Urban impacts on faecal bacterial pollution of the near-shore zone of the St. Clair River in Sarnia were studied by means of field observations and computer modelling. Towards this end, water samples were collected at various sources of faecal pollution and at nine sampling stations in the river, and analysed for faecal coliform, faecal streptococci, Escherichia coli, Pseudomonas aeruginosa and coliphage. Probabilistic distributions of the observed microorganism densities were used to assess the levels of faecal bacteria pollution, describe the impacts of urban sources on faecal bacteria concentrations in the river, and make inferences about compliance with the recreational water quality guidelines. Relatively high probabilities of guideline violations (p > 30%, at most sites, in dry weather) indicated the need for remedial measures. The screening of such measures was accomplished by preliminary modelling of indicator bacteria in the receiving waters, using a bacteria loading model interfaced with a receiving water model.