New communication and digital image technologies have enabled the public to produce and share large quantities of flood observations. Valuable hydraulic data such as water levels, flow rates, inundated areas, etc., can be extracted from photos and movies taken by citizens and help improve the analysis and modelling of flood hazard. We introduce recent citizen science initiatives which have been launched independently by research organisations to document floods in some catchments and urban areas of France, Argentina and New Zealand. Key drivers for success appear to be: a clear and simple procedure, suitable tools for data collecting and processing, an efficient communication plan, the support of local stakeholders, and the public awareness of natural hazards.