We collected water samples from the river surface using a pre-rinsed HDPE 1 L bottle. During the open water period (June to October), water was sampled by hand either by wading into the river from the shore or from a dock or, during the ice-covered period (roughly November to May), through a hole drilled through the river ice (20 m from shoreline). During the ice break-up period, a pole with a mounted sample bottle was used. Samples labelled with "transect" were sampled from a boat. For DOC, CDOM, and TDN, the samples were filtered through a 0.45 μm cellulose acetate filter (Whatman GF/F syringe filter) which was pre-rinsed with 50 mL of sample water. CDOM samples were collected in pre-rinsed 100 mL amber glass bottles that were stored in the dark at 4°C until analysis. Samples were transported to the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI), in Potsdam, Germany for analysis, except for CDOM, which was measured at the German Research Center for Geosciences (GFZ), Potsdam, Germany. CDOM was determined using a double beam LAMBDA 950 UV/Vis (PerkinElmer) spectrophotometer. The absorbance (A) was measured between 200 and 750 nm in 1 nm steps using a 5 cm or 10 cm cuvette, depending on the expected concentration of dissolved organic matter. Absorption (aCDOM) was calculated from the resulting absorbance measurements via aCDOM(λ) = 2.303 * A(λ) * l, where l is the path length (length of cuvette in meter). Every 5 to 10 samples, the reference sample (Milli-Q water) was exchanged and a blank was measured to avoid instrument drift. DOC and TDN samples were filled into a pre-rinsed 20 mL glass vial, acidified with 25 µL HCl (30%) suprapur and stored in the dark at 4 °C. After transport samples were analyzed at AWI. DOC concentrations were analyzed using high temperature catalytic oxidation and NDIR gas analyzer (TOC-L Shimadzu). TDN concentrations were analyzed using high temperature catalytic oxidation and a chemiluminescence gas analyzer (TOC-L Shimadzu).