I understand that the ship could not be salvaged, however the film does not explain what actually happens to the wreck at the end of the film "Generations".
We don't know. After crash-landing on Veridian III, the saucer isn't mentioned in any further films or TV shows
That being said, Veridian IV hosts a pre-warp civilisation, so a starship (even a broken one) isn't the sort of thing you'd want to leave lying around on a planet they could reach before they've achieved warp drive.
The saucer-section is mentioned in the Expanded Universe novel 'The Return' by William Shatner. In short, Riker was tasked with supervising the salvage of the Enterprise-D's saucer section, precisely to avoid contaminating the Veridianese culture.
As Commander William Riker resolved from the transporter beam beside the grave of that Starfleet legend, he was surprised by the sudden thought that had come to him. Of all that had happened on this desolate world of Veridian Ill only a month ago, inexplicably, the fate of James T. Kirk weighed most heavily on his mind.
Half a planet away, the shattered hulk of the U.S.S. Enterprise lay in ruins, slowly being carved into transporter loads of recyclable scrap by a team of Starfleet engineers. Though the ship was beyond salvage, in accordance with the Prime Directive no trace of it could remain on this world. A primitive civilization existed on Veridian IV, the next planet out from the Veridian sun. If someday voyagers from that world landed here, they must find no trace of advanced technology which might affect the natural development of their science.