As part of the Stratford-Avon River Environmental Management Project, the Southwestern Regional Office of the Ontario Ministry of the Environment collected data on water chemistry and sediments at 13 stations along the Avon River during 1980 and 1981. In addition, the River Systems Unit of the Water Resources Branch, MOE, conducted an intensive inlake survey of Lake Victoria in August 1980. Data from these surveys and other sources were used to assess the impact of Lake Victoria and the John Street weir impoundment on the water quality of the Avon River. The analysis focussed on summer impacts (May to September) since this is the period of greatest concern in this study and since the impoundments have in the past been drawn down over the winter period. Major influences of the impoundments include increases in BOD5 and reductions in dissolved oxygen (CO) concentrations, increases in total phosphorus and TKN, decreases of soluble phosphorus, increases in suspended solids, increases in zinc concentrations and a reduction of fecal coliform and fecal streptococci bacteria. Sane of the processes that may underlie these influences are phytoplankton production, weir de-aeration, contamination from storm-sewer flows and assimilation or decay. An analysis of sedimentation rates revealed that Lake Victoria is very likely an efficient trap for suspended mineral sediments. The observed increase in suspended sediments in flays leaving the lake may therefore be caused by organic debris originating in the lake. From an analysis of bottom sediments, it was found that Lake Victoria sediments display some heavy metal contamination likely related to urban runoff .