This paper presents the governing equations of a tidally-averaged, width-averaged, rectangular estuary in completely nondimensionalized forms. Subsequently, we discover that the dynamics of an estuary is entirely controlled by only two variables: (i) the Estuarine Froude number, and (ii) a nondimensional number related to the Estuarine Aspect ratio and the Tidal Froude number. Motivated by this new observation, the problem of estuary classification is re-investigated. Our analysis shows that

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... two control variables are capable of completely determining the stratification at the estuary mouth, and therefore can specify the estuary type. The theoretical estuary classification scheme proposed in this paper is validated against real estuarine data collected from existing literature. Our classification scheme on comparison with the state-of-the-art theory shows significant improvement.