1. Denitrification, net oxygen consumption and net nitrous oxide flux to the atmosphere were measured in three small rivers (discharge approximately 2–27 m3 s−1) at the whole reach scale during Spring and Summer, 2002. Two of these rivers (Iroquois River and Sugar Creek in north-west Indiana – north-east Illinois, U.S.A.) drained agricultural catchments and the other (Millstone River in central New Jersey, U.S.A.) drained a mixed suburban–agricultural catchment.
2. Denitrification, oxygen consumption and N2O flux were measured based on net changes in dissolved gas concentrations (N2, O2, and N2O) during riverine transport, correcting for atmospheric exchange. On each date, measurements were made during both light and dark periods.
3. Denitrification rates in these rivers ranged from 0.31 to 15.91 mmol N m−2 h−1, and rates within each river reach were consistently higher during the day than during the night. This diurnal pattern could be related to cyclic patterns of nitrification driven by diurnal variations in water column pH and temperature.
4. Oxygen consumption ranged from 2.56 to 241 mmol O2 m−2 h−1. In contrast to denitrification, net oxygen consumption was generally higher during the night than during the day.
5. River water was consistently supersaturated with N2O, ranging from 102 to 209% saturated. Net flux of N2O to the atmosphere ranged from 0.4 to 60 μmol N m−2 h−1. Net flux of N2O was generally higher at night than during the day. The high flux of N2O from these rivers strengthens the argument that rivers are an important contributor to anthropogenic emissions of this greenhouse gas.