A simple barotropic model is used to establish the basic nature of the hydraulics of the various shelf modes in coastal flows. It shows that there is a major difference in behavior between flows with an even number of subcritical modes and those with an odd number of subcritical modes. It is suggested that a poleward flowing western boundary current exhibiting an odd number of subcritical shelf modes loses contact with the coast at that latitude at which the flow becomes critical to the next higher shelf mode. The East Australian Current and the East Cape Current are discussed as examples of this behavior.