Using a simple laboratory model we believe that we have clarified the mechanisms whereby a train of solitary waves can be generated by the barotropic tidal flow of a stratified fluid over a three-dimensional obstacle. As the tidal flow reaches critical value of the internal Froude number a downstream depression is formed in the mixed layer. When the tide slackens and turns, this depression moves upstream and evolves into a sequence of solitary waves. Under some circumstances the depression becomes turbulent, and intense mixing takes place. In this case it is also collapse of the mixed region that generates solitary waves which mainly propagate upstream. Available field data are consistent with this explanation, and we can estimate the number of waves formed using existing theory.