During summer, the basins of shallow estuaries in mediterranean climates are usually vertically mixed, hypersaline, and sometimes inverse (in the sense of their densities being elevated above ocean values). These estuaries are commonly controlled by heat and salt exchange with the ocean. Because of the compensating effect of estuarine heating, the magnitude of the density inversion (due to hypersalinity) depends on the net surface buoyancy flux. The surface heat and evaporative fluxes are functions of the estuarine temperature and so the estuarine state (and its associated surface buoyancy flux) depends on the strength of the ocean exchange processes. A simple Stommel model of a summer mediterranean estuary is presented here in which this exchange is controlled by density-driven processes. The Stommel model has two possible stable stationary states and it is shown that the estuary can be characterized by four types of steady state. They include the familiar classical and inverse estuaries plus a solution, called quasi-neutral, in which the estuary is close to neutrality but just marginally inverse; that is, the net buoyancy flux is small and negative. This quasi-neutral solution resembles the steady state in Tomales Bay (California) in late summer and shows the characteristic high level of thermal compensation of the density inversion from hypersalinity. It also has the observed very long flushing time which is responsible for some of the environmental fragility of this class of estuary. This appears to be the first detailed confirmation of the applicability of the Stommel model to estuaries. The fourth type of estuarine state arises from the simultaneous existence of both stable states. It is here called intermittent and contains both the classical and a quasi-neutral Stommel solution. The type of estuarine state achieved by a narrow basin in autumn is highly dependent on its length and depth. This produces extreme sensitivity to changes in basin geometry from human influences. ¿ 1998 American Geophysical Union