What is the impact of urbanisation on the vulnerability of a watershed? This vulnerability can be addressed for each area by calculating the river risk after urbanisation. This analysis is based on the watersheds of the Seine and Thames rivers. To analyse the flooding risk, the whole watershed morphology needs to be observed. Then can come the catchment basin (or part of it) model calculations. This river risk factor proceeds from the calculation of the flow increases and of the urban and local G.D.P. and demographic growth. This fluvial risk factor allows one to have a prompt idea of the vulnerability of a city crossed by a river, provided local data is taken into account. It also gives a short, medium and long term impact prospective concerning any urban project, beyond the apparent safety of artificial structures to protect against flooding.