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Great cities built on rivers have and have had with their streams very different relations. With three samples volontarily chosen for their différencies : Edmonton, Montreal and Lyons, the author illustrates that idea with the survey of the evolution of those relations. Edmonton, very young city, has preserved the natural landscape of the North Saskatchewan valley. Montreal and Lyons, after having for long times sacrified natural aspects to the benefit of economic activities, mortorways and parkings, have rediscovered in different manners others utilisations of the river banks : sports, public walk and bicycling, parks... Then, they are in the world evolution of reconquest of the edges of waters on rivers, lakes and seas.

Within the two last years, the « Communauté Urbaine de Lyon » has been revising the plans of the reglementary submersible areas. From the new topographic map at the 1/2 000e - the map of submersibility - new regulation documents are propounded on the basis of the flood periodicity and the elevation of submersion. These plans linked to the P.O.S. (Land Use Survey) are required for new buildings.

This paper presents a method allowing : — an evaluation of the E.T.P. in a point deprived of temperature measurement — the calculation of water budget of a catchment aera including the losses in porous and gently sloping ground, the interseasonnal water retention or restitution by the ground water tables or snow melting — the prevision of low water stage with the research of the minimum ressource of the following mouth in the case where the later would be dry.