Unsteady natural convection in a reservoir model subject to constant cooling at the water surface is considered in this paper. In such a case, the unequal heat loss resulting from a varying water depth creates a horizontal temperature gradient which in turn drives a horizontal exchange flow between the coastal region and the main water body of the reservoir. Understanding of the flow mechanisms pertinent to this flow is important for predicting the transport of nutrients and pollutants across the reservoir. The present numerical simulations impose random perturbations at the water surface in order to trigger flow instabilities. First, numerical tests are carried out to investigate the flow response to different perturbation amplitudes. These tests reveal a linear range of the flow response to the perturbations. Subsequently, the transient flow development at different stages is described in details based on numerical data, and the effects of the Grashof number on the unsteady flow are examined. (C) 2008 Elsevier Ltd. All rights reserved.