Shark Bay is a large coastal embayment(length similar to 250 km, width similar to 100 km) located on the central west coast of Australia. The Bay is comprised of two major reaches, which are characterised by average depths of 10 m and salinities which increase with longitudinal distance away from the Bay entrance. Maximum salinities in the Bay exceed 60 (Practical Salinity Scale), and occur in Hamelin Pool at the southern end of Hopeless Reach, the eastern region of the Bay. Exchange between Hamelin Pool and Hopeless Reach is severely restricted by the presence of a sill, and occurs predominantly through a single 2 km wide, 6 m deep channel (Herald Loop). CTD measurements taken in Hopeless Reach show variable stratification: vertically well-mixed in summer and strongly stratified during the winter survey. A shallow cavity natural convection model was applied to the Herald Loop channel to determine the contribution of the saline discharge from Hamelin Pool to the observed variable stratification. The model shows that the discharge from Hamelin Pool is a diffusive process. Thus while the discharge is an important salinity source, it will not directly contribute to the variability observed in Hopeless Reach. (C) 1999 Elsevier Science Ltd. All rights reserved.