The mouths of many barrier estuaries open and close intermittently under the influence of a wide range of environmental parameters. While there have been systematic observations of this behaviour, particularly of the South African estuaries, there has been no overarching theory to explain or to guide the observation and management of these resources. Using a simple dynamic process model, the evolution of the estuary entrance has been simulated for hundreds of years for thousands of cases. The long termstates (‘attractors’) have been identified as functions of the environmental conditions and the initial depth of the entrance. For a given estuary these attractors may be shown on a simple diagram, revealing the response under steady conditions and the vulnerability to catchment or coastal storms. The attractors predict and explain the occurrence of intermittent behaviour and the observation that intermittent estuaries fall into one of two classes: normally closed or normally open.