That line is a reference to Psalm 137, in which people of Israel, exiled to Babylon, cry by the river as they remember Jerusalem. In this poem, the people of Israel represent Europeans, and Jerusalem represents Europe before WWI. After the devastated war, Europeans went through an extremely hard time, and they could not do anything but wept for the good memories of the past, the glorious time of Europe before the war.
Leman is the French name of Lake Geneva (Lac Léman); the reference is to the convalescent leave from Lloyd's Bank that Eliot spent in Lausanne, on the shores of the lake, in order to receive psychological treatment. A large part of The Waste Land was written in this time.