Mersey, THE Riveb, is formed from the junction of the Goyt and Etherow in the north-east of Cheshire. During the greater part of its course of 68 miles it forms the boundary between Lancashire and Cheshire. Its chief affluent is the Irwell, from the point of its junction with which stream it is navigable to the mouth. Navigation is aided by sea-walls and good pilotage. The estuary is entered on the Cheshire side by the Manchester ship-canal, and a tunnel connecting Liverpool and Birkenhead was made under the water in 1886.