This paper presents the results of an experimental and numerical investigation of a flow in which salt water is purged from a square cavity by an overflow of fresh water. Two numerical simulations are presented, one two-dimensional simulation and one three-dimensional large eddy simulation. The results are used to describe the important transport mechanisms that occur during the purging process. In particular, we propose a mechanism for the formation of the streamers observed in the experiment. We also discuss the performance of the numerical models for flows of this type. (C) 2004 Elsevier Ltd. All rights reserved.