Behavior of metals during estuarine mixing can significantly influence the chemical mass balance between rivers and seas or lakes. Present investigation describes the results of mixing of a sample of filtered (0.45 mu m) Caspian Sea water with it filtered water sample taken front the Babolrud River in 9 different salinity proportions. Flocculation of colloidal size fraction of heavy metals was investigated oil it series Of mixtures with water salinities ranging from 1.5 to 9.5 parts per thousand during mixing of the Babolrud River with the Caspian Sea water. The flocculation trend of Cu (77.9%) > Zn (74.6%) > Mn (64.97%) > Pb (38.2%) > Ni ( 12.2%) inducates that Cu, Zn, Pb and Mn have non-conservative behavior and Ni has relatively conservative behavior during(I estuarine mixing, The highest flocculation of heavy metals occurs between the salinities of 1.5-3.5 parts per thousand. Statistical analysis indicates that the flocculation of Cu is governed by pH and PO4. The flocculation rate of the Studied metals showed that the overall colloidal metal pollution loads call be reduced by various percentages (ranging from its low its 12 to as high as 78) during estuarine mixing.