The report describes results of a study of water quality budgets in the Lake St. Clair - Detroit River system. In two water quality surveys profiles of contaminants and other chemical factors were measured every 12 hours across transects of the upper and lower Detroit River. Average concentrations of the same parameters were also determined for stations near the mouths of four main tributaries during this period. Parameters included suspended solids, chloride, silicate, phosphorus, hardness, organic carbon, iron, lead, cadmium, zinc, copper, nickel, mercury, hexachlorobenzene (HCB), octachlorostyrene (OCS) and PCB congeners. Concentrations of all parameters were multiplied by river and tributary flows to compute chemical mass loadings into and out of the Detroit River system. Mass imbalances, calculated as the difference between river head and mouth loadings, indicated significant gains for nearly all parameters within a given water mass during its 18 hour passage through the system. Tributary loadings appeared to account for only a minor part of the observed gains in Detroit River loadings. Other point and non-point sources are being evaluated by the Upper Great Lake Connecting Channel Study (UGLCCS) committees as potential sources of the unexplained loading. Findings will be used to prioritize regulatory actions for reducing contaminant loads.