The Turbidity and Gastroenteritis Study aims to establish and characterize the relationship between the turbidity level of distributed water and the number of cases of acute gastroenteritis requiring medical care (AGEC) observed in the service area. This part of the study covers three geographic sectors in the Paris suburbs, including 109 municipalities in the Île-de-France region and more than 3 million people. The three water treatment plants in Méry-sur-Oise (95), Neuilly-sur-Marne (93), and Choisy-le-Roi (94) are supplied by water from the Oise, Marne, and Seine rivers, respectively. This raw surface water is treated through standard stages of clarification, settling, filtration, and disinfection at the three plants, except at Méry-sur-Oise, where an additional nanofiltration stage is in place. The incidence of GEAm was derived from health insurance data covering the period from 2002 to 2007. The method involved regressing, using a generalized additive model, the incidence of acute gastroenteritis against daily water turbidity levels, and then, in a second step, against several other plant operating parameters to identify risk factors for acute gastroenteritis. The results do not show a convincing relationship between turbidity levels and the risk of GEAm. Particle counting proved more useful for characterizing the risk of gastroenteritis linked to tap water in two of the three sectors studied. Risk levels, however, vary greatly from one sector to another. The effect of water temperature was found to be a key factor in better understanding the role of distributed water on the risk of WEG, with stronger correlations observed at higher temperatures. The results of this study confirm that periods of low water flow or flooding are when the risk of WEG is highest for plants using river water.