The distributions of Δ18O, salinity, temperature, and nutrients have been used to quantify water sources to the Mackenzie shelf in the Beaufort Sea. Comparison of water mass analyses with satellite imagery confirms that the meteoric (runoff) water is associated with the Mackenzie plume. The seasonally variable surface layer for the shelf is viewed as cycling between a ''reverse estuary'' in winter, when the polar mixed layer (PML) is formed, and a positive estuary in summer when the shelf waters respond to freshwater inputs (runoff and ice melt). We infer a standing stock of 3.7 m fresh water at the end of summer 1986, of which 30% owes its origin to the melting of sea ice; our data coupled with river flow imply a freshwater flushing time for the Mackenzie shelf at about 150 days. To re-form the PML during winter requires the removal of this seasonal fresh water through the combined processes of flushing and ice formation: once this fresh water has been removed, continued ice growth can produce ''new'' brine which would be observed as a deeper and saltier PML from the previous year. A simple geochemical model shows that autumm conditions (freshwater accumulation) and the rate of flushing are important controls on the potential of the shelf to produce ''new'' brine and that winter runoff, were it to distribute evenly across the shelf, is sufficient to inhibit brine production.