Bacteriological water monitoring carried out during extreme precipitation events can identify higher or lower vulnerability of hydric resources to bacterial contamination. This hypothesis was tested in nine lotic and lentic environments of the floodplain of the Paraná River, one of the largest rivers in the world, after the rainiest month of the century (November 2000; 338 mm). To measure this impact, the concentration of Escherichia coli was compared with that obtained during dry weather (220 samples). Aquatic environments far from urban areas were relatively less affected. In closer areas, in contrast, the concentration of E. coli was 200 to 350 times higher than the average (P<0.0001). The concentration of the faecal contamination indicator was 8 to 13 times higher than the average (P<0.0001) in the middle course of the Paraná River (31 degrees 42' 04"S; 60 degrees 29' 39"W), but lakes isolated from rivers and urban areas were not affected by the event (concentrations lower than the average).