Mersey, an important river of England, separates, in its lower course, the counties of Chester and Lancaster, and has its origin in the junction of the Etherow and Goyt, on the borders of Derbyshire. It flows in a west-south-west direction, and is joined on the right by the Irwell 6 miles below Manchester, from which point it was made navigable to Liverpool for large vessels in the year 1720. Besides the Irwell the chief affluents are the Bollin and the Weaver from Cheshire. At its junction with the Weaver the Mersey expands into an estuary which forms the Liverpool channel, and which is 16 miles long and 1 to 3 miles broad (1 1/4 opposite Liverpool). In this estuary on the Cheshire side is the entrance to the Manchester (q.v.) Ship-canal. The estuary is much obstructed by sandbanks, but excellent pilotage, combined with the admirable construction of the sea-walls, renders the navigation comparatively secure. Entire length, with the estuary, 70 miles. A railway Mersey tunnel between Liverpool and Birkenhead was opened on January 20,1886. The alluvial meadows along the Mersey are famous for their fertility ; and by embanking the river, many thousands of acres of most valuable land have been reclaimed.