The Hydra set is a set of models to determine the hydraulic boundary conditions for flood defences in the Netherlands. Hydra-zoet is the model to be used for the fresh water enviroment (main rivers and fresh water lakes). This report describes the development and working of this model. part one gives a general discription on the physics of the model and a description of the fresh water systems, part 2 describes the physical, statistical and probabilistic aspects of the model Hydra-Zoet for a sea delta, in this case the delta of the rivers Rhine and Meuse. This delta is also denoted as the tidal-river area or simply the tidal area. Part 3 describes the physical, statistical and probabilistic aspects of the model Hydra-Zoet for a lake delta, in this case the delta of the Vecht and IJssel.