An arm of the Irish Sea, and in its upper part forming the estuary of the river Esk, separating Cumberland from the S. of Scotland (Kirkcudbright and Dumfries); stretches inland from Balcarry Point 36 m., and from 2 to 20 m. broad; receives the Annan, Dee, Nith, Eden, and Derwent, and has valuable salmon-fishings; the spring tides ebb and flow with remarkable rapidity, the "bore" often reaching a speed of from 8 to 10 m. an hour; is spanned near Annan by a railway viaduct 1960 yards long.