Dissolved organic carbon (DOC) is of environmental and biological significance to Precambrian shield streams. Our objective was to determine the extent of transformations to the concentration and molecular weight distribution (MWD) of DOC along the length of Harp 4A stream, from the headwater swamp to the outflow into Harp Lake. The average MWD shifted to a smaller weight by 0.48 kDa. A rapid decline in the concentration and MWD of DOC was observed within 120 m from the main DOC source, a headwater swamp. The most significant decline in concentration and MWD was observed within the first of a series of three beaver ponds. DOC within waters downstream of the beaver ponds was resistant to further changes in concentration and MWD. The MWD within each of four stream sections with Hp 4a remained stable over 6 months, from May to October 2001.