A hybrid model incorporating process and system modelling characteristics has been developed and applied to the evolution of a theoretical estuary with a small lake basin, partially enclosed by a barrier with an entrance open to the sea. The one-dimensional model is capable of modelling changes in sedimentation both spatially and temporally and hence, tracks changes in crosssection dimensions along the estuary. The model structure is a series of linked modules to solve the hydrodynamics, the sediment transport equation and a distribution of the sediment to bed and bank. An evolution simulation was conducted for a generic coastal lagoon over an approximate time span of 2000 years with the simplifying assumption of regular sediment supply from the catchment. A further assumption is present for the depositional module in that the distribution is affected by the operation of an attractor state governed by river regime relationships.