Study area Teno river (Finnish: Tenojoki, Norwegian: Tana, Northern Sámi: Deatnu) at the border between Norway and Finland (70.092°N, 27.959°E) Boreal river flowing into the Arctic Ocean Pristine ecosystem mostly unaffected by industry, damming or other human influences Strahler order at measurement locations ranged between 1 and 5 Measurement overview Measurements were conducted for fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) using different chamber setups, and for respective partial pressures, and hydrochemistry in August 2023, June 2024, and September 2024 at various stages of river discharge (Table 1). Table 1: Overview of measurements in the river channel and at banks, which refer to the transitional area bordering the river channel. Measurements Target Chamber type Time CO2 and CH4 fluxes, partial pressures, hydrochemistry River channel Floating chambers August 2023 CO2 and CH4 fluxes, substrate type, vegetation coverage, sediment analysis Banks Static chambers August 2023 CO2, CH4 and N2O fluxes, partial pressures, hydrochemistry River channel Anchored chambers August 2023, June and September 2024 CO2 fluxes Banks Static chambers June and September 2024 Greenhouse gas flux measurements Greenhouse gas fluxes were determined from chamber measurements conducted at the water surface and at adjacent banks. Mole fractions of CO2, CH4 and N2O within the chamber headspace were measured from a boat, from the shore or at confluences with tributaries, with different greenhouse gas analyzers (portable and laboratory analyzers), at different sites and sections of the river all indicated in the dataset. Water-air and soil-air fluxes were derived from changes of mole fractions over time using linear regressions. The atmospheric pressure was measured with an RM Young barometric pressure sensor or a Multilogger Thermo-hygro barometer. Ambient temperatures were determined with a WTW multiprobe or the Multilogger Thermo-hygro barometer. For terrestrial measurements at banks, the substrate type and percentage of vegetation coverage at the measurement location were visually determined. Furthermore, about 300 g of the top three centimeters of the sediment were sampled and analyzed for water content, pH, electrical conductivity and grain size. Partial pressure measurements Partial pressures of CO2, CH4 and N2O were measured at 5-10 cm depth below the water surface in-situ using an equilibrator connected to a portable greenhouse gas analyzer (LI-7810, LI-COR) or in-lab using gas chromatography after water samples were collected. Detection limits for the gas chromatograph were 200 ppm for CO2, 1 ppm for CH4, and 0.4 ppm for N2O. Continuous CO2 and CH4 partial pressure measurements conducted in August 2023 were averaged and assigned to individual flux measurements, but are also published here as a 1 Hz timeseries. Hydrochemical measurements Measurements of water temperature and specific conductivity, pH, turbidity, chlorophyll, phycocyanin, fluorescent dissolved organic matter (fDOM), and dissolved oxygen (DO) were conducted at approximately 10 cm below the water surface with an EXO2 Multiparameter Sonde (YSI) equipped with a wiper to clean the sensors once an hour. Besides, electric conductivity (EC) at the water surface was measured with an EC meter (WTW pH/cond 340i). Chloride, phosphate and nitrate were measured using ion chromatography (Dionex ICS-2100, Thermo Scientific). Dissolved organic carbon (DOC) and dissolved nitrogen (DN) concentrations were measured with a TOC analyzer (TOC-VCPH, Shimadzu). Corresponding detection limits are given in Table 2. If values were below the detection limits, they were set to zero. Wavelength-specific UV-absorbance at 254 nm (SUVA254) was determined from spectral absorbance measured with a spectrophotometer (UV-1800, Shimadzu) divided by the DOC concentration. The chromophoric dissolved organic matter (CDOM) content was derived from adsorption at 350 nm. Table 2: Detection limits of DOC, TN, chloride, phosphate and nitrate. Analysis Detection limit (mg/L) DOC 0.1 TN 0.3636 Chloride 0.13 Phosphate 0.19 Nitrate 0.16