Given observed trends in the concentration of DOC in surface waters in the northern hemisphere the degradation of DOC to CO2 could represent a major and increasing source of greenhouse gas to the atmosphere. However, studies of DOC turnover in rivers have been predominantly based upon mass balance studies and empirical studies have focused upon lakes and estuaries which have far longer residence times than would be the case for transit via rivers. The study measured DOC loss in unfiltered river water samples across an 818 km2 catchment every month for a year and considered total loss, photo and aphotic degradation as well as the rates of each process. The study found: (i) Rate of total DOC change in daylight varied from loss of 30.1 mg C/l/day to an increase of 3.5 mg C/l/day: the average loss was 73% over 10 days. (ii) Rate of change due to photic processes varied from decrease of 19.4 mg C/l/day to an increase of 6 mg C/l/day, i.e. net photo-induced production was possible. (iii) Activation energy of the degradation was estimated as 2.6 ± 1.2 kJ/g C. (iv) The apparent quantum yield varied from 9.6 to −1.7 mmol C/mol photons. (v) Coupling models of total loss of DOC with estimates of in-stream residence times showed that annual loss rates of DOC across the 818 km2 catchment would be between 48% and 69%, in line with estimates from mass balance studies, implying that in-stream DOC degradation represents a large, indirect source of CO2 emissions from peats and other organic soils. (vi) Annual rate of removal was increasing in line with increasing loss of DOC at source, implying that observed DOC trends are leading to increased CO2 emissions.