Please explain the line... from poem lochinvar... Love swells like the solway, but ebbs like its tide

IT MEANS WHEN LOCHINVAR USED TO LOVE ELLEN HIS LOVE WAS LIKE THE GREAT HIGH TIDE OF THE SOLWAY RIVER BUT NOW HE HAS LOST LOVE FOR HER AND HE HAS JUST WENT

TO NETHERBY HALL TO PARTICIPATE IN THE WEDDING CEREMONY