Complex sentence: One theory suggests that Cuerdale was chosen as the site to ury the hoard because it was the extent of the river that was navigable though the river could be navigated up to Ribchester in Roman times.
Split into simpler sentences: One theory suggests that Cuerdale was chosen as the site to bury the hoard because it was the limit of the river that was navigable. The river could be navigated up to Ribchester in Roman times.