A methodology is proposed to calculate statistical average and standard deviation of long time water quality parameter series along a river network. The method considers the water network as a graph consisting of straight sessions and junctions. With a Taylor-series approximation, statistical values of an arbitrary point of the network can be calculated from upstream ones without the need to calculate the single downstream values. According to preliminary results of the first calculations on a pilot area, mean value of the downstream biological oxygen demand and the so called ‘transfer coefficient’ can be approximated with a relative accuracy of 10%
