/></svg:svg></span></span>, equivalent to an annual <span class="inline-formula">CH₄</span> flux of <span class="inline-formula">1.24×10⁷</span> <span class="inline-formula">g CH₄ yr⁻¹</span> emitted during a 146 d open water season. Our study highlights the importance of high-resolution continuous <span class="inline-formula">CH₄</span> measurements in Arctic rivers for identifying spatial and temporal variations, as well as providing a glimpse of the magnitude of riverine <span class="inline-formula">CH₄</span> emissions in the Arctic and their potential relevance to regional <span class="inline-formula">CH₄</span> budgets.</p>