The Mackenzie shelf is a broad, estuarine region bordering the southeastern Beaufort Sea in the Arctic Ocean. Its fields of temperature and salinity result from the modification of offshore water masses by river inflow, ice melting and freeze, solar insolation, and air-sea exchange. We here relate water masses resident on the Mackenzie shelf to the large-scale oceanography of the Arctic mediterranean. The summertime exchange between the shelf and open ocean is largely confined to waters lying above the main halocline (S34.2 psu). Cross-shelf property distributions show that individual water masses maintain their structural identity (i.e., core properties and buoyancy frequency) as they move across the shelf and participate in the estuarine circulation. Shelf waters are strongly influenced by river inflow; however, the concept of a single ''plume'' issuing from the incoming river and forming a strictly two-layered structure over uniform shelf water is misleading, since a variety of temperature, salinity, and turbidity fronts co-exist on the shelf at any given time. ¿ American Geophysical Union 1989