Introduction The Seine River rises at the northeast of France and flows through Paris before emptying into the English Channel. On June 2016 (week 22) and January 2018 (weeks 4 to 6), major floods occurred in the Basin of Seine River, after important rainy periods. The second period was also marked by the occurrence on the same area of a cold wave including heavy snowfall and ice conditions on week 6. Floods are known to have potential health impacts on population living in those regions, both at short-, medium- and long-term both on physical and mental health. The objective of the study is to present the results of the daily monitoring of health indicators conducted by the French public health agency (SpFrance) during the two major floods, in order to early identify potential impact of those disasters on the population. Methods Since 2004, SpFrance set up a national syndromic surveillance system SurSaUD, enabling to ensure morbidity and mortality surveillance. Morbidity data are daily collected from a network involving emergency departments (ED) and emergency general practitioners’ associations SOS Medecins. Both administrative (age, gender, date and location of consultation) and medical information (medical diagnosis using ICD10 codes in ED and specific thesauri in SOS Medecins associations) are recorded for each patient. The daily and weekly evolution of the number of all-cause ED attendances and SOS Medecins consultations during the flood periods were compared to the evolution on the two previous years. The number of hospitalisations after ED discharge was also monitored. The immediate health impact of floods was assessed by monitoring eight syndromic indicators related to flood exposure: gastroenteritis, carbon monoxide poisoning, burnt, stress, faintness, drowning, injuries and hypothermia. Analyses were performed by age group and at different geographical levels (national, Paris region and districts located in the Basin of Seine River). Results During week 22-2016, a decrease of total number of ED attendances all ages and more specifically for children aged less than 15 years old was observed in region Centre (−15% compared with weeks 21 and 23) and to a lesser extent in Paris area (−4% and −12% respectively compared with week 21 and week 23). Locally, in the most impacted cities, an increase of total ED attendances was also observed for people aged 65 years and more. At the opposite, the number of all-cause SOS Medecins consultations remained stable in all age groups. Analysis by syndromic indicator did not show unusual variations during the first flood period. In 2018, syndromic surveillance did not show any major impact on all-cause ED attendances and SOS Medecins consultations from week 4 to week 6, neither in Paris area nor in other areas along the Seine River. The numbers were comparable to the two precedent years in all age groups. During week 6 in Paris area, an increase of ED attendances was observed for injuries (+4% compared to the past weeks) and to a lesser extent for hypothermia (16 attendances compared to less than 9 for the past weeks). Conclusion For both flood episodes the rising water level was slow with foreseeable evolution, compared to other sudden flood events occurring in south of France in 2010 due to violent thunderstorms. This progressive evolution allows French authority to deploy wide specific organization in order to mitigate impact on concerned populations. That may explain the limited impact observed during the two flood disasters. The evolution of injuries during 2018 episode is attributable to the cold wave that occurred simultaneously. As the French syndromic surveillance system is implemented on the whole territory and collects emergency data routinely since several years, it constitutes a reactive tool to assess the potential public health impact of both sudden and predictable disasters. It can either contribute to adapt management action or reassure decision makers if no major impact is observed.