Calculating downstream concentration

[dabraham at chief.ecn.uoknor.edu]

I have a map of a stream network(tributaries and the main river) with pointdischarges of chlorides into the various tributaries at different points. Now there are a lot of formulas which help calculate the downstream conc. ofconservative substances like chlorides. Is there anyway in grass by which Ican arrive at a map of the stream network giving me the downstream conc. 
along different points of the tributaries and the main river? Any comments or suggestions will be appreciated.
David.