The urban hydrology as a scientific discipline has been gradually developped in France through networks gathering engineers from local authorities' departments and local research teams. The "observatories of urban hydrology" (OPUR in Paris area, OTHU in Lyon, and Onevu in Nantes) is the last step in this institutionalization's process. This article shows how these new institutions influence the innovation process in stormwater management. Indeed, they impact on the way these major cities' public services are run. Observatories tend to spread among them a culture of "monitoring" and "evaluation" which relies on a "rational analysis". Out of these scientific and technical networks, smaller cities (Rennes and Le Douaisis) show other ways to produce innovation: they favour "pragmatic and functional" approaches and the implementation of local solutions. We point how Rennes and Le Douaisis tend to promote a quite radical innovation, which can be implemented without scientific backing and guarantee, while major cities, thanks to the observatories, develop innovation in much more cautious way. However, the reality is more balanced: the expertise produced by OPUR, OTHU and Onevu is spread far beyond their own network; it reduces the risks really taken by local communities "without observatories".