The carbon isotopic composition of the organic matter in fluvial sediments and in fluvial suspended matter falls in the range valid for continental organic matter (−25 to −28‰). The δ¹³C values in sediments from marine sedimentation areas and in marine suspended matter varies between −20 and −25‰. The rather low mean values for the deposited sediments of −13.2‰ suggest the presence of relatively large amounts of continental organic matter. This may be a direct supply by the present-day rivers and/or organic matter derived from the erosion of peat layers, even in the marine environment. A distinction between these two sources of detrital continental organic matter is not possible with stable isotopes studies.

In the estuaries studied the organic matter contents decrease and the δ¹³C values of the organic matter increase in the seaward direction. These changes are primarily caused by the mixing of fluvial and marine sediments.